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  • 1. Ackley, K.
    et al.
    Amati, L.
    Barbieri, C.
    Bauer, F. E.
    Benetti, S.
    Bernardini, M. G.
    Bhirombhakdi, K.
    Botticella, M. T.
    Branchesi, M.
    Brocato, E.
    Bruun, S. H.
    Bulla, Mattia
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Campana, S.
    Cappellaro, E.
    Castro-Tirado, A. J.
    Chambers, K. C.
    Chaty, S.
    Chen, Ting-Wan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Max-Planck-Institut für Extraterrestrische Physik, Germany.
    Ciolfi, R.
    Coleiro, A.
    Copperwheat, C. M.
    Covino, S.
    Cutter, R.
    D'Ammando, F.
    D'Avanzo, P.
    De Cesare, G.
    D'Elia, V.
    Della Valle, M.
    Denneau, L.
    De Pasquale, M.
    Dhillon, V. S.
    Dyer, M. J.
    Elias-Rosa, N.
    Evans, P. A.
    Eyles-Ferris, R. A. J.
    Fiore, A.
    Fraser, M.
    Fruchter, A. S.
    Fynbo, J. P. U.
    Galbany, L.
    Gall, C.
    Galloway, D. K.
    Getman, F.
    Ghirlanda, G.
    Gillanders, J. H.
    Gomboc, A.
    Gompertz, B. P.
    Gonzalez-Fernandez, C.
    Gonzalez-Gaitan, S.
    Grado, A.
    Greco, G.
    Gromadzki, M.
    Groot, P. J.
    Gutierrez, C. P.
    Heikkila, T.
    Heintz, K. E.
    Hjorth, J.
    Hu, Y.-D.
    Huber, M. E.
    Inserra, C.
    Izzo, L.
    Japelj, J.
    Jerkstrand, Anders
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Jin, Z. P.
    Jonker, P. G.
    Kankare, E.
    Kann, D. A.
    Kennedy, M.
    Kim, S.
    Klose, S.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kotak, R.
    Kuncarayakti, H.
    Lamb, G. P.
    Leloudas, G.
    Levan, A. J.
    Longo, F.
    Lowe, T. B.
    Lyman, J. D.
    Magnier, E.
    Maguire, K.
    Maiorano, E.
    Mandel, I.
    Mapelli, M.
    Mattila, S.
    McBrien, O. R.
    Melandri, A.
    Michalowski, M. J.
    Milvang-Jensen, B.
    Moran, S.
    Nicastro, L.
    Nicholl, M.
    Nicuesa Guelbenzu, A.
    Nuttal, L.
    Oates, S. R.
    O'Brien, P. T.
    Onori, F.
    Palazzi, E.
    Patricelli, B.
    Perego, A.
    Torres, M. A. P.
    Perley, D. A.
    Pian, E.
    Pignata, G.
    Piranomonte, S.
    Poshyachinda, S.
    Possenti, A.
    Pumo, M. L.
    Quirola-Vasquez, J.
    Ragosta, F.
    Ramsay, G.
    Rau, A.
    Rest, A.
    Reynolds, T. M.
    Rosetti, S. S.
    Rossi, A.
    Rosswog, Stephan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sabha, N. B.
    Sagués Carracedo, Ana
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Salafia, O. S.
    Salmon, L.
    Salvaterra, R.
    Savaglio, S.
    Sbordone, L.
    Schady, P.
    Schipani, P.
    Schultz, A. S. B.
    Schweyer, Tassilo
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Max-Planck-Institut für Extraterrestrische Physik, Germany.
    Smartt, S. J.
    Smith, K. W.
    Smith, M.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Srivastav, S.
    Stanway, E. R.
    Starling, R. L. C.
    Steeghs, D.
    Stratta, G.
    Stubbs, C. W.
    Tanvir, N. R.
    Testa, V.
    Thrane, E.
    Tonry, J. L.
    Turatto, M.
    Ulaczyk, K.
    van der Horst, A. J.
    Vergani, S. D.
    Walton, N. A.
    Watson, D.
    Wiersema, K.
    Wiik, K.
    Wyrzykowski, L.
    Yang, Sheng
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Yi, S.-X.
    Young, D. R.
    Observational constraints on the optical and near-infrared emission from the neutron star-black hole binary merger candidate S190814bv2020In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 643, article id A113Article in journal (Refereed)
    Abstract [en]

    Context. Gravitational wave (GW) astronomy has rapidly reached maturity, becoming a fundamental observing window for modern astrophysics. The coalescences of a few tens of black hole (BH) binaries have been detected, while the number of events possibly including a neutron star (NS) is still limited to a few. On 2019 August 14, the LIGO and Virgo interferometers detected a high-significance event labelled S190814bv. A preliminary analysis of the GW data suggests that the event was likely due to the merger of a compact binary system formed by a BH and a NS.

    Aims. In this paper, we present our extensive search campaign aimed at uncovering the potential optical and near infrared electromagnetic counterpart of S190814bv. We found no convincing electromagnetic counterpart in our data. We therefore use our non-detection to place limits on the properties of the putative outflows that could have been produced by the binary during and after the merger.

    Methods. Thanks to the three-detector observation of S190814bv, and given the characteristics of the signal, the LIGO and Virgo Collaborations delivered a relatively narrow localisation in low latency - a 50% (90%) credible area of 5 deg(2) (23 deg(2)) - despite the relatively large distance of 26752 Mpc. ElectromagNetic counterparts of GRAvitational wave sources at the VEry Large Telescope collaboration members carried out an intensive multi-epoch, multi-instrument observational campaign to identify the possible optical and near infrared counterpart of the event. In addition, the ATLAS, GOTO, GRAWITA-VST, Pan-STARRS, and VINROUGE projects also carried out a search on this event. In this paper, we describe the combined observational campaign of these groups.

    Results. Our observations allow us to place limits on the presence of any counterpart and discuss the implications for the kilonova (KN), which was possibly generated by this NS-BH merger, and for the strategy of future searches. The typical depth of our wide-field observations, which cover most of the projected sky localisation probability (up to 99.8%, depending on the night and filter considered), is r similar to 22 (resp. K similar to 21) in the optical (resp. near infrared). We reach deeper limits in a subset of our galaxy-targeted observations, which cover a total similar to 50% of the galaxy-mass-weighted localisation probability. Altogether, our observations allow us to exclude a KN with large ejecta mass M greater than or similar to 0.1 M-circle dot to a high (> 90%) confidence, and we can exclude much smaller masses in a sub-sample of our observations. This disfavours the tidal disruption of the neutron star during the merger.

    Conclusions. Despite the sensitive instruments involved in the campaign, given the distance of S190814bv, we could not reach sufficiently deep limits to constrain a KN comparable in luminosity to AT 2017gfo on a large fraction of the localisation probability. This suggests that future (likely common) events at a few hundred megaparsecs will be detected only by large facilities with both a high sensitivity and large field of view. Galaxy-targeted observations can reach the needed depth over a relevant portion of the localisation probability with a smaller investment of resources, but the number of galaxies to be targeted in order to get a fairly complete coverage is large, even in the case of a localisation as good as that of this event.

  • 2. Agudo, I.
    et al.
    Chen, T. -W.
    Stockholm Univ, Oskar Klein Ctr, Dept Astron, AlbaNova, Stockholm, Sweden.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Schulze, Steve
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Yang, S.
    Stockholm Univ, Oskar Klein Ctr, Dept Astron, AlbaNova, Stockholm, Sweden.
    Young, D. R.
    Panning for gold, but finding helium: Discovery of the ultra-stripped supernova SN 2019wxt from gravitational-wave follow-up observations2023In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 675, article id A201Article in journal (Refereed)
    Abstract [en]

    We present the results from multi-wavelength observations of a transient discovered during an intensive follow-up campaign of S191213g, a gravitational wave (GW) event reported by the LIGO-Virgo Collaboration as a possible binary neutron star merger in a low latency search. This search yielded SN 2019wxt, a young transient in a galaxy whose sky position (in the 80% GW contour) and distance (similar to SIM;150 Mpc) were plausibly compatible with the localisation uncertainty of the GW event. Initially, the transient's tightly constrained age, its relatively faint peak magnitude (M-i similar to -16.7 mag), and the r-band decline rate of similar to 1 mag per 5 days appeared suggestive of a compact binary merger. However, SN 2019wxt spectroscopically resembled a type Ib supernova, and analysis of the optical-near-infrared evolution rapidly led to the conclusion that while it could not be associated with S191213g, it nevertheless represented an extreme outcome of stellar evolution. By modelling the light curve, we estimated an ejecta mass of only similar to 0.1 M circle dot, with Ni-56 comprising similar to 20% of this. We were broadly able to reproduce its spectral evolution with a composition dominated by helium and oxygen, with trace amounts of calcium. We considered various progenitor channels that could give rise to the observed properties of SN 2019wxt and concluded that an ultra-stripped origin in a binary system is the most likely explanation. Disentangling genuine electromagnetic counterparts to GW events from transients such as SN 2019wxt soon after discovery is challenging: in a bid to characterise this level of contamination, we estimated the rate of events with a volumetric rate density comparable to that of SN 2019wxt and found that around one such event per week can occur within the typical GW localisation area of O4 alerts out to a luminosity distance of 500 Mpc, beyond which it would become fainter than the typical depth of current electromagnetic follow-up campaigns.

  • 3. Ahumada, Tomas
    et al.
    Anand, Shreya
    Coughlin, Michael W.
    Andreoni, Igor
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kumar, Harsh
    Reusch, Simeon
    Sagués Carracedo, Ana
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Stein, Robert
    Cenko, S. Bradley
    Kasliwal, Mansi M.
    Singer, Leo P.
    Dunwoody, Rachel
    Mangan, Joseph
    Bhalerao, Varun
    Bulla, Mattia
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Burns, Eric
    Graham, Matthew J.
    Kaplan, David L.
    Perley, Daniel
    Almualla, Mouza
    Bloom, Joshua S.
    Cunningham, Virginia
    De, Kishalay
    Gatkine, Pradip
    Ho, Anna Y. Q.
    Karambelkar, Viraj
    Kong, Albert K. H.
    Yao, Yuhan
    Anupama, G. C.
    Barway, Sudhanshu
    Ghosh, Shaon
    Itoh, Ryosuke
    McBreen, Sheila
    Bellm, Eric C.
    Fremling, Christoffer
    Laher, Russ R.
    Mahabal, Ashish A.
    Riddle, Reed L.
    Rosnet, Philippe
    Rusholme, Ben
    Smith, Roger
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bissaldi, Elisabetta
    Fletcher, Corinne
    Hamburg, Rachel
    Mailyan, Bagrat
    Malacaria, Christian
    Roberts, Oliver
    In Search of Short Gamma-Ray Burst Optical Counterparts with the Zwicky Transient Facility2022In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 932, no 1, article id 40Article in journal (Refereed)
    Abstract [en]

    The Fermi Gamma-ray Burst Monitor (GBM) triggers on-board in response to ∼40 short gamma-ray bursts (SGRBs) per year; however, their large localization regions have made the search for optical counterparts a challenging endeavour. We have developed and executed an extensive program with the wide field of view of the Zwicky Transient Facility (ZTF) camera, mounted on the Palomar 48 inch Oschin telescope (P48), to perform target-of-opportunity (ToO) observations on 10 Fermi-GBM SGRBs during 2018 and 2020–2021. Bridging the large sky areas with small field-of-view optical telescopes in order to track the evolution of potential candidates, we look for the elusive SGRB afterglows and kilonovae (KNe) associated with these high-energy events. No counterpart has yet been found, even though more than 10 ground-based telescopes, part of the Global Relay of Observatories Watching Transients Happen (GROWTH) network, have taken part in these efforts. The candidate selection procedure and the follow-up strategy have shown that ZTF is an efficient instrument for searching for poorly localized SGRBs, retrieving a reasonable number of candidates to follow up and showing promising capabilities as the community approaches the multi-messenger era. Based on the median limiting magnitude of ZTF, our searches would have been able to retrieve a GW170817-like event up to ∼200 Mpc and SGRB afterglows to z = 0.16 or 0.4, depending on the assumed underlying energy model. Future ToOs will expand the horizon to z = 0.2 and 0.7, respectively.

  • 4. Ahumada, Tomas
    et al.
    Singer, Leo P.
    Anand, Shreya
    Coughlin, Michael W.
    Kasliwal, Mansi M.
    Ryan, Geoffrey
    Andreoni, Igor
    Cenko, S. Bradley
    Fremling, Christoffer
    Kumar, Harsh
    Pang, Peter T. H.
    Burns, Eric
    Cunningham, Virginia
    Dichiara, Simone
    Dietrich, Tim
    Svinkin, Dmitry S.
    Almualla, Mouza
    Castro-Tirado, Alberto J.
    De, Kishalay
    Dunwoody, Rachel
    Gatkine, Pradip
    Hammerstein, Erica
    Iyyani, Shabnam
    Mangan, Joseph
    Perley, Dan
    Purkayastha, Sonalika
    Bellm, Eric
    Bhalerao, Varun
    Bolin, Bryce
    Bulla, Mattia
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Cannella, Christopher
    Chandra, Poonam
    Duev, Dmitry A.
    Frederiks, Dmitry
    Gal-Yam, Avishay
    Graham, Matthew
    Ho, Anna Y. Q.
    Hurley, Kevin
    Karambelkar, Viraj
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kulkarni, S. R.
    Mahabal, Ashish
    Masci, Frank
    McBreen, Sheila
    Pandey, Shashi B.
    Reusch, Simeon
    Ridnaia, Anna
    Rosnet, Philippe
    Rusholme, Benjamin
    Carracedo, A. Sagués
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Smith, Roger
    Soumagnac, Maayane
    Stein, Robert
    Troja, Eleonora
    Tsvetkova, Anastasia
    Walters, Richard
    Valeev, Azamat F.
    Discovery and confirmation of the shortest gamma-ray burst from a collapsar2021In: Nature Astronomy, E-ISSN 2397-3366, Vol. 5, no 9, p. 917-927Article in journal (Refereed)
    Abstract [en]

    Gamma-ray bursts (GRBs) are among the brightest and most energetic events in the Universe. The duration and hardness distribution of GRBs has two clusters(1), now understood to reflect (at least) two different progenitors(2). Short-hard GRBs (SGRBs; T-90 < 2 s) arise from compact binary mergers, and long-soft GRBs (LGRBs; T-90 > 2 s) have been attributed to the collapse of peculiar massive stars (collapsars)(3). The discovery of SN 1998bw/GRB 980425 (ref. (4)) marked the first association of an LGRB with a collapsar, and AT 2017gfo (ref. (5))/GRB 170817A/GW170817 (ref. (6)) marked the first association of an SGRB with a binary neutron star merger, which also produced a gravitational wave. Here, we present the discovery of ZTF20abwysqy (AT2020scz), a fast-fading optical transient in the Fermi satellite and the Interplanetary Network localization regions of GRB 200826A; X-ray and radio emission further confirm that this is the afterglow. Follow-up imaging (at rest-frame 16.5 days) reveals excess emission above the afterglow that cannot be explained as an underlying kilonova, but which is consistent with being the supernova. Although the GRB duration is short (rest-frame T-90 of 0.65 s), our panchromatic follow-up data confirm a collapsar origin. GRB 200826A is the shortest LGRB found with an associated collapsar; it appears to sit on the brink between a successful and a failed collapsar. Our discovery is consistent with the hypothesis that most collapsars fail to produce ultra-relativistic jets.

  • 5. Anand, Shreya
    et al.
    Coughlin, Michael W.
    Kasliwal, Mansi M.
    Bulla, Mattia
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Ahumada, Tomás
    Sagués Carracedo, Ana
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Almualla, Mouza
    Andreoni, Igor
    Stein, Robert
    Foucart, Francois
    Singer, Leo P.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bellm, Eric C.
    Bolin, Bryce
    Caballero-García, M. D.
    Castro-Tirado, Alberto J.
    Cenko, S. Bradley
    De, Kishalay
    Dekany, Richard G.
    Duev, Dmitry A.
    Feeney, Michael
    Fremling, Christoffer
    Goldstein, Daniel A.
    Golkhou, V. Zach
    Graham, Matthew J.
    Guessoum, Nidhal
    Hankins, Matthew J.
    Hu, Youdong
    Kong, Albert K. H.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kulkarni, S. R.
    Kumar, Harsh
    Laher, Russ R.
    Masci, Frank J.
    Mróz, Przemek
    Nissanke, Samaya
    Porter, Michael
    Reusch, Simeon
    Riddle, Reed
    Rosnet, Philippe
    Rusholme, Ben
    Serabyn, Eugene
    Sánchez-Ramírez, R.
    Rigault, Mickael
    Shupe, David L.
    Smith, Roger
    Soumagnac, Maayane T.
    Walters, Richard
    Valeev, Azamat F.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). California Institute of Technology, USA; Russian Academy of Sciences, Russia.
    Optical follow-up of the neutron star-black hole mergers S200105ae and S200115j2021In: Nature Astronomy, E-ISSN 2397-3366, Vol. 5, p. 46-53Article in journal (Refereed)
    Abstract [en]

    LIGO and Virgo's third observing run revealed the first neutron star-black hole (NSBH) merger candidates in gravitational waves. These events are predicted to synthesize r-process elements(1,2)creating optical/near-infrared 'kilonova' emission. The joint gravitational wave and electromagnetic detection of an NSBH merger could be used to constrain the equation of state of dense nuclear matter(3), and independently measure the local expansion rate of the Universe(4). Here, we present the optical follow-up and analysis of two of the only three high-significance NSBH merger candidates detected to date, S200105ae and S200115j, with the Zwicky Transient Facility(5). The Zwicky Transient Facility observed similar to 48% of S200105ae and similar to 22% of S200115j's localization probabilities, with observations sensitive to kilonovae brighter than -17.5 mag fading at 0.5 mag d(-1)in the g- and r-bands; extensive searches and systematic follow-up of candidates did not yield a viable counterpart. We present state-of-the-art kilonova models tailored to NSBH systems that place constraints on the ejecta properties of these NSBH mergers. We show that with observed depths of apparent magnitude similar to 22 mag, attainable in metre-class, wide-field-of-view survey instruments, strong constraints on ejecta mass are possible, with the potential to rule out low mass ratios, high black hole spins and large neutron star radii.

  • 6. Andreoni, Igor
    et al.
    Coughlin, Michael W.
    Almualla, Mouza
    Bellm, Eric C.
    Bianco, Federica B.
    Bulla, Mattia
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Cucchiara, Antonino
    Dietrich, Tim
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Li, Xiaolong
    Ragosta, Fabio
    Sagués Carracedo, Ana
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Singer, Leo P.
    Optimizing Cadences with Realistic Light-curve Filtering for Serendipitous Kilonova Discovery with Vera Rubin Observatory2022In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 258, no 1, article id 5Article in journal (Refereed)
    Abstract [en]

    Current and future optical and near-infrared wide-field surveys have the potential to find kilonovae, the optical and infrared counterparts to neutron star mergers, independently of gravitational-wave or high-energy gamma-ray burst triggers. The ability to discover fast and faint transients such as kilonovae largely depends on the area observed, the depth of those observations, the number of revisits per field in a given time frame, and the filters adopted by the survey; it also depends on the ability to perform rapid follow-up observations to confirm the nature of the transients. In this work, we assess kilonova detectability in existing simulations of the Legacy Survey of Space and Time strategy for the Vera C. Rubin Wide Fast Deep survey, with focus on comparing rolling to baseline cadences. Although currently available cadences can enable the detection of >300 kilonovae out to ∼1400 Mpc over the 10 year survey, we can expect only 3–32 kilonovae similar to GW170817 to be recognizable as fast-evolving transients. We also explore the detectability of kilonovae over the plausible parameter space, focusing on viewing angle and ejecta masses. We find that observations in redder izy bands are crucial for identification of nearby (within 300 Mpc) kilonovae that could be spectroscopically classified more easily than more distant sources. Rubin's potential for serendipitous kilonova discovery could be increased by gain of efficiency with the employment of individual 30 s exposures (as opposed to 2 × 15 s snap pairs), with the addition of red-band observations coupled with same-night observations in g or r bands, and possibly with further development of a new rolling-cadence strategy.

  • 7. Andreoni, Igor
    et al.
    Coughlin, Michael W.
    Kool, Erik C.
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Astronomy.
    Kasliwal, Mansi M.
    Kumar, Harsh
    Bhalerao, Varun
    Carracedo, Ana Sagués
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Ho, Anna Y. Q.
    Pang, Peter T. H.
    Saraogi, Divita
    Sharma, Kritti
    Shenoy, Vedant
    Burns, Eric
    Ahumada, Tomás
    Anand, Shreya
    Singer, Leo P.
    Perley, Daniel A.
    De, Kishalay
    Fremling, U. C.
    Bellm, Eric C.
    Bulla, Mattia
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Astronomy.
    Crellin-Quick, Arien
    Dietrich, Tim
    Drake, Andrew
    Duev, Dmitry A.
    Goobar, Ariel
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Graham, Matthew J.
    Kaplan, David L.
    Kulkarni, S. R.
    Laher, Russ R.
    Mahabal, Ashish A.
    Shupe, David L.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Astronomy.
    Walters, Richard
    Yao, Yuhan
    Fast-transient Searches in Real Time with ZTFReST: Identification of Three Optically Discovered Gamma-Ray Burst Afterglows and New Constraints on the Kilonova Rate2021In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 918, no 2, article id 63Article in journal (Refereed)
    Abstract [en]

    The most common way to discover extragalactic fast transients, which fade within a few nights in the optical, is via follow-up of gamma-ray burst and gravitational-wave triggers. However, wide-field surveys have the potential to identify rapidly fading transients independently of such external triggers. The volumetric survey speed of the Zwicky Transient Facility (ZTF) makes it sensitive to objects as faint and fast fading as kilonovae, the optical counterparts to binary neutron star mergers, out to almost 200 Mpc. We introduce an open-source software infrastructure, the ZTF REaltime Search and Triggering, ZTFReST, designed to identify kilonovae and fast transients in ZTF data. Using the ZTF alert stream combined with forced point-spread-function photometry, we have implemented automated candidate ranking based on their photometric evolution and fitting to kilonova models. Automated triggering, with a human in the loop for monitoring, of follow-up systems has also been implemented. In 13 months of science validation, we found several extragalactic fast transients independently of any external trigger, including two supernovae with post-shock cooling emission, two known afterglows with an associated gamma-ray burst (ZTF20abbiixp, ZTF20abwysqy), two known afterglows without any known gamma-ray counterpart (ZTF20aajnksq, ZTF21aaeyldq), and three new fast-declining sources (ZTF20abtxwfx, ZTF20acozryr, ZTF21aagwbjr) that are likely associated with GRB200817A, GRB201103B, and GRB210204A. However, we have not found any objects that appear to be kilonovae. We constrain the rate of GW170817-like kilonovae to R < 900 Gpc(-3) yr(-1) (95% confidence). A framework such as ZTFReST could become a prime tool for kilonova and fast-transient discovery with the Vera Rubin Observatory.

  • 8. Andreoni, Igor
    et al.
    Goldstein, Daniel A.
    Anand, Shreya
    Coughlin, Michael W.
    Singer, Leo P.
    Ahumada, Tomás
    Medford, Michael
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Webb, Sara
    Bulla, Mattia
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bloom, Joshua S.
    Kasliwal, Mansi M.
    Nugent, Peter E.
    Bagdasaryan, Ashot
    Barnes, Jennifer
    Cook, David O.
    Cooke, Jeff
    Duev, Dmitry A.
    Fremling, U. Christoffer
    Gatkine, Pradip
    Golkhou, V. Zach
    Kong, Albert K. H.
    Mahabal, Ashish
    Martínez-Palomera, Jorge
    Tao, Duo
    Zhang, Keming
    GROWTH on S190510g: DECam Observation Planning and Follow-up of a Distant Binary Neutron Star Merger Candidate2019In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 881, no 1, article id L16Article in journal (Refereed)
    Abstract [en]

    The first two months of the third Advanced LIGO and Virgo observing run (2019 April-May) showed that distant gravitational-wave (GW) events can now be readily detected. Three candidate mergers containing neutron stars (NS) were reported in a span of 15 days, all likely located more than 100 Mpc away. However, distant events such as the three new NS mergers are likely to be coarsely localized, which highlights the importance of facilities and scheduling systems that enable deep observations over hundreds to thousands of square degrees to detect the electromagnetic counterparts. On 2019 May 10 02: 59:39.292 UT the GW candidate S190510g was discovered and initially classified as a binary neutron star (BNS) merger with 98% probability. The GW event was localized within an area of 3462 deg(2), later refined to 1166 deg(2) (90%) at a distance of 227 +/- 92 Mpc. We triggered Target-of-Opportunity observations with the Dark Energy Camera (DECam), a wide-field optical imager mounted at the prime focus of the 4 m Blanco Telescope at Cerro Tololo Inter-American Observatory in Chile. This Letter describes our DECam observations and our real-time analysis results, focusing in particular on the design and implementation of the observing strategy. Within 24 hr of the merger time, we observed 65% of the total enclosed probability of the final skymap with an observing efficiency of 94%. We identified and publicly announced 13 candidate counterparts. S190510g was reclassified 1.7 days after the merger, after our observations were completed, with a BNS merger probability reduced from 98% to 42% in favor of a terrestrial classification.

  • 9. Andreoni, Igor
    et al.
    Goldstein, Daniel A.
    Kasliwal, Mansi M.
    Nugent, Peter E.
    Zhou, Rongpu
    Newman, Jeffrey A.
    Bulla, Mattia
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Foucart, Francois
    Hotokezaka, Kenta
    Nakar, Ehud
    Nissanke, Samaya
    Raaijmakers, Geert
    Bloom, Joshua S.
    De, Kishalay
    Jencson, Jacob E.
    Ward, Charlotte
    Ahumada, Tomas
    Anand, Shreya
    Buckley, David A. H.
    Caballero-Garcia, Maria D.
    Castro-Tirado, Alberto J.
    Copperwheat, Christopher M.
    Coughlin, Michael W.
    Cenko, S. Bradley
    Gromadzki, Mariusz
    Hu, Youdong
    Karambelkar, Viraj R.
    Perley, Daniel A.
    Sharma, Yashvi
    Valeev, Azamat F.
    Cook, David O.
    Fremling, U. Christoffer
    Kumar, Harsh
    Taggart, Kirsty
    Bagdasaryan, Ashot
    Cooke, Jeff
    Dahiwale, Aishwarya
    Dhawan, Suhail
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Dobie, Dougal
    Gatkine, Pradip
    Golkhou, V. Zach
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Chaves, Andreas Guerra
    Hankins, Matthew
    Kaplan, David L.
    Kong, Albert K. H.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mohite, Siddharth
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Tzanidakis, Anastasios
    Webb, Sara
    Zhang, Keming
    GROWTH on S190814bv: Deep Synoptic Limits on the Optical/Near-infrared Counterpart to a Neutron Star-Black Hole Merger2020In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 890, no 2, article id 131Article in journal (Refereed)
    Abstract [en]

    On 2019 August 14, the Advanced LIGO and Virgo interferometers detected the high-significance gravitational wave (GW) signal S190814bv. The GW data indicated that the event resulted from a neutron star-black hole (NSBH) merger, or potentially a low-mass binary BH merger. Due to the low false-alarm rate and the precise localization (23 deg(2) at 90%), S190814bv presented the community with the best opportunity yet to directly observe an optical/near-infrared counterpart to an NSBH merger. To search for potential counterparts, the GROWTH Collaboration performed real-time image subtraction on six nights of public Dark Energy Camera images acquired in the 3 weeks following the merger, covering >98% of the localization probability. Using a worldwide network of follow-up facilities, we systematically undertook spectroscopy and imaging of optical counterpart candidates. Combining these data with a photometric redshift catalog, we ruled out each candidate as the counterpart to S190814bv and placed deep, uniform limits on the optical emission associated with S190814bv. For the nearest consistent GW distance, radiative transfer simulations of NSBH mergers constrain the ejecta mass of S190814bv to be M-ej < 0.04 M-circle dot at polar viewing angles, or M-ej < 0.03 M-circle dot if the opacity is kappa < 2 cm(2)g(-1). Assuming a tidal deformability for the NS at the high end of the range compatible with GW170817 results, our limits would constrain the BH spin component aligned with the orbital momentum to be chi < 0.7 for mass ratios Q < 6, with weaker constraints for more compact NSs.

  • 10. Andreoni, Igor
    et al.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sagués Carracedo, Ana
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kasliwal, Mansi M.
    Bulla, Mattia
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Ahumada, Tomás
    Coughlin, Michael W.
    Anand, Shreya
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kaplan, David L.
    Loveridge, Tegan T.
    Karambelkar, Viraj
    Cooke, Jeff
    Bagdasaryan, Ashot
    Bellm, Eric C.
    Cenko, S. Bradley
    Cook, David O.
    De, Kishalay
    Dekany, Richard
    Delacroix, Alexandre
    Drake, Andrew
    Duev, Dmitry A.
    Fremling, Christoffer
    Golkhou, V. Zach
    Graham, Matthew J.
    Hale, David
    Kulkarni, S. R.
    Kupfer, Thomas
    Laher, Russ R.
    Mahabal, Ashish A.
    Masci, Frank J.
    Rusholme, Ben
    Smith, Roger M.
    Tzanidakis, Anastasios
    Sistine, Angela Van
    Yao, Yuhan
    Constraining the Kilonova Rate with Zwicky Transient Facility Searches Independent of Gravitational Wave and Short Gamma-Ray Burst Triggers2020In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 904, no 2, article id 155Article in journal (Refereed)
    Abstract [en]

    The first binary neutron star merger, GW170817, was accompanied by a radioactivity-powered optical/infrared transient called a kilonova. To date, no compelling kilonova has been found in all-sky optical surveys, independently of short gamma-ray burst and gravitational-wave triggers. In this work, we searched the first 23 months of the Zwicky Transient Facility (ZTF) data stream for candidate kilonovae in the form of rapidly evolving transients. We combined ZTF alert queries with forced point-spread-function photometry and nightly flux stacking to increase our sensitivity to faint and fast transients. Automatic queries yielded >11,200 candidates, 24 of which passed quality checks and selection criteria based on a grid of kilonova models tailored for both binary neutron star and neutron star-black hole mergers. None of the candidates in our sample was deemed a possible kilonova after thorough vetting. The sources that passed our selection criteria are dominated by Galactic cataclysmic variables. We identified two fast transients at high Galactic latitude, one of which is the confirmed afterglow of long-duration GRB.190106A, the other is a possible cosmological afterglow. Using a survey simulation code, we constrained the kilonova rate for a range of models including top-hat, linearly decaying light curves, and synthetic light curves obtained with radiative transfer simulations. For prototypical GW170817-like kilonovae, we constrain the rate to be R < 1775 Gpc(-3) yr(-1) (95% confidence). By assuming a population of kilonovae with the same geometry and composition of GW170817 observed under a uniform viewing angle distribution, we obtained a constraint on the rate of R.<.4029 Gpc(-3) yr(-1).

  • 11. Andreoni, Igor
    et al.
    Margutti, Raffaella
    Salafia, Om Sharan
    Parazin, B.
    Villar, V. Ashley
    Coughlin, Michael W.
    Yoachim, Peter
    Mortensen, Kris
    Brethauer, Daniel
    Smartt, S. J.
    Kasliwal, Mansi M.
    Alexander, Kate D.
    Anand, Shreya
    Berger, E.
    Bernardini, Maria Grazia
    Bianco, Federica B.
    Blanchard, Peter K.
    Bloom, Joshua S.
    Brocato, Enzo
    Bulla, Mattia
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Cartier, Regis
    Cenko, S. Bradley
    Chornock, Ryan
    Copperwheat, Christopher M.
    Corsi, Alessandra
    D'Ammando, Filippo
    D'Avanzo, Paolo
    Datrier, Laurence Elise Helene
    Foley, Ryan J.
    Ghirlanda, Giancarlo
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Grindlay, Jonathan
    Hajela, Aprajita
    Holz, Daniel E.
    Karambelkar, Viraj
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Lamb, Gavin P.
    Laskar, Tanmoy
    Levan, Andrew
    Maguire, Kate
    May, Morgan
    Melandri, Andrea
    Milisavljevic, Dan
    Miller, A. A.
    Nicholl, Matt
    Nissanke, Samaya M.
    Palmese, Antonella
    Piranomonte, Silvia
    Rest, Armin
    Sagués Carracedo, Ana
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Siellez, Karelle
    Singer, Leo P.
    Smith, Mathew
    Steeghs, D.
    Tanvir, Nial
    Target-of-opportunity Observations of Gravitational-wave Events with Vera C. Rubin Observatory2022In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 260, no 1, article id 18Article in journal (Refereed)
    Abstract [en]

    The discovery of the electromagnetic counterpart to the binary neutron star (NS) merger GW170817 has opened the era of gravitational-wave multimessenger astronomy. Rapid identification of the optical/infrared kilonova enabled a precise localization of the source, which paved the way to deep multiwavelength follow-up and its myriad of related science results. Fully exploiting this new territory of exploration requires the acquisition of electromagnetic data from samples of NS mergers and other gravitational-wave sources. After GW170817, the frontier is now to map the diversity of kilonova properties and provide more stringent constraints on the Hubble constant, and enable new tests of fundamental physics. The Vera C. Rubin Observatory's Legacy Survey of Space and Time can play a key role in this field in the 2020s, when an improved network of gravitational-wave detectors is expected to reach a sensitivity that will enable the discovery of a high rate of merger events involving NSs (∼tens per year) out to distances of several hundred megaparsecs. We design comprehensive target-of-opportunity observing strategies for follow-up of gravitational-wave triggers that will make the Rubin Observatory the premier instrument for discovery and early characterization of NS and other compact-object mergers, and yet unknown classes of gravitational-wave events.

  • 12.
    Biswas, Rahul
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Dhawan, Suhail
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). University of Cambridge, UK.
    Schulze, Steve
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Johansson, Joel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bellm, Eric C.
    Dekany, Richard
    Drake, Andrew J.
    Duev, Dmitry A.
    Fremling, Christoffer
    Graham, Matthew
    Kim, Young-Lo
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kulkarni, Shrinivas R.
    Mahabal, Ashish A.
    Perley, Daniel
    Rigault, Mickael
    Rusholme, Ben
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm Univ, Oskar Klein Ctr, Dept Astron, SE-10691 Stockholm, Sweden.
    Shupe, David L.
    Smith, Matthew
    Walters, Richard S.
    Two c's in a pod: cosmology-independent measurement of the Type Ia supernova colour-luminosity relation with a sibling pair2022In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 509, no 4, p. 5340-5356Article in journal (Refereed)
    Abstract [en]

    Using Zwicky Transient Facility (ZTF) observations, we identify a pair of ‘sibling’ Type Ia supernovae (SNe Ia), i.e. hosted by the same galaxy at z = 0.0541. They exploded within 200 d from each other at a separation of 0.6arcsec0.6arcsec corresponding to a projected distance of only 0.6 kpc. Performing SALT2 light-curve fits to the gri ZTF photometry, we show that for these equally distant ‘standardizable candles’, there is a difference of 2 mag in their rest-frame B-band peaks, and the fainter supernova (SN) has a significantly red SALT2 colour c = 0.57 ± 0.04, while the stretch values x1 of the two SNe are similar, suggesting that the fainter SN is attenuated by dust in the interstellar medium of the host galaxy. We use these measurements to infer the SALT2 colour standardization parameter, β = 3.5 ± 0.3, independent of the underlying cosmology and Malmquist bias. Assuming the colour excess is entirely due to dust, the result differs by 2σ from the average Milky Way total-to-selective extinction ratio, but is in good agreement with the colour–brightness corrections empirically derived from the most recent SN Ia Hubble–Lemaitre diagram fits. Thus we suggest that SN ‘siblings’, which will increasingly be discovered in the coming years, can be used to probe the validity of the colour and light-curve shape corrections using in SN Ia cosmology while avoiding important systematic effects in their inference from global multiparameter fits to inhomogeneous data sets, and also help constrain the role of interstellar dust in SN Ia cosmology.

  • 13. Burdge, Kevin B.
    et al.
    Marsh, Thomas R.
    Fuller, Jim
    Bellm, Eric C.
    Caiazzo, Ilaria
    Chakrabarty, Deepto
    Coughlin, Michael W.
    De, Kishalay
    Dhillon, V. S.
    Graham, Matthew J.
    Rodríguez-Gil, Pablo
    Jaodand, Amruta D.
    Kaplan, David L.
    Kara, Erin
    Kong, Albert K. H.
    Kulkarni, S. R.
    Li, Kwan-Lok
    Littlefair, S. P.
    Majid, Walid A.
    Mróz, Przemek
    Pearlman, Aaron B.
    Phinney, E. S.
    van Roestel, Jan
    Simcoe, Robert A.
    Andreoni, Igor
    Drake, Andrew J.
    Dekany, Richard G.
    Duev, Dmitry A.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mahabal, Ashish A.
    Medford, Michael S.
    Riddle, Reed
    Prince, Thomas A.
    A 62-minute orbital period black widow binary in a wide hierarchical triple2022In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 605, no 7908, p. 41-45Article in journal (Refereed)
    Abstract [en]

    Over a dozen millisecond pulsars are ablating low-mass companions in close binary systems. In the original 'black widow', the eight-hour orbital period eclipsing pulsar PSR J1959+2048 (PSR B1957+20)(1), high-energy emission originating from the pulsar2 is irradiating and may eventually destroy(3) a low-mass companion. These systems are not only physical laboratories that reveal the interesting results of exposing a close companion star to the relativistic energy output of a pulsar, but are also believed to harbour some of the most massive neutron stars(4), allowing for robust tests of the neutron star equation of state. Here we report observations of ZTF J1406+1222, a wide hierarchical triple hosting a 62-minute orbital period black widow candidate, the optical flux of which varies by a factor of more than ten. ZTF J1406+1222 pushes the boundaries of evolutionary models(5), falling below the 80-minute minimum orbital period of hydrogen-rich systems. The wide tertiary companion is a rare low-metallicity cool subdwarf star, and the system has a Galactic halo orbit consistent with passing near the Galactic Centre, making it a probe of formation channels, neutron star kick physics(6) and binary evolution.

  • 14. Chomiuk, Laura
    et al.
    Linford, Justin D.
    Aydi, Elias
    Bannister, Keith W.
    Krauss, Miriam
    Mioduszewski, Amy J.
    Mukai, Koji
    Nelson, Thomas J.
    Rupen, Michael P.
    Ryder, Stuart D.
    Sokoloski, Jennifer L.
    Sokolovsky, Kirill
    Strader, Jay
    Filipović, Miroslav D.
    Finzell, Tom
    Kawash, Adam
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Metzger, Brian D.
    Nyamai, Miriam M.
    Ribeiro, Valério A. R. M.
    Roy, Nirupam
    Urquhart, Ryan
    Weston, Jennifer
    Classical Novae at Radio Wavelengths2021In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 257, no 2, article id 49Article in journal (Refereed)
    Abstract [en]

    We present radio observations (1-40 GHz) for 36 classical novae, representing data from over five decades compiled from the literature, telescope archives, and our own programs. Our targets display a striking diversity in their optical parameters (e.g., spanning optical fading timescales, t (2) = 1-263 days), and we find a similar diversity in the radio light curves. Using a brightness temperature analysis, we find that radio emission from novae is a mixture of thermal and synchrotron emission, with nonthermal emission observed at earlier times. We identify high brightness temperature emission (T ( B ) > 5 x 10(4) K) as an indication of synchrotron emission in at least nine (25%) of the novae. We find a class of synchrotron-dominated novae with mildly evolved companions, exemplified by V5589 Sgr and V392 Per, that appear to be a bridge between classical novae with dwarf companions and symbiotic binaries with giant companions. Four of the novae in our sample have two distinct radio maxima (the first dominated by synchrotron and the later by thermal emission), and in four cases the early synchrotron peak is temporally coincident with a dramatic dip in the optical light curve, hinting at a common site for particle acceleration and dust formation. We publish the light curves in a machine-readable table and encourage the use of these data by the broader community in multiwavelength studies and modeling efforts.

  • 15.
    Fransson, Claes
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Strotjohann, Nora L.
    Yang, Sheng
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Schulze, Steve
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ofek, Eran O.
    Crellin-Quick, Arien
    De, Kishalay
    Drake, Andrew J.
    Fremling, Christoffer
    Gal-Yam, Avishay
    Ho, Anna Y. Q.
    Kasliwal, Mansi M.
    SN 2019zrk, a bright SN 2009ip analog with a precursor2022In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 666, article id A79Article in journal (Refereed)
    Abstract [en]

    We present photometric and spectroscopic observations of the Type IIn supernova SN 2019zrk (also known as ZTF 20aacbyec). The SN shows a > 100 day precursor, with a slow rise, followed by a rapid rise to M ≈ −19.2 in the r and g bands. The post-peak light-curve decline is well fit with an exponential decay with a timescale of ∼39 days, but it shows prominent undulations, with an amplitude of ∼1 mag. Both the light curve and spectra are dominated by an interaction with a dense circumstellar medium (CSM), probably from previous mass ejections. The spectra evolve from a scattering-dominated Type IIn spectrum to a spectrum with strong P-Cygni absorptions. The expansion velocity is high, ∼16 000 km s−1, even in the last spectra. The last spectrum ∼110 days after the main eruption reveals no evidence for advanced nucleosynthesis. From analysis of the spectra and light curves, we estimate the mass-loss rate to be ∼4 × 10−2M yr−1 for a CSM velocity of 100 km s−1, and a CSM mass of 1 M. We find strong similarities for both the precursor, general light curve, and spectral evolution with SN 2009ip and similar SNe, although SN 2019zrk displays a brighter peak magnitude. Different scenarios for the nature of the 09ip-class of SNe, based on pulsational pair instability eruptions, wave heating, and mergers, are discussed.

  • 16. Fremling, Christoffer
    et al.
    Hall, Xander J.
    Coughlin, Michael W.
    Dahiwale, Aishwarya S.
    Duev, Dmitry A.
    Graham, Matthew J.
    Kasliwal, Mansi M.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mahabal, Ashish A.
    Miller, Adam A.
    Neill, James D.
    Perley, Daniel A.
    Rigault, Mickael
    Rosnet, Philippe
    Rusholme, Ben
    Sharma, Yashvi
    Shin, Kyung Min
    Shupe, David L.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Walters, Richard S.
    Kulkarni, S. R.
    SNIascore: Deep-learning Classification of Low-resolution Supernova Spectra2021In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 917, no 1, article id L2Article in journal (Refereed)
    Abstract [en]

    We present SNIascore, a deep-learning-based method for spectroscopic classification of thermonuclear supernovae (SNe Ia) based on very low-resolution (R similar to 100) data. The goal of SNIascore is the fully automated classification of SNe Ia with a very low false-positive rate (FPR) so that human intervention can be greatly reduced in large-scale SN classification efforts, such as that undertaken by the public Zwicky Transient Facility (ZTF) Bright Transient Survey (BTS). We utilize a recurrent neural network architecture with a combination of bidirectional long short-term memory and gated recurrent unit layers. SNIascore achieves a SNIascore simultaneously performs binary classification and predicts the redshifts of secure SNe Ia via regression (with a typical uncertainty of z = 0.01 to z = 0.12). For the magnitude-limited ZTF BTS survey (approximate to 70% SNe Ia), deploying SNIascore reduces the amount of spectra in need of human classification or confirmation by approximate to 60%. Furthermore, SNIascore allows SN Ia classifications to be automatically announced in real time to the public immediately following a finished observation during the night.

  • 17. Gal-Yam, A.
    et al.
    Bruch, R.
    Schulze, Steve
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Weizmann Institute of Science, Israel.
    Yang, Y.
    Perley, D. A.
    Irani, I.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Soumagnac, M. T.
    Yaron, O.
    Strotjohann, N. L.
    Zimmerman, E.
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kulkarni, S. R.
    Kasliwal, M. M.
    De, K.
    Yao, Y.
    Fremling, C.
    Yan, L.
    Ofek, E. O.
    Fransson, Claes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Filippenko, A. V.
    Zheng, W.
    Brink, T. G.
    Copperwheat, C. M.
    Foley, R. J.
    Brown, J.
    Siebert, M.
    Leloudas, G.
    Cabrera-Lavers, A. L.
    Garcia-Alvarez, D.
    Marante-Barreto, A.
    Frederick, S.
    Hung, T.
    Wheeler, J. C.
    Vinkó, J.
    Thomas, B. P.
    Graham, M. J.
    Duev, D. A.
    Drake, A. J.
    Dekany, R.
    Bellm, E. C.
    Rusholme, B.
    Shupe, D. L.
    Andreoni, I.
    Sharma, Y.
    Riddle, R.
    van Roestel, J.
    Knezevic, N.
    A WC/WO star exploding within an expanding carbon-oxygen-neon nebula2022In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 601, no 7892, p. 201-204Article in journal (Refereed)
    Abstract [en]

    The final fate of massive stars, and the nature of the compact remnants they leave behind (black holes and neutron stars), are open questions in astrophysics. Many massive stars are stripped of their outer hydrogen envelopes as they evolve. Such Wolf-Rayet stars(1) emit strong and rapidly expanding winds with speeds greater than 1,000 kilometres per second. A fraction of this population is also helium-depleted, with spectra dominated by highly ionized emission lines of carbon and oxygen (types WC/WO). Evidence indicates that the most commonly observed supernova explosions that lack hydrogen and helium (types Ib/Ic) cannot result from massive WC/WO stars(2,3), leading some to suggest that most such stars collapse directly into black holes without a visible supernova explosion(4). Here we report observations of SN 2019hgp, beginning about a day after the explosion. Its short rise time and rapid decline place it among an emerging population of rapidly evolving transients(5-8). Spectroscopy reveals a rich set of emission lines indicating that the explosion occurred within a nebula composed of carbon, oxygen and neon. Narrow absorption features show that this material is expanding at high velocities (greater than 1,500 kilometres per second), requiring a compact progenitor. Our observations are consistent with an explosion of a massive WC/WO star, and suggest that massive Wolf-Rayet stars may be the progenitors of some rapidly evolving transients.

  • 18. Hammerstein, Erica
    et al.
    van Velzen, Sjoert
    Gezari, Suvi
    Cenko, S. Bradley
    Yao, Yuhan
    Ward, Charlotte
    Frederick, Sara
    Villanueva, Natalia
    Somalwar, Jean J.
    Graham, Matthew J.
    Kulkarni, Shrinivas R.
    Stern, Daniel
    Andreoni, Igor
    Bellm, Eric C.
    Dekany, Richard
    Dhawan, Suhail
    Drake, Andrew J.
    Fremling, Christoffer
    Gatkine, Pradip
    Groom, Steven L.
    Ho, Anna Y. Q.
    Kasliwal, Mansi M.
    Karambelkar, Viraj
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Masci, Frank J.
    Medford, Michael S.
    Perley, Daniel A.
    Purdum, Josiah
    Roestel, Jan van
    Sharma, Yashvi
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Taggart, Kirsty
    Yan, Lin
    The Final Season Reimagined: 30 Tidal Disruption Events from the ZTF-I Survey2023In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 942, no 1, article id 9Article in journal (Refereed)
    Abstract [en]

    Tidal disruption events (TDEs) offer a unique way to study dormant black holes. While the number of observed TDEs has grown thanks to the emergence of wide-field surveys in the past few decades, questions regarding the nature of the observed optical, UV, and X-ray emission remain. We present a uniformly selected sample of 30 spectroscopically classified TDEs from the Zwicky Transient Facility Phase I survey operations with follow-up Swift UV and X-ray observations. Through our investigation into correlations between light-curve properties, we recover a shallow positive correlation between the peak bolometric luminosity and decay timescales. We introduce a new spectroscopic class of TDE, TDE-featureless, which are characterized by featureless optical spectra. The new TDE-featureless class shows larger peak bolometric luminosities, peak blackbody temperatures, and peak blackbody radii. We examine the differences between the X-ray bright and X-ray faint populations of TDEs in this sample, finding that X-ray bright TDEs show higher peak blackbody luminosities than the X-ray faint subsample. This sample of optically selected TDEs is the largest sample of TDEs from a single survey yet, and the systematic discovery, classification, and follow-up of this sample allows for robust characterization of TDE properties, an important stepping stone looking forward toward the Rubin era.

  • 19. Ho, Anna Y. Q.
    et al.
    Perley, Daniel A.
    Gal-Yam, Avishay
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Schulze, Steve
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Das, Kaustav K.
    Dobie, Dougal
    Yao, Yuhan
    Fremling, Christoffer
    Adams, Scott
    Anand, Shreya
    Andreoni, Igor
    Bellm, Eric C.
    Bruch, Rachel J.
    Burdge, Kevin B.
    Castro-Tirado, Alberto J.
    Dahiwale, Aishwarya
    De, Kishalay
    Dekany, Richard
    Drake, Andrew J.
    Duev, Dmitry A.
    Graham, Matthew J.
    Helou, George
    Kaplan, David L.
    Karambelkar, Viraj
    Kasliwal, Mansi M.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kulkarni, S. R.
    Mahabal, Ashish A.
    Medford, Michael S.
    Miller, A. A.
    Nordin, Jakob
    Ofek, Eran
    Petitpas, Glen
    Riddle, Reed
    Sharma, Yashvi
    Smith, Roger
    Stewart, Adam J.
    Taggart, Kirsty
    Tartaglia, Leonardo
    Tzanidakis, Anastasios
    Winters, Jan Martin
    A Search for Extragalactic Fast Blue Optical Transients in ZTF and the Rate of AT2018cow-like Transients2023In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 949, no 2, article id 120Article in journal (Refereed)
    Abstract [en]

    We present a search for extragalactic fast blue optical transients (FBOTs) during Phase I of the Zwicky Transient Facility (ZTF). We identify 38 candidates with durations above half-maximum light 1 day < t (1/2) < 12 days, of which 28 have blue (g - r less than or similar to -0.2 mag) colors at peak light. Of the 38 transients (28 FBOTs), 19 (13) can be spectroscopically classified as core-collapse supernovae (SNe): 11 (8) H- or He-rich (Type II/IIb/Ib) SNe, 6 (4) interacting (Type IIn/Ibn) SNe, and 2 (1) H&He-poor (Type Ic/Ic-BL) SNe. Two FBOTs (published previously) had predominantly featureless spectra and luminous radio emission: AT2018lug (The Koala) and AT2020xnd (The Camel). Seven (five) did not have a definitive classification: AT 2020bdh showed tentative broad H alpha in emission, and AT 2020bot showed unidentified broad features and was 10 kpc offset from the center of an early-type galaxy. Ten (eight) have no spectroscopic observations or redshift measurements. We present multiwavelength (radio, millimeter, and/or X-ray) observations for five FBOTs (three Type Ibn, one Type IIn/Ibn, one Type IIb). Additionally, we search radio-survey (VLA and ASKAP) data to set limits on the presence of radio emission for 24 of the transients. All X-ray and radio observations resulted in nondetections; we rule out AT2018cow-like X-ray and radio behavior for five FBOTs and more luminous emission (such as that seen in the Camel) for four additional FBOTs. We conclude that exotic transients similar to AT2018cow, the Koala, and the Camel represent a rare subset of FBOTs and use ZTF's SN classification experiments to measure the rate to be at most 0.1% of the local core-collapse SN rate.

  • 20. Irani, I.
    et al.
    Prentice, S. J.
    Schulze, Steve
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Weizmann Institute of Science, Israel.
    Gal-Yam, A.
    Teffs, Jacob
    Mazzali, Paolo
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Gonzalez, E. P.
    Taggart, K.
    De, Kishalay
    Fremling, Christoffer
    Perley, Daniel A.
    Strotjohann, Nora L.
    Kasliwal, Mansi M.
    Howell, A.
    Dhawan, Suhail
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). University of Cambridge, UK.
    Tzanidakis, Anastasios
    Hiramatsu, Daichi
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Anderson, J. P.
    Müller-Bravo, T. E.
    Dekany, Richard
    Gromadzki, Mariusz
    Carini, Roberta
    Galbany, L.
    Drake, Andrew J.
    Burke, Jamison
    Pellegrino, Craig
    Della Valle, Massimo
    Medford, Michael S.
    Rusholme, Ben
    Young, D. R.
    Gutiérrez, Claudia P.
    Inserra, Cosimo
    Omer, Rafia
    Shupe, David L.
    Chen, Ting-Wan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Shin, Kyung Min
    Yaron, Ofer
    McCully, Curtis
    Nicholl, Matt
    Riddle, Reed
    Less Than 1% of Core-collapse Supernovae in the Local Universe Occur in Elliptical Galaxies2022In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 927, no 1, article id 10Article in journal (Refereed)
    Abstract [en]

    We present observations of three core-collapse supernovae (CCSNe) in elliptical hosts, detected by the Zwicky Transient Facility Bright Transient Survey (BTS). SN 2019ape is a SN Ic that exploded in the main body of a typical elliptical galaxy. Its properties are consistent with an explosion of a regular SN Ic progenitor. A secondary g-band light-curve peak could indicate interaction of the ejecta with circumstellar material (CSM). An Ha-emitting source at the explosion site suggests a residual local star formation origin. SN 2018fsh and SN 2020uik are SNe II which exploded in the outskirts of elliptical galaxies. SN 2020uik shows typical spectra for SNe II, while SN 2018fsh shows a boxy nebular Ha profile, a signature of CSM interaction. We combine these 3 SNe with 7 events from the literature and analyze their hosts as a sample. We present multi-wavelength photometry of the hosts, and compare this to archival photometry of all BTS hosts. Using the spectroscopically complete BTS, we conclude that 0.3%(+0.3)(-0.1) of all CCSNe occur in elliptical galaxies. We derive star formation rates and stellar masses for the host galaxies and compare them to the properties of other SN hosts. We show that CCSNe in ellipticals have larger physical separations from their hosts compared to SNe Ia in elliptical galaxies, and discuss implications for star-forming activity in elliptical galaxies.

  • 21. Kankare, E.
    et al.
    Efstathiou, A.
    Kotak, R.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kangas, T.
    O'Neill, D.
    Mattila, S.
    Väisänen, P.
    Ramphul, R.
    Mogotsi, M.
    Ryder, S. D.
    Parker, S.
    Reynolds, T.
    Fraser, M.
    Pastorello, A.
    Cappellaro, E.
    Mazzali, P. A.
    Ochner, P.
    Tomasella, L.
    Turatto, M.
    Kotilainen, J.
    Kuncarayakti, H.
    Pérez-Torres, M. A.
    Randriamanakoto, Z.
    Romero-Cañizales, C.
    Berton, M.
    Cartier, R.
    Chen, Ting-Wan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Galbany, L.
    Gromadzki, M.
    Inserra, C.
    Maguire, K.
    Moran, S.
    Müller-Bravo, T. E.
    Nicholl, M.
    Reguitti, A.
    Young, D. R.
    Core-collapse supernova subtypes in luminous infrared galaxies2021In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 649, article id A134Article in journal (Refereed)
    Abstract [en]

    The fraction of core-collapse supernovae (CCSNe) occurring in the central regions of galaxies is not well constrained at present. This is partly because large-scale transient surveys operate at optical wavelengths, making it challenging to detect transient sources that occur in regions susceptible to high extinction factors. Here we present the discovery and follow-up observations of two CCSNe that occurred in the luminous infrared galaxy (LIRG) NGC 3256. The first, SN 2018ec, was discovered using the ESO HAWK-I /GRAAL adaptive optics seeing enhancer, and was classified as a Type Ic with a host galaxy extinction of AV = 2:1+0:3 0:1 mag. The second, AT 2018cux, was discovered during the course of follow-up observations of SN 2018ec, and is consistent with a subluminous Type IIP classification with an AV = 2:1 +/- 0:4 mag of host extinction. A third CCSN, PSN J10275082 4354034 in NGC 3256, was previously reported in 2014, and we recovered the source in late-time archival Hubble Space Telescope imaging. Based on template light curve fitting, we favour a Type IIn classification for it with modest host galaxy extinction of AV = 0:3+0:4 0:3 mag. We also extend our study with follow-up data of the recent Type IIb SN 2019lqo and Type Ib SN 2020fkb that occurred in the LIRG system Arp 299 with host extinctions of AV = 2:1 +0:1 0 :3 and AV = 0:4 +0:1 0 :2 mag, respectively. Motivated by the above, we inspected, for the first time, a sample of 29 CCSNe located within a projected distance of 2.5 kpc from the host galaxy nuclei in a sample of 16 LIRGs. We find, if star formation within these galaxies is modelled assuming a global starburst episode and normal IMF, that there is evidence of a correlation between the starburst age and the CCSN subtype. We infer that the two subgroups of 14 H-poor (Type IIb /Ib /Ic /Ibn) and 15 H-rich (Type II /IIn) CCSNe have di fferent underlying progenitor age distributions, with the H-poor progenitors being younger at 3 sigma significance. However, we note that the currently available sample sizes of CCSNe and host LIRGs are small, and the statistical comparisons between subgroups do not take into account possible systematic or model errors related to the estimated starburst ages.

  • 22. Karambelkar, Viraj R.
    et al.
    Kasliwal, Mansi M.
    Maguire, Kate
    Anand, Shreya G.
    Andreoni, Igor
    De, Kishalay
    Drake, Andrew
    Duev, Dmitry A.
    Graham, Matthew J.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Laher, Russ R.
    Magee, Mark R.
    Mahabal, Ashish A.
    Medford, Michael S.
    Perley, Daniel
    Rigault, Mickael
    Rusholme, Ben
    Schulze, Steve
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sharma, Yashvi
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Tzanidakis, Anastasios
    Walters, Richard
    Yao, Yuhan
    Faintest of Them All: ZTF 21aaoryiz/SN 2021fcg-Discovery of an Extremely Low Luminosity Type Iax Supernova2021In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 921, no 1, article id L6Article in journal (Refereed)
    Abstract [en]

    We present the discovery of ZTF 21aaoryiz/SN 2021fcg-an extremely low luminosity Type Tax supernova. SN 2021fcg was discovered by the Zwicky Transient Facility in the star-forming galaxy IC0512 at a distance of approximate to 27 Mpc. It reached a peak absolute magnitude of M-r = -12.66 +/- 0.20 mag, making it the least luminous thermonuclear supernova discovered to date. The E(B - V) contribution from the underlying host galaxy is unconstrained. However, even if it were as large as 0.5 mag, the peak absolute magnitude would be M-r = -13.78 +/- 0.20 mag-still consistent with being the lowest-luminosity SN. Optical spectra of SN 2021fcg taken at 37 and 65 days post-maximum show strong [Ca II], Ca II, and Na I D emission and several weak [Fe II] emission lines. The [Ca II] emission in the two spectra has extremely low velocities of approximate to 1300 and 1000 km s(-1), respectively. The spectra very closely resemble those of the very low luminosity Type Tax supernovae SN 2008 ha, SN 2010ae, and SN 2019gsc taken at similar phases. The peak bolometric luminosity of SN 2021fcg is approximate to 2.5(-0.3)(+1.5) x 10(40) erg s(-1), which is a factor of 3 lower than that for SN 2008 ha. The bolometric lightcurve of SN 2021fcg is consistent with a very low ejected nickel mass (M-Ni approximate to 0.8(-0.5)(+0.4) x 10(-3) M-circle dot). The low luminosity and nickel mass of SN 2021fcg pose a challenge to the picture that low-luminosity SNe Tax originate from deflagrations of near-M-ch hybrid carbon-oxygen-neon white dwarfs. Instead, the merger of a carbon-oxygen and oxygen-neon white dwarf is a promising model to explain SN 2021fcg.

  • 23. Kasliwal, Mansi M.
    et al.
    Anand, Shreya
    Ahumada, Tomás
    Stein, Robert
    Sagués Carracedo, Ana
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Andreoni, Igor
    Coughlin, Michael W.
    Singer, Leo P.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    De, Kishalay
    Kumar, Harsh
    AlMualla, Mouza
    Yao, Yuhan
    Bulla, Mattia
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Dobie, Dougal
    Reusch, Simeon
    Perley, Daniel A.
    Cenko, S. Bradley
    Bhalerao, Varun
    Kaplan, David L.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Goobar, Ariel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Copperwheat, Christopher M.
    Bellm, Eric C.
    Anupama, G. C.
    Corsi, Alessandra
    Nissanke, Samaya
    Agudo, Iván
    Bagdasaryan, Ashot
    Barway, Sudhanshu
    Belicki, Justin
    Bloom, Joshua S.
    Bolin, Bryce
    Buckley, David A. H.
    Burdge, Kevin B.
    Burruss, Rick
    Caballero-García, Maria D.
    Cannella, Chris
    Castro-Tirado, Alberto J.
    Cook, David O.
    Cooke, Jeff
    Cunningham, Virginia
    Dahiwale, Aishwarya
    Deshmukh, Kunal
    Dichiara, Simone
    Duev, Dmitry A.
    Dutta, Anirban
    Feeney, Michael
    Franckowiak, Anna
    Frederick, Sara
    Fremling, Christoffer
    Gal-Yam, Avishay
    Gatkine, Pradip
    Ghosh, Shaon
    Goldstein, Daniel A.
    Golkhou, V. Zach
    Graham, Matthew J.
    Graham, Melissa L.
    Hankins, Matthew J.
    Helou, George
    Hu, Youdong
    Ip, Wing-Huen
    Jaodand, Amruta
    Karambelkar, Viraj
    Kong, Albert K. H.
    Kowalski, Marek
    Khandagale, Maitreya
    Kulkarni, S. R.
    Kumar, Brajesh
    Laher, Russ R.
    Li, K. L.
    Mahabal, Ashish
    Masci, Frank J.
    Miller, Adam A.
    Mogotsi, Moses
    Mohite, Siddharth
    Mooley, Kunal
    Mroz, Przemek
    Newman, Jeffrey A.
    Ngeow, Chow-Choong
    Oates, Samantha R.
    Patil, Atharva Sunil
    Pandey, Shashi B.
    Pavana, M.
    Pian, Elena
    Riddle, Reed
    Sánchez-Ramírez, Rubén
    Sharma, Yashvi
    Singh, Avinash
    Smith, Roger
    Soumagnac, Maayane T.
    Taggart, Kirsty
    Tan, Hanjie
    Tzanidakis, Anastasios
    Troja, Eleonora
    Valeev, Azamat F.
    Walters, Richard
    Waratkar, Gaurav
    Webb, Sara
    Yu, Po-Chieh
    Zhang, Bin-Bin
    Zhou, Rongpu
    Zolkower, Jeffry
    Kilonova Luminosity Function Constraints Based on Zwicky Transient Facility Searches for 13 Neutron Star Merger Triggers during O32020In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 905, no 2, article id 145Article in journal (Refereed)
    Abstract [en]

    We present a systematic search for optical counterparts to 13 gravitational wave (GW) triggers involving at least one neutron star during LIGO/Virgo's third observing run (O3). We searched binary neutron star (BNS) and neutron star black hole (NSBH) merger localizations with the Zwicky Transient Facility (ZTF) and undertook follow-up with the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. The GW triggers had a median localization area of 4480 deg(2), a median distance of 267 Mpc, and false-alarm rates ranging from 1.5 to 10(-25) yr(-1). The ZTF coverage in the g and r bands had a median enclosed probability of 39%, median depth of 20.8 mag, and median time lag between merger and the start of observations of 1.5 hr. The O3 follow-up by the GROWTH team comprised 340 UltraViolet/Optical/InfraRed (UVOIR) photometric points, 64 OIR spectra, and three radio images using 17 different telescopes. We find no promising kilonovae (radioactivity-powered counterparts), and we show how to convert the upper limits to constrain the underlying kilonova luminosity function. Initially, we assume that all GW triggers are bona fide astrophysical events regardless of false-alarm rate and that kilonovae accompanying BNS and NSBH mergers are drawn from a common population; later, we relax these assumptions. Assuming that all kilonovae are at least as luminous as the discovery magnitude of GW170817 (-16.1 mag), we calculate that our joint probability of detecting zero kilonovae is only 4.2%. If we assume that all kilonovae are brighter than -16.6 mag (the extrapolated peak magnitude of GW170817) and fade at a rate of 1 mag day(-1) (similar to GW170817), the joint probability of zero detections is 7%. If we separate the NSBH and BNS populations based on the online classifications, the joint probability of zero detections, assuming all kilonovae are brighter than -16.6 mag, is 9.7% for NSBH and 7.9% for BNS mergers. Moreover, no more than <57% (<89%) of putative kilonovae could be brighter than -16.6 mag assuming flat evolution (fading by 1 mag day(-1)) at the 90% confidence level. If we further take into account the online terrestrial probability for each GW trigger, we find that no more than <68% of putative kilonovae could be brighter than -16.6 mag. Comparing to model grids, we find that some kilonovae must have M-ej M, X-lan > 10(-4), or > 30 degrees to be consistent with our limits. We look forward to searches in the fourth GW observing run; even 17 neutron star mergers with only 50% coverage to a depth of -16 mag would constrain the maximum fraction of bright kilonovae to <25%.

  • 24.
    Kool, Erik C.
    et al.
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Astronomy.
    Karamehmetoglu, E.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Schulze, S.
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Reynolds, T. M.
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bellm, E. C.
    De, K.
    Duev, D. A.
    Fremling, C.
    Golkhou, V. Z.
    Graham, M. L.
    Green, D. A.
    Horesh, A.
    Kaye, S.
    Kim, Y. -L.
    Laher, R. R.
    Masci, F. J.
    Nordin, J.
    Perley, D. A.
    Phinney, E. S.
    Porter, M.
    Reiley, D.
    Rodriguez, H.
    van Roestel, J.
    Rusholme, B.
    Sharma, Y.
    Sfaradi, I.
    Soumagnac, M. T.
    Taggart, K.
    Tartaglia, Leonardo
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Williams, D. R. A.
    Yan, L.
    SN 2020bqj: A Type Ibn supernova with a long-lasting peak plateau2021In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 652, article id A136Article in journal (Refereed)
    Abstract [en]

    Context. Type Ibn supernovae (SNe Ibn) are a rare class of stripped envelope supernovae interacting with a helium-rich circumstellar medium (CSM). The majority of the SNe Ibn reported in the literature display a surprising homogeneity in their fast-evolving lightcurves and are typically found in actively starforming spiral galaxies.

    Aims. We present the discovery and the study of SN 2020bqj (ZTF20aalrqbu), a SN Ibn with a long-duration peak plateau lasting 40 days and hosted by a faint low-mass galaxy. We aim to explain its peculiar properties using an extensive photometric and spectroscopic data set.

    Methods. We compare the photometric and spectral evolution of SN 2020bqj with regular SNe Ibn from the literature, as well as with other outliers in the SN Ibn subclass. We fit the bolometric and multi-band lightcurves with powering mechanism models such as radioactive decay and CSM interaction. We also model the host galaxy of SN 2020bqj.

    Results. The risetime, peak magnitude and spectral features of SN 2020bqj are consistent with those of most SNe Ibn, but the SN is a clear outlier in the subclass based on its bright, long-lasting peak plateau and the low mass of its faint host galaxy. We show through modeling that the lightcurve of SN 2020bqj can be powered predominantly by shock heating from the interaction of the SN ejecta and a dense CSM, combined with radioactive decay. The peculiar Type Ibn SN 2011hw is a close analog to SN 2020bqj in terms of lightcurve and spectral evolution, suggesting a similar progenitor and CSM scenario. In this scenario a very massive progenitor star in the transitional phase between a luminous blue variable and a compact Wolf-Rayet star undergoes core-collapse, embedded in a dense helium-rich CSM with an elevated opacity compared to normal SNe Ibn, due to the presence of residual hydrogen. This scenario is consistent with the observed properties of SN 2020bqj and the modeling results.

    Conclusions. SN 2020bqj is a compelling example of a transitional SN Ibn/IIn based on not only its spectral features, but also its lightcurve, host galaxy properties and the inferred progenitor properties. The strong similarity with SN 2011hw suggests this subclass may be the result of a progenitor in a stellar evolution phase that is distinct from those of progenitors of regular SNe Ibn.

  • 25.
    Kool, Erik C.
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Pearson Johansson, Joel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Astronomy.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Moldón, Javier
    Moriya, Takashi J.
    Mattila, Seppo
    Schulze, Steve
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Chomiuk, Laura
    Pérez-Torres, Miguel
    Harris, Chelsea
    Lundqvist, Peter
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Graham, Matthew
    Yang, Sheng
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Henan Academy of Sciences, China.
    Perley, Daniel A.
    Strotjohann, Nora Linn
    Fremling, Christoffer
    Gal-Yam, Avishay
    Lezmy, Jeremy
    Maguire, Kate
    Omand, Conor M. B.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Smith, Mathew
    Andreoni, Igor
    Bellm, Eric C.
    Bloom, Joshua S.
    De, Kishalay
    Groom, Steven L.
    Kasliwal, Mansi M.
    Masci, Frank J.
    Medford, Michael S.
    Park, Sungmin
    Purdum, Josiah
    Reynolds, Thomas M.
    Riddle, Reed
    Robert, Estelle
    Ryder, Stuart D.
    Sharma, Yashvi
    Stern, Daniel
    A radio-detected type Ia supernova with helium-rich circumstellar material2023In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 617, no 7961, p. 477-482Article in journal (Refereed)
    Abstract [en]

    Type Ia supernovae (SNe Ia) are thermonuclear explosions of degenerate white dwarf stars destabilized by mass accretion from a companion star1, but the nature of their progenitors remains poorly understood. A way to discriminate between progenitor systems is through radio observations; a non-degenerate companion star is expected to lose material through winds2 or binary interaction3 before explosion, and the supernova ejecta crashing into this nearby circumstellar material should result in radio synchrotron emission. However, despite extensive efforts, no type Ia supernova (SN Ia) has ever been detected at radio wavelengths, which suggests a clean environment and a companion star that is itself a degenerate white dwarf star4,5. Here we report on the study of SN 2020eyj, a SN Ia showing helium-rich circumstellar material, as demonstrated by its spectral features, infrared emission and, for the first time in a SN Ia to our knowledge, a radio counterpart. On the basis of our modelling, we conclude that the circumstellar material probably originates from a single-degenerate binary system in which a white dwarf accretes material from a helium donor star, an often proposed formation channel for SNe Ia (refs. 6,7). We describe how comprehensive radio follow-up of SN 2020eyj-like SNe Ia can improve the constraints on their progenitor systems.

  • 26.
    Kool, Erik C.
    et al.
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Astronomy. Macquarie University, Australia.
    Reynolds, T. M.
    Mattila, S.
    Kankare, E.
    Pérez-Torres, M. A.
    Efstathiou, A.
    Ryder, S.
    Romero-Cañizales, C.
    Lu, W.
    Heikkilä, T.
    Anderson, G. E.
    Berton, M.
    Bright, J.
    Cannizzaro, G.
    Eappachen, D.
    Fraser, M.
    Gromadzki, M.
    Jonker, P. G.
    Kuncarayakti, H.
    Lundqvist, Peter
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Astronomy.
    Maeda, K.
    McDermid, R. M.
    Medling, A. M.
    Moran, S.
    Reguitti, A.
    Shahbandeh, M.
    Tsygankov, S.
    U, V
    Wevers, T.
    AT 2017gbl: a dust obscured TDE candidate in a luminous infrared galaxy2020In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 498, no 2, p. 2167-2195Article in journal (Refereed)
    Abstract [en]

    We present the discovery with Keck of the extremely infrared (IR) luminous transient AT 2017gbl, coincident with the Northern nucleus of the luminous infrared galaxy (LIRG) IRAS 23436+5257. Our extensive multiwavelength follow-up spans ∼900 d, including photometry and spectroscopy in the optical and IR, and (very long baseline interferometry) radio and X-ray observations. Radiative transfer modelling of the host galaxy spectral energy distribution and long-term pre-outburst variability in the mid-IR indicate the presence of a hitherto undetected dust obscured active galactic nucleus (AGN). The optical and near-IR spectra show broad ∼2000 km s−1 hydrogen, He i, and O i emission features that decrease in flux over time. Radio imaging shows a fast evolving compact source of synchrotron emission spatially coincident with AT 2017gbl. We infer a lower limit for the radiated energy of 7.3 × 1050 erg from the IR photometry. An extremely energetic supernova would satisfy this budget, but is ruled out by the radio counterpart evolution. Instead, we propose AT 2017gbl is related to an accretion event by the central supermassive black hole, where the spectral signatures originate in the AGN broad line region and the IR photometry is consistent with re-radiation by polar dust. Given the fast evolution of AT 2017gbl, we deem a tidal disruption event (TDE) of a star a more plausible scenario than a dramatic change in the AGN accretion rate. This makes AT 2017gbl the third TDE candidate to be hosted by a LIRG, in contrast to the so far considered TDE population discovered at optical wavelengths and hosted preferably by post-starburst galaxies.

  • 27. Kumar, Harsh
    et al.
    Gupta, Rahul
    Saraogi, Divita
    Ahumada, Tomas
    Andreoni, Igor
    Anupama, G. C.
    Aryan, Amar
    Barway, Sudhanshu
    Bhalerao, Varun
    Chandra, Poonam
    Coughlin, Michael W.
    Dimple,
    Dutta, Anirban
    Ghosh, Ankur
    Ho, Anna Y. Q.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kumar, Amit
    Medford, Michael S.
    Misra, Kuntal
    Pandey, Shashi B.
    Perley, Daniel A.
    Riddle, Reed
    Ror, Amit Kumar
    Setiadi, Jason M.
    Yao, Yuhan
    The long-active afterglow of GRB 210204A: detection of the most delayed flares in a gamma-ray burst2022In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 513, no 2, p. 2777-2793Article in journal (Refereed)
    Abstract [en]

    We present results from extensive broadband follow-up of GRB 210204A over the period of 30 d. We detect optical flares in the afterglow at 7.6 x 10(5) s and 1.1 x 10(6) s after the burst: the most delayed flaring ever detected in a GRB afterglow. At the source redshift of 0.876, the rest-frame delay is 5.8 x 10(5) s (6.71 d). We investigate possible causes for this flaring and conclude that the most likely cause is a refreshed shock in the jet. The prompt emission of the GRB is within the range of typical long bursts: it shows three disjoint emission episodes, which all follow the typical GRB correlations. This suggests that GRB 210204A might not have any special properties that caused late-time flaring, and the lack of such detections for other afterglows might be resulting from the paucity of late-time observations. Systematic late-time follow-up of a larger sample of GRBs can shed more light on such afterglow behaviour. Further analysis of the GRB 210204A shows that the late-time bump in the light curve is highly unlikely due to underlying SNe at redshift (z) = 0.876 and is more likely due to the late-time flaring activity. The cause of this variability is not clearly quantifiable due to the lack of multiband data at late-time constraints by bad weather conditions. The flare of GRB 210204A is the latest flare detected to date.

  • 28.
    Lundqvist, Peter
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kundu, Esha
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Curtin University, Australia.
    Perez-Torres, Miguel A.
    Ryder, Stuart D.
    Björnsson, Claes-Ingvar
    Stockholm University, Faculty of Science, Department of Astronomy.
    Moldon, Javier
    Argo, Megan K.
    Beswick, Robert J.
    Alberdi, Antxon
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Macquarie University, Australia; Australian Astronomical Observatory, Australia.
    The Deepest Radio Observations of Nearby SNe Ia: Constraining Progenitor Types and Optimizing Future Surveys2020In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 890, no 2, article id 159Article in journal (Refereed)
    Abstract [en]

    We report deep radio observations of nearby Type Ia supernovae (SNe Ia) with the electronic Multi-Element Radio Linked Interferometer Network and the Australia Telescope Compact Array. No detections were made. With standard assumptions for the energy densities of relativistic electrons going into a power-law energy distribution and the magnetic field strength (epsilon(e) = epsilon(B)=.0.1), we arrive at upper limits on mass-loss rate for the progenitor system of SN.2013dy.(SN 2016coj, SN 2018gv, SN 2018pv, SN 2019np) of M less than or similar to 12 (2.8, 1.3, 2.1, 1.7) x 10(-8) M-circle dot yr(-1) (v(w)/100 km s-(1)), where v(w) is the wind speed of the mass loss. To SN.2016coj, SN 2018gv, SN 2018pv, and SN 2019np we add radio data for 17 other nearby SNe.Ia and model their nondetections. With the same model as described, all 21 SNe Ia have M less than or similar to 4 x 10(-8) M-circle dot yr(-1) (v(w)/ 100 km s(-1)). We compare those limits with the expected mass-loss rates in different single-degenerate progenitor scenarios. We also discuss how information on oe and oB can be obtained from late observations of SNe.Ia and the youngest SN.Ia remnant detected in radio, G1.9+0.3, as well as strippedenvelope core-collapse SNe. We highlight SN.2011dh and argue for epsilon e approximate to 0.1 and epsilon(B) approximate to 0.0033. Finally, we discuss strategies to observe at radio frequencies to maximize the chance of detection, given the time since explosion, the distance to the SN, and the telescope sensitivity.

  • 29. Perley, Daniel A.
    et al.
    Ho, Anna Y. Q.
    Yao, Yuhan
    Fremling, Christoffer
    Anderson, Joseph P.
    Schulze, Steve
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kumar, Harsh
    Anupama, G. C.
    Barway, Sudhanshu
    Bellm, Eric C.
    Bhalerao, Varun
    Chen, Ting-Wan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Duev, Dmitry A.
    Galbany, Lluís
    Graham, Matthew J.
    Gromadzki, Mariusz
    Gutiérrez, Claudia P.
    Ihanec, Nada
    Inserra, Cosimo
    Kasliwal, Mansi M.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kulkarni, S. R.
    Laher, Russ R.
    Masci, Frank J.
    Neill, James D.
    Nicholl, Matt
    Pursiainen, Miika
    van Roestel, Joannes
    Sharma, Yashvi
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Walters, Richard
    Wiseman, Philip
    Real-time discovery of AT2020xnd: a fast, luminous ultraviolet transient with minimal radioactive ejecta2021In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 508, no 4, p. 5138-5147Article in journal (Refereed)
    Abstract [en]

    The many unusual properties of the enigmatic AT2018cow suggested that at least some subset of the empirical class of fast blue optical transients (FBOTs) represents a genuinely new astrophysical phenomenon. Unfortunately, the intrinsic rarity and fleeting nature of these events have made it difficult to identify additional examples early enough to acquire the observations necessary to constrain theoretical models. We present here the Zwicky Transient Facility discovery of AT2020xnd (ZTF20acigmel, the 'Camel') at z = 0.243, the first unambiguous AT2018cow analogue to be found and confirmed in real time. AT2018cow and AT2020xnd share all key observational properties: a fast optical rise, sustained high photospheric temperature, absence of a second peak attributable to ejection of a radioactively heated stellar envelope, extremely luminous radio, millimetre, and X-ray emission, and a dwarf-galaxy host. This supports the argument that AT2018cow-like events represent a distinct phenomenon from slower-evolving radio-quiet supernovae, likely requiring a different progenitor or a different central engine. The sample properties of the four known members of this class to date disfavour tidal disruption models but are consistent with the alternative model of an accretion powered jet following the direct collapse of a massive star to a black hole. Contextual filtering of alert streams combined with rapid photometric verification using multiband imaging provides an efficient way to identify future members of this class, even at high redshift.

  • 30. Perley, Daniel A.
    et al.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Schulze, Steve
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Yao, Yuhan
    Fremling, Christoffer
    Gal-Yam, Avishay
    Ho, Anna Y. Q.
    Yang, Yi
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Irani, Ido
    Yan, Lin
    Andreoni, Igor
    Baade, Dietrich
    Bellm, Eric C.
    Brink, Thomas G.
    Chen, Ting-Wan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Cikota, Aleksandar
    Coughlin, Michael W.
    Dahiwale, Aishwarya
    Dekany, Richard
    Duev, Dmitry A.
    Filippenko, Alexei
    Hoeflich, Peter
    Kasliwal, Mansi M.
    Kulkarni, S. R.
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Masci, Frank J.
    Maund, Justyn R.
    Medford, Michael S.
    Riddle, Reed
    Rosnet, Philippe
    Shupe, David L.
    Strotjohann, Nora Linn
    Tzanidakis, Anastasios
    Zheng, WeiKang
    The Type Icn SN 2021csp: Implications for the Origins of the Fastest Supernovae and the Fates of Wolf-Rayet Stars2022In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 927, no 2, article id 180Article in journal (Refereed)
    Abstract [en]

    We present observations of SN 2021csp, the second example of a newly identified type of supernova (SN) hallmarked by strong, narrow, P Cygni carbon features at early times (Type Icn). The SN appears as a fast and luminous blue transient at early times, reaching a peak absolute magnitude of −20 within 3 days due to strong interaction between fast SN ejecta (v ≈ 30,000 km s−1) and a massive, dense, fast-moving C/O wind shed by the WC-like progenitor months before explosion. The narrow-line features disappear from the spectrum 10–20 days after explosion and are replaced by a blue continuum dominated by broad Fe features, reminiscent of Type Ibn and IIn supernovae and indicative of weaker interaction with more extended H/He-poor material. The transient then abruptly fades ∼60 days post-explosion when interaction ceases. Deep limits at later phases suggest minimal heavy-element nucleosynthesis, a low ejecta mass, or both, and imply an origin distinct from that of classical Type Ic SNe. We place SN 2021csp in context with other fast-evolving interacting transients, and discuss various progenitor scenarios: an ultrastripped progenitor star, a pulsational pair-instability eruption, or a jet-driven fallback SN from a Wolf–Rayet (W-R) star. The fallback scenario would naturally explain the similarity between these events and radio-loud fast transients, and suggests a picture in which most stars massive enough to undergo a W-R phase collapse directly to black holes at the end of their lives.

  • 31. Purdum, Josiah N.
    et al.
    Lin, Zhong-Yi
    Bolin, Bryce T.
    Sharma, Kritti
    Choi, Philip I.
    Bhalerao, Varun
    Hanuš, Josef
    Kumar, Harsh
    Quimby, Robert
    van Roestel, Joannes C.
    Zhai, Chengxing
    Fernandez, Yanga R.
    Lisse, Carey M.
    Bodewits, Dennis
    Fremling, Christoffer
    Ryan Golovich, Nathan
    Hsu, Chen-Yen
    Ip, Wing-Huen
    Ngeow, Chow-Choong
    Saini, Navtej S.
    Shao, Michael
    Yao, Yuhan
    Ahumada, Tomás
    Anand, Shreya
    Andreoni, Igor
    Burdge, Kevin B.
    Burruss, Rick
    Chang, Chan-Kao
    Copperwheat, Chris M.
    Coughlin, Michael
    De, Kishalay
    Dekany, Richard
    Delacroix, Alexandre
    Drake, Andrew
    Duev, Dmitry
    Graham, Matthew
    Hale, David
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Macquarie University, Australia.
    Kasliwal, Mansi M.
    Kostadinova, Iva S.
    Kulkarni, Shrinivas R.
    Laher, Russ R.
    Mahabal, Ashish
    Masci, Frank J.
    Mróz, Przemyslaw J.
    Neill, James D.
    Riddle, Reed
    Rodriguez, Hector
    Smith, Roger M.
    Walters, Richard
    Yan, Lin
    Zolkower, Jeffry
    Time-series and Phase-curve Photometry of the Episodically Active Asteroid (6478) Gault in a Quiescent State Using APO, GROWTH, P200, and ZTF2021In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 911, no 2, article id L35Article in journal (Refereed)
    Abstract [en]

    We observed the episodically active asteroid (6478) Gault in 2020 with multiple telescopes in Asia and North America and found that it is no longer active after its recent outbursts at the end of 2018 and the start of 2019. The inactivity during this apparition allowed us to measure the absolute magnitude of Gault of H ( r ) = 14.63 +/- 0.02, G ( r ) = 0.21 +/- 0.02 from our secular phase-curve observations. In addition, we were able to constrain Gault's rotation period using time-series photometric lightcurves taken over 17 hr on multiple days in 2020 August, September, and October. The photometric lightcurves have a repeating less than or similar to 0.05 mag feature suggesting that (6478) Gault has a rotation period of similar to 2.5 hr and may have a semispherical or top-like shape, much like the near-Earth asteroids Ryugu and Bennu. The rotation period of similar to 2.5 hr is near the expected critical rotation period for an asteroid with the physical properties of (6478) Gault, suggesting that its activity observed over multiple epochs is due to surface mass shedding from its fast rotation spin-up by the Yarkovsky-O'Keefe-Radzievskii-Paddack effect.

  • 32. Reusch, Simeon
    et al.
    Stein, Robert
    Kowalski, Marek
    van Velzen, Sjoert
    Franckowiak, Anna
    Lunardini, Cecilia
    Murase, Kohta
    Winter, Walter
    Miller-Jones, James C. A.
    Kasliwal, Mansi M.
    Gilfanov, Marat
    Garrappa, Simone
    Paliya, Vaidehi S.
    Ahumada, Tomás
    Anand, Shreya
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bellm, Eric C.
    Brinnel, Valéry
    Buson, Sara
    Cenko, S. Bradley
    Coughlin, Michael W.
    De, Kishalay
    Dekany, Richard
    Frederick, Sara
    Gal-Yam, Avishay
    Gezari, Suvi
    Giroletti, Marcello
    Graham, Matthew J.
    Karambelkar, Viraj
    Kimura, Shigeo S.
    Kong, Albert K. H.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Laher, Russ R.
    Medvedev, Pavel
    Necker, Jannis
    Nordin, Jakob
    Perley, Daniel A.
    Rigault, Mickael
    Rusholme, Ben
    Schulze, Steve
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Schweyer, Tassilo
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Singer, Leo P.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Astronomy.
    Strotjohann, Nora Linn
    Sunyaev, Rashid
    van Santen, Jakob
    Walters, Richard
    Zhang, B. Theodore
    Zimmerman, Erez
    Candidate Tidal Disruption Event AT2019fdr Coincident with a High-Energy Neutrino2022In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 128, no 22, article id 221101Article in journal (Refereed)
    Abstract [en]

    The origins of the high-energy cosmic neutrino flux remain largely unknown. Recently, one high-energy neutrino was associated with a tidal disruption event (TDE). Here we present AT2019fdr, an exceptionally luminous TDE candidate, coincident with another high-energy neutrino. Our observations, including a bright dust echo and soft late-time x-ray emission, further support a TDE origin of this flare. The probability of finding two such bright events by chance is just 0.034%. We evaluate several models for neutrino production and show that AT2019fdr is capable of producing the observed high-energy neutrino, reinforcing the case for TDEs as neutrino sources.

  • 33. Reynolds, T. M.
    et al.
    Mattila, S.
    Efstathiou, A.
    Kankare, E.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ryder, S.
    Peña-Moñino, L.
    Pérez-Torres, M. A.
    Energetic nuclear transients in luminous and ultraluminous infrared galaxies2022In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 664, article id A158Article in journal (Refereed)
    Abstract [en]

    Energetic nuclear outbursts have been discovered in luminous and ultraluminous infrared galaxies (U/LIRGs) at unexpectedly high rates. To investigate this population of transients, we performed a search in mid-IR data from the Wide-field Infrared Survey Explorer (WISE) satellite and its NEOWISE survey to detect and characterise luminous and smoothly evolving transients in a sample of 215 U/LIRGs. We report three new transients, all with ΔL >  1043 erg s−1, in addition to two previously known cases. Their host galaxies are all part of major galaxy mergers, and through radiative transfer model fitting we find that all have a significant contribution from an active galactic nucleus (AGN). We characterised the transients through measurements of their luminosities and resulting energetics, all of which are between 1050.9 erg and 1052.2 erg. The IR emission of the five transients was found to be consistent with re-radiation by the hot dust of emission at shorter wavelengths, presumably originating from an accretion event, onto the supermassive black hole. The corresponding transient rate of (1.6–4.6) × 10−3 yr−1 galaxy−1 is over an order of magnitude higher than the rate of large amplitude flares shown by AGN in the optical. We suggest that the observed transients are part of a dust-obscured population of tidal disruption events (TDEs) that have remained out of the reach of optical surveys due to the obscuring dust. In one case, this is supported by our radio observations. We also discuss other plausible explanations. The observed rate of events is significantly higher than optical TDE rates, which can be expected in U/LIRG hosts undergoing a major galaxy merger with increased stellar densities in the nuclear regions. Continued searches for such transients and their multi-wavelength follow-up is required to constrain their rate and nature.

  • 34.
    Sagués Carracedo, Ana
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Schulze, Steve
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bulla, Mattia
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    A very luminous jet from the disruption of a star by a massive black hole2022In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 612, no 7940, p. 430-434Article in journal (Refereed)
    Abstract [en]

    Tidal disruption events (TDEs) are bursts of electromagnetic energy that are released when supermassive black holes at the centres of galaxies violently disrupt a star that passes too close1. TDEs provide a window through which to study accretion onto supermassive black holes; in some rare cases, this accretion leads to launching of a relativistic jet2,3,4,5,6,7,8,9, but the necessary conditions are not fully understood. The best-studied jetted TDE so far is Swift J1644+57, which was discovered in γ-rays, but was too obscured by dust to be seen at optical wavelengths. Here we report the optical detection of AT2022cmc, a rapidly fading source at cosmological distance (redshift z = 1.19325) the unique light curve of which transitioned into a luminous plateau within days. Observations of a bright counterpart at other wavelengths, including X-ray, submillimetre and radio, supports the interpretation of AT2022cmc as a jetted TDE containing a synchrotron ‘afterglow’, probably launched by a supermassive black hole with spin greater than approximately 0.3. Using four years of Zwicky Transient Facility10 survey data, we calculate a rate of 0.02+0.04−0.01 Gpc−3 yr−1 for on-axis jetted TDEs on the basis of the luminous, fast-fading red component, thus providing a measurement complementary to the rates derived from X-ray and radio observations11. Correcting for the beaming angle effects, this rate confirms that approximately 1 per cent of TDEs have relativistic jets. Optical surveys can use AT2022cmc as a prototype to unveil a population of jetted TDEs.

  • 35. Sharma, Yashvi
    et al.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fremling, Christoffer
    Kulkarni, Shrinivas R.
    De Kishalay, Kishalay
    Irani, Ido
    Schulze, Steve
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Strotjohann, Nora Linn
    Gal-Yam, Avishay
    Maguire, Kate
    Perley, Daniel A.
    Bellm, Eric C.
    Kool, Erik C.
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Brink, Thomas G.
    Bruch, Rachel
    Deckers, Maxime
    Dekany, Richard
    Dugas, Alison
    Filippenko, Alexei V.
    Goldwasser, Samantha
    Graham, Matthew J.
    Graham, Melissa L.
    Groom, Steven L.
    Hankins, Matt
    Jencson, Jacob
    Pearson Johansson, Joel
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Karambelkar, Viraj
    Kasliwal, Mansi M.
    Masci, Frank J.
    Medford, Michael S.
    Neill, James D.
    Nir, Guy
    Riddle, Reed L.
    Rigault, Mickael
    Schweyer, Tassilo
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Terwel, Jacco H.
    Yan, Lin
    Yang, Yi
    Yao, Yuhan
    A Systematic Study of Ia-CSM Supernovae from the ZTF Bright Transient Survey2023In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 948, no 1, article id 52Article in journal (Refereed)
    Abstract [en]

    Among the supernovae (SNe) that show strong interaction with a circumstellar medium (CSM), there is a rare subclass of Type Ia supernovae, SNe Ia-CSM, which show strong narrow hydrogen emission lines much like SNe IIn but on top of a diluted Type Ia spectrum. The only previous systematic study of this class identified 16 SNe Ia-CSM, eight historic and eight from the Palomar Transient Factory (PTF). Now using the successor survey to PTF, the Zwicky Transient Facility (ZTF), we have classified 12 additional SNe Ia-CSM through the systematic Bright Transient Survey (BTS). Consistent with previous studies, we find these SNe to have slowly evolving optical light curves with peak absolute magnitudes between -19.1 and -21, spectra having weak H ss and large Balmer ldecrements of similar to 7. Out of the 10 SNe from our sample observed by NEOWISE, nine have 3 sigma detections, with some SNe showing a reduction in the red wing of Ha, indicative of newly formed dust. We do not find our SN Ia-CSM sample to have a significantly different distribution of equivalent widths of He I.5876 than SNe IIn as observed in Silverman et al. The hosts tend to be late-type galaxies with recent star formation. We derive a rate estimate of 29+(27)(21) Gpc(-3) yr(-1) for SNe Ia-CSM, which is similar to 0.02%-0.2% of the SN Ia rate. We also identify six ambiguous SNe IIn/Ia-CSM in the BTS sample and including them gives an upper limit rate of 0.07%-0.8%. This work nearly doubles the sample of well-studied Ia-CSM objects in Silverman et al., increasing the total number to 28.

  • 36.
    Sollerman, Jesper
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fransson, Claes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fremling, C.
    Horesh, A.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Schulze, S.
    Sfaradi, I.
    Yang, Sheng
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bellm, E. C.
    Burruss, R.
    Cunningham, V.
    De, K.
    Drake, A. J.
    Golkhou, V. Z.
    Green, D. A.
    Kasliwal, M.
    Kulkarni, S.
    Kupfer, T.
    Laher, R. R.
    Masci, F. J.
    Rodriguez, H.
    Rusholme, B.
    Williams, D. R. A.
    Yan, L.
    Zolkower, J.
    Two stripped envelope supernovae with circumstellar interaction: But only one really shows it2020In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 643, article id A79Article in journal (Refereed)
    Abstract [en]

    Context. We present observations of SN 2019tsf (ZTF19ackjszs) and SN 2019oys (ZTF19abucwzt). These two stripped envelope (SE) Type Ib supernovae (SNe) suddenly showed a (re-)brightening in their late light curves. We investigate this in the context of circumstellar material (CSM) interaction with previously ejected material, a phenomenon that is unusual among SE SNe.

    Aims. We use our follow-up photometry and spectroscopy for these supernovae to demonstrate the presence of CSM interaction, estimate the properties of the CSM, and discuss why the signals are so different for the two objects.

    Methods. We present and analyze observational data, consisting of optical light curves and spectra. For SN 2019oys, we also have detections in radio as well as limits from UV and X-rays.

    Results. Both light curves show spectacular re-brightening after about 100 days. In the case of SN 2019tsf, the re-brightening is followed by a new period of decline, and the spectra never show signs of narrow emission lines that would indicate CSM interaction. On the contrary, SN 2019oys made a spectral makeover from a Type Ib to a spectrum clearly dominated by CSM interaction at the light curve brightening phase. Deep Keck spectra reveal a plethora of narrow high-ionization lines, including coronal lines, and the radio observations show strong emission.

    Conclusions. The rather similar light curve behavior – with a late linear re-brightening – of these two Type Ib SE SNe indicate CSM interaction as the powering source. For SN 2019oys the evidence for a phase where the ejecta hit H-rich material, likely ejected from the progenitor star, is conspicuous. We observe strong narrow lines of H and He, but also a plethora of high-ionization lines, including coronal lines, revealing shock interaction. Spectral simulations of SN 2019oys show two distinct density components, one with density ≳109 cm−3, dominated by somewhat broader, low-ionization lines of H I, He I, Na I, and Ca II, and one with narrow, high-ionization lines at a density ∼106 cm−3. The former is strongly affected by electron scattering, while the latter is unaffected. The evidence for CSM interaction in SN 2019oys is corroborated by detections in radio. On the contrary, for SN 2019tsf, we find little evidence in the spectra for any CSM interaction.

     

  • 37.
    Sollerman, Jesper
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Yang, Sheng
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Schulze, Steve
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Strotjohann, N. L.
    Jerkstrand, Anders
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Van Dyk, S. D.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Tel Aviv University, Israel.
    Brink, T. G.
    Bruch, R.
    De, K.
    Filippenko, A. V.
    Fremling, C.
    Patra, K. C.
    Perley, D.
    Yan, L.
    Yang, Y.
    Andreoni, I.
    Campbell, R.
    Coughlin, M.
    Kasliwal, M.
    Kim, Y. -L.
    Rigault, M.
    Shin, K.
    Tzanidakis, A.
    Ashley, M. C. B.
    Moore, A. M.
    Travouillon, T.
    The Type II supernova SN 2020jfo in M 61, implications for progenitor system, and explosion dynamics2021In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 655, article id A105Article in journal (Refereed)
    Abstract [en]

    We present the discovery and extensive follow-up observations of SN 2020jfo, a Type IIP supernova (SN) in the nearby (14.5 Mpc) galaxy M 61. Optical light curves (LCs) and spectra from the Zwicky Transient Facility (ZTF), complemented with data from Swift/UVOT and near-infrared photometry is presented. These were used to model the 350-day duration bolometric light curve, which exhibits a relatively short (∼65 days) plateau. This implies a moderate ejecta mass (∼5 M⊙) at the time of explosion, whereas the deduced amount of ejected radioactive nickel is ∼0.025 M⊙. An extensive series of spectroscopy is presented, including spectropolarimetric observations. The nebular spectra are dominated by Hα, but also reveal emission lines from oxygen and calcium. Comparisons to synthetic nebular spectra indicate an initial progenitor mass of ∼12 M⊙. We also note the presence of stable nickel in the nebular spectrum, and SN 2020jfo joins a small group of SNe that have inferred super-solar Ni/Fe ratios. Several years of prediscovery data were examined, but no signs of precursor activity were found. Pre-explosion Hubble Space Telescope imaging reveals a probable progenitor star, detected only in the reddest band (MF814W ≈ −5.8) and it is fainter than expected for stars in the 10−15 M⊙ range. There is thus some tension between the LC analysis, the nebular spectral modeling, and the pre-explosion imaging. To compare and contrast, we present two additional core-collapse SNe monitored by the ZTF, which also have nebular Hα-dominated spectra. This illustrates how the absence or presence of an interaction with circumstellar material (CSM) affect both the LCs and in particular the nebular spectra. Type II SN 2020amv has a LC powered by CSM interaction, in particular after ∼40 days when the LC is bumpy and slowly evolving. The late-time spectra show strong Hα emission with a structure suggesting emission from a thin, dense shell. The evolution of the complex three-horn line profile is reminiscent of that observed for SN 1998S. Finally, SN 2020jfv has a poorly constrained early-time LC, but it is of interest because of the transition from a hydrogen-poor Type IIb to a Type IIn, where the nebular spectrum after the light-curve rebrightening is dominated by Hα, although with an intermediate line width.

  • 38. Strotjohann, Nora L.
    et al.
    Ofek, Eran O.
    Gal-Yam, Avishay
    Bruch, Rachel
    Schulze, Steve
    Shaviv, Nir
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Filippenko, Alexei V.
    Yaron, Ofer
    Fremling, Christoffer
    Nordin, Jakob
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Perley, Dan A.
    Ho, Anna Y. Q.
    Yang, Yi
    Yao, Yuhan
    Soumagnac, Maayane T.
    Graham, Melissa L.
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Tartaglia, Leonardo
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). INAF—Osservatorio Astronomico di Padova, Italy.
    De, Kishalay
    Goldstein, Daniel A.
    Cook, David O.
    Brink, Thomas G.
    Taggart, Kirsty
    Yan, Lin
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kasliwal, Mansi
    Kulkarni, Shri R.
    Nugent, Peter E.
    Masci, Frank J.
    Rosnet, Philippe
    Adams, Scott M.
    Andreoni, Igor
    Bagdasaryan, Ashot
    Bellm, Eric C.
    Burdge, Kevin
    Duev, Dmitry A.
    Dugas, Alison
    Frederick, Sara
    Goldwasser, Samantha
    Hankins, Matthew
    Irani, Ido
    Karambelkar, Viraj
    Kupfer, Thomas
    Liang, Jingyi
    Neill, James D.
    Porter, Michael
    Riddle, Reed L.
    Sharma, Yashvi
    Short, Phil
    Taddia, Francesco
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Tzanidakis, Anastasios
    Roestel, Jan van
    Walters, Richard
    Zhuang, Zhuyun
    Bright, Months-long Stellar Outbursts Announce the Explosion of Interaction-powered Supernovae2021In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 907, no 2, article id 99Article in journal (Refereed)
    Abstract [en]

    Interaction-powered supernovae (SNe) explode within an optically thick circumstellar medium (CSM) that could be ejected during eruptive events. To identify and characterize such pre-explosion outbursts, we produce forced-photometry light curves for 196 interacting SNe, mostly of Type IIn, detected by the Zwicky Transient Facility between early 2018 and 2020 June. Extensive tests demonstrate that we only expect a few false detections among the 70,000 analyzed pre-explosion images after applying quality cuts and bias corrections. We detect precursor eruptions prior to 18 Type IIn SNe and prior to the Type Ibn SN 2019uo. Precursors become brighter and more frequent in the last months before the SN and month-long outbursts brighter than magnitude -13 occur prior to 25% (5-69%, 95% confidence range) of all Type IIn SNe within the final three months before the explosion. With radiative energies of up to 10(49) erg, precursors could eject similar to 1 M of material. Nevertheless, SNe with detected precursors are not significantly more luminous than other SNe IIn, and the characteristic narrow hydrogen lines in their spectra typically originate from earlier, undetected mass-loss events. The long precursor durations require ongoing energy injection, and they could, for example, be powered by interaction or by a continuum-driven wind. Instabilities during the neon- and oxygen-burning phases are predicted to launch precursors in the final years to months before the explosion; however, the brightest precursor is 100 times more energetic than anticipated.

  • 39.
    Tartaglia, Leonardo
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Pastorello, A.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fransson, Claes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mattila, S.
    Fraser, M.
    Taddia, Francesco
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Tomasella, L.
    Turatto, M.
    Morales-Garoffolo, A.
    Elias-Rosa, N.
    Lundqvist, Peter
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Harmanen, J.
    Reynolds, T.
    Cappellaro, E.
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Nyholm, Anders
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kool, Erik
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ofek, E.
    Gao, X.
    Jin, Z.
    Tan, H.
    Sand, D. J.
    Ciabattari, F.
    Wang, X.
    Zhang, J.
    Huang, F.
    Li, W.
    Mo, J.
    Rui, L.
    Xiang, D.
    Zhang, T.
    Hosseinzadeh, G.
    Howell, D. A.
    McCully, C.
    Valenti, S.
    Benetti, S.
    Callis, E.
    Sagués Carracedo, Ana
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fremling, C.
    Kangas, T.
    Rubin, A.
    Somero, A.
    Terreran, G.
    The long-lived Type IIn SN 2015da: Infrared echoes and strong interaction within an extended massive shell star star star2020In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 635, article id A39Article in journal (Refereed)
    Abstract [en]

    In this paper we report the results of the first similar to four years of spectroscopic and photometric monitoring of the Type IIn supernova SN 2015da (also known as PSN J13522411+3941286, or iPTF16tu). The supernova exploded in the nearby spiral galaxy NGC 5337 in a relatively highly extinguished environment. The transient showed prominent narrow Balmer lines in emission at all times and a slow rise to maximum in all bands. In addition, early observations performed by amateur astronomers give a very well-constrained explosion epoch. The observables are consistent with continuous interaction between the supernova ejecta and a dense and extended H-rich circumstellar medium. The presence of such an extended and dense medium is difficult to reconcile with standard stellar evolution models, since the metallicity at the position of SN 2015da seems to be slightly subsolar. Interaction is likely the mechanism powering the light curve, as confirmed by the analysis of the pseudo bolometric light curve, which gives a total radiated energy greater than or similar to 10(51) erg. Modeling the light curve in the context of a supernova shock breakout through a dense circumstellar medium allowed us to infer the mass of the prexisting gas to be similar or equal to 8 M-circle dot, with an extreme mass-loss rate for the progenitor star similar or equal to 0.6 M-circle dot yr(-1), suggesting that most of the circumstellar gas was produced during multiple eruptive events. Near- and mid-infrared observations reveal a fluxexcess in these domains, similar to those observed in SN 2010jl and other interacting transients, likely due to preexisting radiatively heated dust surrounding the supernova. By modeling the infrared excess, we infer a mass greater than or similar to 0.4 x 10(-3) M-circle dot for the dust.

  • 40. van Velzen, Sjoert
    et al.
    Stein, Robert
    Gilfanov, Marat
    Kowalski, Marek
    Hayasaki, Kimitake
    Reusch, Simeon
    Yao, Yuhan
    Garrappa, Simone
    Franckowiak, Anna
    Gezari, Suvi
    Nordin, Jakob
    Fremling, Christoffer
    Sharma, Yashvi
    Yan, Lin
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Stern, Daniel
    Veres, Patrik M.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Medvedev, Pavel
    Sunyaev, Rashid
    Bellm, Eric C.
    Dekany, Richard G.
    Duev, Dimitri A.
    Graham, Matthew J.
    Kasliwal, Mansi M.
    Kulkarni, Shrinivas R.
    Laher, Russ R.
    Riddle, Reed L.
    Rusholme, Ben
    Establishing accretion flares from supermassive black holes as a source of high-energy neutrinos2024In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 529, no 3, p. 2559-2576Article in journal (Refereed)
    Abstract [en]

    The origin of cosmic high-energy neutrinos remains largely unexplained. For high-energy neutrino alerts from IceCube, a coincidence with time-variable emission has been seen for three different types of accreting black holes: (1) a gamma-ray flare from a blazar (TXS 0506+056), (2) an optical transient following a stellar tidal disruption event (TDE; AT2019dsg), and (3) an optical outburst from an active galactic nucleus (AGN; AT2019fdr). For the latter two sources, infrared follow-up observations revealed a powerful reverberation signal due to dust heated by the flare. This discovery motivates a systematic study of neutrino emission from all supermassive black hole with similar dust echoes. Because dust reprocessing is agnostic to the origin of the outburst, our work unifies TDEs and high-amplitude flares from AGN into a population that we dub accretion flares. Besides the two known events, we uncover a third flare that is coincident with a PeV-scale neutrino (AT2019aalc). Based solely on the optical and infrared properties, we estimate a significance of 3.6σ for this association of high-energy neutrinos with three accretion flares. Our results imply that at least ∼10 per cent of the IceCube high-energy neutrino alerts could be due to accretion flares. This is surprising because the sum of the fluence of these flares is at least three orders of magnitude lower compared to the total fluence of normal AGN. It thus appears that the efficiency of high-energy neutrino production in accretion flares is increased compared to non-flaring AGN. We speculate that this can be explained by the high Eddington ratio of the flares.

  • 41. Ward, Charlotte
    et al.
    Gezari, Suvi
    Nugent, Peter
    Bellm, Eric C.
    Dekany, Richard
    Drake, Andrew
    Duev, Dmitry A.
    Graham, Matthew J.
    Kasliwal, Mansi M.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Masci, Frank J.
    Riddle, Reed L.
    Variability-selected Intermediate-mass Black Hole Candidates in Dwarf Galaxies from ZTF and WISE2022In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 936, no 2, article id 104Article in journal (Refereed)
    Abstract [en]

    While it is difficult to observe the first black hole seeds in the early universe, we can study intermediate-mass black holes (IMBHs) in local dwarf galaxies for clues about their origins. In this paper we present a sample of variability-selected active galactic nuclei (AGN) in dwarf galaxies using optical photometry from the Zwicky Transient Facility (ZTF) and forward-modeled mid-IR photometry of time-resolved Wide-field Infrared Survey Explorer (WISE) co-added images. We found that 44 out of 25,714 dwarf galaxies had optically variable AGN candidates and 148 out of 79,879 dwarf galaxies had mid-IR variable AGN candidates, corresponding to active fractions of 0.17% ± 0.03% and 0.19% ± 0.02%, respectively. We found that spectroscopic approaches to AGN identification would have missed 81% of our ZTF IMBH candidates and 69% of our WISE IMBH candidates. Only nine candidates have been detected previously in radio, X-ray, and variability searches for dwarf galaxy AGN. The ZTF and WISE dwarf galaxy AGN with broad Balmer lines have virial masses of 105 M < MBH < 107 M, but for the rest of the sample, BH masses predicted from host galaxy mass range between 105.2 M < MBH < 107.25 M. We found that only 5 of 152 previously reported variability-selected AGN candidates from the Palomar Transient Factory in common with our parent sample were variable in ZTF. We also determined a nuclear supernova fraction of 0.05% ± 0.01% yr−1 for dwarf galaxies in ZTF. Our ZTF and WISE IMBH candidates show the promise of variability searches for the discovery of otherwise hidden low-mass AGN.

  • 42.
    Yang, Sheng
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Chen, Janet Ting-Wan
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kool, Erik C.
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Astronomy.
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Schulze, S.
    Strotjohann, N.
    Horesh, A.
    Kasliwal, M.
    Kupfer, T.
    Mahabal, A. A.
    Masci, F. J.
    Nugent, P.
    Perley, D. A.
    Riddle, R.
    Rusholme, B.
    Sharma, Y.
    Is supernova SN 2020faa an iPTF14hls look-alike?2021In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 646, article id A22Article in journal (Refereed)
    Abstract [en]

    Context. We present observations of ZTF20aatqesi (SN 2020faa). This Type II supernova (SN) displays a luminous light curve (LC) that started to rebrighten from an initial decline. We investigate this in relation to the famous SN iPTF14hls, which received a great deal of attention and multiple interpretations in the literature, but whose nature and source of energy still remain unknown.

    Aims. We demonstrate the great similarity between SN 2020faa and iPTF14hls during the first 6 months, and use this comparison to forecast the evolution of SN 2020faa and to reflect on the less well observed early evolution of iPTF14hls.

    Methods. We present and analyse our observational data, consisting mainly of optical LCs from the Zwicky Transient Facility in the gri bands and of a sequence of optical spectra. We construct colour curves and a bolometric lc, and we compare ejecta-velocity and black-body radius evolutions for the two supernovae (SNe) and for more typical Type II SNe.

    Results. The LCs show a great similarity with those of iPTF14hls over the first 6 months in luminosity, timescale, and colour. In addition, the spectral evolution of SN 2020faa is that of a Type II SN, although it probes earlier epochs than those available for iPTF14hls.

    Conclusions. The similar LC behaviour is suggestive of SN 2020faa being a new iPTF14hls. We present these observations now to advocate follow-up observations, since most of the more striking evolution of SN iPTF14hls came later, with LC undulations and a spectacular longevity. On the other hand, for SN 2020faa we have better constraints on the explosion epoch than we had for iPTF14hls, and we have been able to spectroscopically monitor it from earlier phases than was done for the more famous sibling.

  • 43.
    Yang, Sheng
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Strotjohann, N. L.
    Schulze, Steve
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Physics.
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fremling, C.
    Perley, D.
    Ofek, E.
    Schweyer, Tassilo
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bellm, E. C.
    Kasliwal, M. M.
    Masci, F. J.
    Rigault, M.
    Yang, Y.
    A low-energy explosion yields the underluminous Type IIP SN 2020cxd2021In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 655, article id A90Article in journal (Refereed)
    Abstract [en]

    Context. We present our observations and analysis of SN 2020cxd, a low-luminosity (LL), long-lived Type IIP supernova (SN). This object is a clear outlier in the magnitude-limited SN sample recently presented by the Zwicky Transient Facility’s (ZTF) Bright Transient Survey.

    Aims. We demonstrate that SN 2020cxd is an additional member of the group of LL SNe and we discuss the rarity of LL SNe in the context of the ZTF survey. We consider how further studies of these faintest members of the core-collapse (CC) SN family might help improve the general understanding of the underlying initial mass function for stars that explode.

    Methods. We used optical light curves (LCs) from the ZTF in the gri bands and several epochs of ultraviolet data from the Neil Gehrels Swift observatory as well as a sequence of optical spectra. We constructed the colour curves and a bolometric LC. Then we compared the evolution of the ejecta velocity and black-body temperature for LL SNe as well as for typical Type II SNe. Furthermore, we adopted a Monte Carlo code that fits semi-analytic models to the LC of SN 2020cxd, which allows for the estimation of the physical parameters. Using our late-time nebular spectra, we also make a comparison against SN II spectral synthesis models from the literature to constrain the progenitor properties of SN 2020cxd.

    Results. The LCs of SN 2020cxd show a great similarity with those of LL SNe IIP in terms of luminosity, timescale, and colours. Also, the spectral evolution of SN 2020cxd is that of a Type IIP SN. The spectra show prominent and narrow P-Cygni lines, indicating low expansion velocities. This is one of the faintest LL SNe observed, with an absolute plateau magnitude of Mr = −14.5 mag and also one with the longest plateau lengths, with a duration of 118 days. Finally, the velocities measured from the nebular emission lines are among the lowest ever seen in a SN, with an intrinsic full width at half maximum value of 478 km s−1. The underluminous late-time exponential LC tail indicates that the mass of 56Ni ejected during the explosion is much smaller than the average of normal SNe IIP, we estimate M56Ni = 0.003 M. The Monte Carlo fitting of the bolometric LC suggests that the progenitor of SN 2020cxd had a radius of R0 = 1.3 × 1013 cm, kinetic energy of Ekin = 4.3 × 1050 erg, and ejecta mass of Mej = 9.5 M. From the bolometric LC, we estimated the total radiated energy Erad = 1.52 × 1048 erg. Using our late-time nebular spectra, we compared these results against SN II spectral synthesis models to constrain the progenitor zero-age main sequence mass and found that it is likely to be ≲15 M.

    Conclusions. SN 2020cxd is a LL Type IIP SN. The inferred progenitor parameters and the features observed in the nebular spectrum favour a low-energy, Ni-poor, iron CC SN from a low-mass (∼12 M) red supergiant.

  • 44. Yao, Yuhan
    et al.
    Lu, Wenbin
    Guolo, Muryel
    Pasham, Dheeraj R.
    Gezari, Suvi
    Gilfanov, Marat
    Gendreau, Keith C.
    Harrison, Fiona
    Cenko, S. Bradley
    Kulkarni, S. R.
    Miller, Jon M.
    Walton, Dominic J.
    García, Javier A.
    van Velzen, Sjoert
    Alexander, Kate D.
    Miller-Jones, James C. A.
    Nicholl, Matt
    Hammerstein, Erica
    Medvedev, Pavel
    Stern, Daniel
    Ravi, Vikram
    Sunyaev, R.
    Bloom, Joshua S.
    Graham, Matthew J.
    Kool, Erik C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Mahabal, Ashish A.
    Masci, Frank J.
    Purdum, Josiah
    Rusholme, Ben
    Sharma, Yashvi
    Smith, Roger
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    The Tidal Disruption Event AT2021ehb: Evidence of Relativistic Disk Reflection, and Rapid Evolution of the Disk-Corona System2022In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 937, no 1, article id 8Article in journal (Refereed)
    Abstract [en]

    We present X-ray, UV, optical, and radio observations of the nearby (≈78 Mpc) tidal disruption event AT2021ehb/ZTF21aanxhjv during its first 430 days of evolution. AT2021ehb occurs in the nucleus of a galaxy hosting a≈107 M black hole (MBH inferred from host galaxy scaling relations). High-cadence Swift and Neutron Star Interior Composition Explorer (NICER) monitoring reveals a delayed X-ray brightening. The spectrum first undergoes a gradual soft → hard transition and then suddenly turns soft again within 3 days at δt≈272 days during which the X-ray flux drops by a factor of 10. In the joint NICER+NuSTAR observation (δt = 264 days, harder state), we observe a prominent nonthermal component up to 30 keV and an extremely broad emission line in the iron K band. The bolometric luminosity of AT2021ehb reaches a maximum of 6.0+10.4-3.8%LEdd when the X-ray spectrum is the hardest. During the dramatic X-ray evolution, no radio emission is detected, the UV/optical luminosity stays relatively constant, and the optical spectra are featureless. We propose the following interpretations: (i) the soft → hard transition may be caused by the gradual formation of a magnetically dominated corona; (ii) hard X-ray photons escape from the system along solid angles with low scattering optical depth (∼a few) whereas the UV/optical emission is likely generated by reprocessing materials with much larger column density—the system is highly aspherical; and (iii) the abrupt X-ray flux drop may be triggered by the thermal–viscous instability in the inner accretion flow, leading to a much thinner disk.

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