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  • 1. Bellm, Eric C.
    et al.
    Kulkarni, Shrinivas R.
    Graham, Matthew J.
    Dekany, Richard
    Smith, Roger M.
    Riddle, Reed
    Masci, Frank J.
    Helou, George
    Prince, Thomas A.
    Adams, Scott 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).
    Barlow, Tom
    Bauer, James
    Beck, Ron
    Belicki, Justin
    Biswas, Rahul
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Blagorodnova, Nadejda
    Bodewits, Dennis
    Bolin, Bryce
    Brinnel, Valery
    Brooke, Tim
    Bue, Brian
    Bulla, Mattia
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Burruss, Rick
    Cenko, S. Bradley
    Chang, Chan-Kao
    Connolly, Andrew
    Coughlin, Michael
    Cromer, John
    Cunningham, Virginia
    De, Kishalay
    Delacroix, Alex
    Desai, Vandana
    Duev, Dmitry A.
    Eadie, Gwendolyn
    Farnham, Tony L.
    Feeney, Michael
    Feindt, Ulrich
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Flynn, David
    Franckowiak, Anna
    Frederick, S.
    Fremling, C.
    Gal-Yam, Avishay
    Gezari, Suvi
    Giomi, Matteo
    Goldstein, Daniel A.
    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).
    Groom, Steven
    Hacopians, Eugean
    Hale, David
    Henning, John
    Ho, Anna Y. Q.
    Hover, David
    Howell, Justin
    Hung, Tiara
    Huppenkothen, Daniela
    Imel, David
    Ip, Wing-Huen
    Ivezic, Zeljko
    Jackson, Edward
    Jones, Lynne
    Juric, Mario
    Kasliwal, Mansi M.
    Kaspi, S.
    Kaye, Stephen
    Kelley, Michael S. P.
    Kowalski, Marek
    Kramer, Emily
    Kupfer, Thomas
    Landry, Walter
    Laher, Russ R.
    Lee, Chien-De
    Lin, Hsing Wen
    Lin, Zhong-Yi
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Giomi, Matteo
    Mahabal, Ashish
    Mao, Peter
    Miller, Adam A.
    Monkewitz, Serge
    Murphy, Patrick
    Ngeow, Chow-Choong
    Nordin, Jakob
    Nugent, Peter
    Ofek, Eran
    Patterson, Maria T.
    Penprase, Bryan
    Porter, Michael
    Rauch, Ludwig
    Rebbapragada, Umaa
    Reiley, Dan
    Rigault, Mickael
    Rodriguez, Hector
    van Roestel, Jan
    Rusholme, Ben
    van Santen, Jakob
    Schulze, S.
    Shupe, David L.
    Singer, Leo P.
    Soumagnac, Maayane T.
    Stein, Robert
    Surace, Jason
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Szkody, Paula
    Taddia, Francesco
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Terek, Scott
    Van Sistine, Angela
    van Velzen, Sjoert
    Vestrand, W. Thomas
    Walters, Richard
    Ward, Charlotte
    Ye, Quan-Zhi
    Yu, Po-Chieh
    Yan, Lin
    Zolkower, Jeffry
    The Zwicky Transient Facility: System Overview, Performance, and First Results2019In: Publications of the Astronomical Society of the Pacific, ISSN 0004-6280, E-ISSN 1538-3873, Vol. 131, no 995, article id 018002Article in journal (Refereed)
    Abstract [en]

    The Zwicky Transient Facility (ZTF) is a new optical time-domain survey that uses the Palomar 48 inch Schmidt telescope. A custom-built wide-field camera provides a 47 deg(2) field of view and 8 s readout time, yielding more than an order of magnitude improvement in survey speed relative to its predecessor survey, the Palomar Transient Factory. We describe the design and implementation of the camera and observing system. The ZTF data system at the Infrared Processing and Analysis Center provides near-real-time reduction to identify moving and varying objects. We outline the analysis pipelines, data products, and associated archive. Finally, we present on-sky performance analysis and first scientific results from commissioning and the early survey. ZTF's public alert stream will serve as a useful precursor for that of the Large Synoptic Survey Telescope.

  • 2. Blagorodnova, N.
    et al.
    Gezari, S.
    Hung, T.
    Kulkarni, S. R.
    Cenko, S. B.
    Pasham, D. R.
    Yan, L.
    Arcavi, I.
    Ben-Ami, S.
    Bue, B. D.
    Cantwell, T.
    Cao, Y.
    Castro-Tirado, A. J.
    Fender, R.
    Fremling, C.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Gal-Yam, A.
    Ho, A. Y. Q.
    Horesh, A.
    Hosseinzadeh, G.
    Kasliwal, M. M.
    Kong, A. K. H.
    Laher, R. R.
    Leloudas, G.
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). California Institute of Technology, USA.
    Masci, F. J.
    Mooley, K.
    Neill, J. D.
    Nugent, P.
    Powell, M.
    Valeev, A. F.
    Vreeswijk, P. M.
    Walters, R.
    Wozniak, P.
    iPTF16fnl: A Faint and Fast Tidal Disruption Event in an E plus A Galaxy2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 844, no 1, article id 46Article in journal (Refereed)
    Abstract [en]

    We present ground-based and Swift observations of iPTF16fnl, a likely tidal disruption event (TDE) discovered by the intermediate Palomar Transient Factory (iPTF) survey at 66.6 Mpc. The light curve of the object peaked at an absolute mag M-g = -17.2. The maximum bolometric luminosity (from optical and UV) was L-p similar or equal to (1.0 +/- 0.15) x 10(43) erg s(-1), an order of magnitude fainter than any other optical TDE discovered so far. The luminosity in the first 60 days is consistent with an exponential decay, with L proportional to e(-(t-t0)/T), where t(0) = 57631.0 (MJD) and tau similar or equal to 15 days. The X-ray shows a marginal detection at L-X = 2.4(-1.1)(1.9) x 10(39) erg s(-1) (Swift X-ray Telescope). No radio counterpart was detected down to 3s, providing upper limits for monochromatic radio luminosities of nu L-nu < 2.3 x 10(36) erg s(-1) and nLn < 1.7 x 10(37) erg s(-1) (Very Large Array, 6.1 and 22 GHz). The blackbody temperature, obtained from combined Swift UV and optical photometry, shows a constant value of 19,000 K. The transient spectrum at peak is characterized by broad He II and Ha emission lines, with FWHMs of about 14,000 km s(-1) and 10,000 km s(-1), respectively. He. I lines are also detected at lambda lambda 5875 and 6678. The spectrum of the host is dominated by strong Balmer absorption lines, which are consistent with a post-starburst (E+A) galaxy with an age of similar to 650 Myr and solar metallicity. The characteristics of iPTF16fnl make it an outlier on both luminosity and decay timescales, as compared to other optically selected TDEs. The discovery of such a faint optical event suggests a higher rate of tidal disruptions, as low-luminosity events may have gone unnoticed in previous searches.

  • 3. Cowperthwaite, P. S.
    et al.
    Berger, E.
    Rest, A.
    Chornock, R.
    Scolnic, D. M.
    Williams, P. K. G.
    Fong, W.
    Drout, M. R.
    Foley, R. J.
    Margutti, 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).
    Metzger, B. D.
    Quataert, E.
    An Empirical Study of Contamination in Deep, Rapid, and Wide-field Optical Follow-up of Gravitational Wave Events2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 858, no 1, article id 18Article in journal (Refereed)
    Abstract [en]

    We present an empirical study of contamination in wide-field optical follow-up searches of gravitational wave sources from Advanced LIGO/Virgo using dedicated observations with the Dark Energy Camera. Our search covered similar to 56 deg(2), with two visits per night, in the i and z bands, followed by an additional set of griz images three weeks later to serve as reference images for subtraction. We achieve 5 sigma point-source limiting magnitudes of i approximate to 23.5 and z approximate to 22.4 mag in the coadded single-epoch images. We conduct a search for transient objects that mimic the i - z color behavior of both red (i-z > 0.5 mag) and blue (i-z < 0 mag) kilonova emission, finding 11 and 10 contaminants, respectively. Independent of color, we identify 48 transients of interest. Additionally, we leverage the rapid cadence of our observations to search for sources with characteristic timescales of approximate to 1 day and approximate to 3 hr, finding no potential contaminants. We assess the efficiency of our search with injected point sources, finding that we are 90% (60%) efficient when searching for red (blue) kilonova-like sources to a limiting magnitude of i less than or similar to 22.5 mag. Using our efficiencies, we derive sky rates for kilonova contaminants of R-red approximate to 0.16 deg(-2) and R-blue approximate to 0.80 deg(-2). The total contamination rate is R-all approximate to 1.79 deg(-2). We compare our results to previous optical follow-up efforts and comment on the outlook for gravitational wave follow-up searches as additional detectors (e.g., KAGRA, LIGO India) come online in the next decade.

  • 4. De, K.
    et al.
    Kasliwal, M. M.
    Ofek, E. O.
    Moriya, T. J.
    Burke, J.
    Cao, Y.
    Cenko, S. B.
    Doran, G. B.
    Duggan, G. E.
    Fender, R. P.
    Fransson, Claes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Gal-Yam, A.
    Horesh, A.
    Kulkarni, S. R.
    Laher, R. 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).
    Manulis, I.
    Masci, F.
    Mazzali, P. A.
    Nugent, P. E.
    Perley, D. A.
    Petrushevska, Tanja
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). University of Nova Gorica, Slovenia.
    Piro, A. L.
    Rumsey, C.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sullivan, 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).
    A hot and fast ultra-stripped supernova that likely formed a compact neutron star binary2018In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 362, no 6411, p. 201-206Article in journal (Refereed)
    Abstract [en]

    Compact neutron star binary systems are produced from binary massive stars through stellar evolution involving up to two supernova explosions. The final stages in the formation of these systems have not been directly observed. We report the discovery of iPTF 14gqr (SN 2014ft), a type Ic supernova with a fast-evolving light curve indicating an extremely low ejecta mass (approximate to 0.2 solar masses) and low kinetic energy (approximate to 2 x 10(50) ergs). Early photometry and spectroscopy reveal evidence of shock cooling of an extended helium-rich envelope, likely ejected in an intense pre-explosion mass-loss episode of the progenitor. Taken together, we interpret iPTF 14gqr as evidence for ultra-stripped supernovae that form neutron stars in compact binary systems.

  • 5. De, Kishalay
    et al.
    Kasliwal, Mansi M.
    Cantwell, Therese
    Cao, Yi
    Cenko, S. Bradley
    Gal-Yam, Avishay
    Johansson, Joel
    Kong, Albert
    Kulkarni, Shrinivas 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
    Matuszewski, Matt
    Mooley, Kunal P.
    Neill, James D.
    Nugent, Peter E.
    Ofek, Eran O.
    Perrott, Yvette
    Rebbapragada, Umaa D.
    Rubin, Adam
    O' Sullivan, Donal
    Yaron, Ofer
    iPTF 16hgs: A Double-peaked Ca-rich Gap Transient in a Metal-poor, Star-forming Dwarf Galaxy2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 866, no 1, article id 72Article in journal (Refereed)
    Abstract [en]

    Calcium-rich gap transients represent an intriguing new class of faint and fast-evolving supernovae that exhibit strong [Ca II] emission in their nebular phase spectra. In this paper, we present the discovery and follow-up observations of a faint and fast-evolving transient, iPTF 16hgs, that exhibited a double-peaked light curve. Exhibiting a Type Ib spectrum in the photospheric phase and an early transition to a [Ca II] dominated nebular phase, iPTF 16hgs shows properties consistent with the class of Ca-rich gap transients, with two important exceptions. First, while the second peak of the light curve is similar to other Ca-rich gap transients, the first blue and fast-fading peak (declining over approximate to 2 days) is unique to this source. Second, we find that iPTF 16hgs occurred in the outskirts (projected offset of approximate to 6 kpc approximate to 1.9 R-eff) of a low-metallicity (approximate to 0.4 Z(circle dot)), star-forming, dwarf spiral galaxy. Deep limits from late-time radio observations suggest a low-density environment for the source. If iPTF 16hgs shares explosion physics with the class of Ca-rich gap transients, the first peak can be explained by the presence of 0.01 M-circle dot of Ni-56 in the outer layers the ejecta, reminiscent of some models of He-shell detonations on WDs. However, if iPTF 16hgs is physically unrelated to the class, the first peak is consistent with shock cooling emission (of an envelope with a mass of approximate to 0.08 M-circle dot and radius of approximate to 13 R-circle dot) in a core-collapse explosion of a highly stripped massive star in a close binary system.

  • 6.
    Dhawan, Suhail
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bulla, Mattia
    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).
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Johansson, J.
    Fransson, Claes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kulkarni, R.
    Papadogiannakis, Seméli
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Miller, A. A.
    iPTF16abc and the population of Type Ia supernovae: comparing the photospheric, transitional, and nebular phases2018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 480, no 2, p. 1445-1456Article in journal (Refereed)
    Abstract [en]

    Key information about the progenitor system and the explosion mechanism of Type la supernovae (SNe Ia) can be obtained from early observations, within a few days from explosion. iPTF16abc was discovered as a young SN la with excellent early time data. Here, we present photometry and spectroscopy of the SN in the nebular phase. A comparison of the early time data with a sample of SNe la shows distinct features, differing from normal SNe la at early phases but similar to normal SNe Ia at a few weeks after maximum light (i.e. the transitional phase) and well into the nebular phase. The transparency time-scales (t(0)) for this sample of SNe Ia range between similar to 25 and 41 d indicating a diversity in the ejecta masses. t(0) also weakly correlates with the peak bolometric luminosity, consistent with the interpretation that SNe with higher ejecta masses would produce more Ni-56. Comparing the to and the maximum luminosity, L-max distribution of a sample of SNe Ia to predictions from a wide range of explosion models we find an indication that the sub-Chandrasekhar mass models span the range of observed values. However, the bright end of the distribution can be better explained by Chandrasekhar mass delayed detonation models, hinting at multiple progenitor channels to explain the observed bolometric properties of SNe Ia. iPTF16abc appears to be consistent with the predictions from the M-ch models.

  • 7. Eftekhari, T.
    et al.
    Berger, E.
    Margalit, B.
    Blanchard, P. K.
    Patton, L.
    Demorest, P.
    Williams, P. K. G.
    Chatterjee, S.
    Cordes, J. M.
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Metzger, B. D.
    Nicholl, M.
    A Radio Source Coincident with the Superluminous Supernova PTF10hgi: Evidence for a Central Engine and an Analog of the Repeating FRB 121102?2019In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 876, no 1, article id L10Article in journal (Refereed)
    Abstract [en]

    We present the detection of an unresolved radio source coincident with the position of the Type I superluminous supernova (SLSN) PTF10hgi (z = 0.098) about 7.5 yr post-explosion, with a flux density of F-nu(6 GHz) approximate to 47.3 mu Jy and a luminosity of L-nu(6 GHz) approximate to 1.1 x 10(28) erg s(-1) Hz(-1). This represents the first detection of radio emission coincident with an SLSN on any timescale. We investigate various scenarios for the origin of the radio emission: star formation activity, an active galactic nucleus, and a non-relativistic supernova blastwave. While any of these would be quite novel if confirmed, none appear likely when considered within the context of the other properties of the host galaxy, previous radio observations of SLSNe, and the general population of hydrogen-poor supernovae (SNe). Instead, the radio emission is reminiscent of the quiescent radio source associated with the repeating FRB 121102, which has been argued to be powered by a magnetar born in a SLSN or long gamma-ray burst explosion several decades ago. We show that the properties of the radio source are consistent with a magnetar wind nebula or an off-axis jet, indicating the presence of a central engine. Our directed search for fast radio bursts from the location of PTF10hgi using 40 minutes of Very Large Array phased-array data reveals no detections to a limit of 22 mJy (10 sigma; 10 ms duration). We outline several follow-up observations that can conclusively establish the origin of the radio emission.

  • 8.
    Fremling, Christoffer
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). California Institute of Technology, USA.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Kasliwal, M. M.
    Kulkarni, S. R.
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Ergon, Mattias
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Karamehmetoglu, Emir
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Taddia, Francesco
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Arcavi, I
    Cenko, S. B.
    Clubb, K.
    De Cia, A.
    Duggan, G.
    Filippenko, A.
    Gal-Yam, A.
    Graham, M. L.
    Horesh, A.
    Hosseinzadeh, G.
    Howell, D. A.
    Kuesters, D.
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). California Institute of Technology, USA.
    Matheson, T.
    Nugent, P. E.
    Perley, D. A.
    Quimby, R. M.
    Saunders, C.
    Oxygen and helium in stripped-envelope supernovae2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 618, article id A37Article in journal (Refereed)
    Abstract [en]

    We present an analysis of 507 spectra of 173 stripped-envelope (SE) supernovae (SNe) discovered by the untargeted Palomar Transient Factory (PTF) and intermediate PTF (iPTF) surveys. Our sample contains 55 Type IIb SNe (SNe 45 Type Ib SNe (SNe IIb), 56 Type Ic SNe (SNe Ic), and 17 Type Ib/c SNe (SNe Ib/c). We have compared the SE SN subtypes via measurements of the pseudo-equivalent widths (pEWs) and velocities of the He I lambda lambda 5876, 7065 and O I lambda 7774 absorption lines. Consistent with previous work, we find that SNe Ic show higher pEWs and velocities in O I lambda 7774 compared to SNe IIb and Ib. The pEWs of the He I lambda lambda 5876, 7065 lines are similar in SNe Ib and IIb after maximum light. The He I lambda lambda 5876, 7065 velocities at maximum light are higher in SNe Ib compared to SNe IIb. We identify an anticorrelation between the He I lambda 7065 pEW and O I lambda 7774 velocity among SNe IIb and Ib. This can be interpreted as a continuum in the amount of He present at the time of explosion. It has been suggested that SNe Ib and Ic have similar amounts of He, and that lower mixing could be responsible for hiding He in SNe Ic. However, our data contradict this mixing hypothesis. The observed difference in the expansion rate of the ejecta around maximum light of SNe Ic (V-m root 2E(k)/M-ej approximate to 15 000 km s(-1)) and SNe Ib (V-m approximate to 9000 km s(-1)) would imply an average He mass difference of similar to 1.4 M-circle dot, if the other explosion parameters are assumed to be unchanged between the SE SN subtypes. We conclude that SNe Ic do not hide He but lose He due to envelope stripping.

  • 9. Graham, Matthew J.
    et al.
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Feindt, Ulrich
    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).
    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).
    Taddia, Francesco
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Biswas, Rahul
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bulla, Mattia
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Zolkower, Jeffry
    The Zwicky Transient Facility: Science Objectives2019In: Publications of the Astronomical Society of the Pacific, ISSN 0004-6280, E-ISSN 1538-3873, Vol. 131, no 1001, article id 078001Article in journal (Refereed)
    Abstract [en]

    The Zwicky Transient Facility (ZTF), a public-private enterprise, is a new time-domain survey employing a dedicated camera on the Palomar 48-inch Schmidt telescope with a 47 deg(2) field of view and an 8 second readout time. It is well positioned in the development of time-domain astronomy, offering operations at 10% of the scale and style of the Large Synoptic Survey Telescope (LSST) with a single 1-m class survey telescope. The public surveys will cover the observable northern sky every three nights in g and r filters and the visible Galactic plane every night in g and r. Alerts generated by these surveys are sent in real time to brokers. A consortium of universities that provided funding (partnership) are undertaking several boutique surveys. The combination of these surveys producing one million alerts per night allows for exploration of transient and variable astrophysical phenomena brighter than r similar to 20.5 on timescales of minutes to years. We describe the primary science objectives driving ZTF, including the physics of supernovae and relativistic explosions, multi-messenger astrophysics, supernova cosmology, active galactic nuclei, and tidal disruption events, stellar variability, and solar system objects.

  • 10. Ho, Anna Y. Q.
    et al.
    Goldstein, Daniel A.
    Schulze, Steve
    Khatami, David K.
    Perley, Daniel A.
    Ergon, Mattias
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Gal-Yam, Avishay
    Corsi, Alessandra
    Andreoni, Igor
    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.
    Blagorodnova, Nadia
    Bright, Joe S.
    Burns, E.
    Cenko, S. Bradley
    Cunningham, Virginia
    De, Kishalay
    Dekany, Richard
    Dugas, Alison
    Fender, Rob P.
    Fransson, Claes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Fremling, Christoffer
    Goldstein, Adam
    Graham, Matthew J.
    Hale, David
    Horesh, Assaf
    Hung, Tiara
    Kasliwal, Mansi M.
    Kuin, N. Paul M.
    Kulkarni, S. R.
    Kupfer, Thomas
    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.
    Ngeow, Chow-Choong
    Nugent, Peter E.
    Ofek, Eran O.
    Patterson, Maria T.
    Petitpas, Glen
    Rusholme, Ben
    Sai, Hanna
    Sfaradi, Itai
    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).
    Soumagnac, Maayane T.
    Tachibana, Yutaro
    Taddia, Francesco
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Walters, Richard
    Wang, Xiaofeng
    Yao, Yuhan
    Zhang, Xinhan
    Evidence for Late-stage Eruptive Mass Loss in the Progenitor to SN2018gep, a Broad-lined Ic Supernova: Pre-explosion Emission and a Rapidly Rising Luminous Transient2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 887, no 2, article id 169Article in journal (Refereed)
    Abstract [en]

    We present detailed observations of ZTF18abukavn (SN2018gep), discovered in high-cadence data from the Zwicky Transient Facility as a rapidly rising (1.4 +/- 0.1 mag hr(-1)) and luminous (M-g,M- peak = -20 mag) transient. It is spectroscopically classified as a broad-lined stripped-envelope supernova (Ic-BL SN). The high peak luminosity (L-bol greater than or similar to 3 x 10(44) erg s(-1)), the short rise time (t(rise) = 3 days in g band), and the blue colors at peak (g-r similar to -0.4) all resemble the high-redshift Ic-BL iPTF16asu, as well as several other unclassified fast transients. The early discovery of SN2018gep (within an hour of shock breakout) enabled an intensive spectroscopic campaign, including the highest-temperature (T-eff greater than or similar to 40,000 K) spectra of a stripped-envelope SN. A retrospective search revealed luminous (M-g similar to M-r approximate to -14 mag) emission in the days to weeks before explosion, the first definitive detection of precursor emission for a Ic-BL. We find a limit on the isotropic gamma-ray energy release E-gamma,E- iso < 4.9 x 10(48) erg, a limit on X-ray emission L-X < 10(40) erg s(-1), and a limit on radio emission nu L-v less than or similar to 10(37) erg s(-1). Taken together, we find that the early (< 10 days) data are best explained by shock breakout in a massive shell of dense circumstellar material (0.02 M-circle dot) at large radii (3 x 10(14) cm) that was ejected in eruptive pre-explosion mass-loss episodes. The late-time (> 10 days) light curve requires an additional energy source, which could be the radioactive decay of Ni-56.

  • 11. Hung, T.
    et al.
    Gezari, S.
    Cenko, S. B.
    van Velzen, S.
    Blagorodnova, N.
    Yan, Lin
    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).
    Kupfer, T.
    Leloudas, G.
    Kong, A. K. H.
    Nugent, P. E.
    Fremling, C.
    Laher, Russ R.
    Masci, F. J.
    Cao, Y.
    Roy, Rupak
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Petrushevska, Tanja
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Sifting for Sapphires: Systematic Selection of Tidal Disruption Events in iPTF2018In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 238, no 2, article id 15Article in journal (Refereed)
    Abstract [en]

    We present results from a systematic selection of tidal disruption events (TDEs) in a wide-area (4800 deg(2)), g+ R band, Intermediate Palomar Transient Factory experiment. Our selection targets typical optically selected TDEs: bright (> 60% flux increase) and blue transients residing in the centers of red galaxies. Using photometric selection criteria to down-select from a total of 493 nuclear transients to a sample of 26 sources, we then use follow-up UV imaging with the Neil Gehrels Swift Telescope, ground-based optical spectroscopy, and light curve fitting to classify them as 14 Type Ia supernovae (SNe Ia), 9 highly variable active galactic nuclei (AGNs), 2 confirmed TDEs, and 1 potential core-collapse supernova. We find it possible to filter AGNs by employing a more stringent transient color cut (g - r < -0.2 mag); further, UV imaging is the best discriminator for filtering SNe, since SNe Ia can appear as blue, optically, as TDEs in their early phases. However, when UV-optical color is unavailable, higher-precision astrometry can also effectively reduce SNe contamination in the optical. Our most stringent optical photometric selection criteria yields a 4.5: 1 contamination rate, allowing for a manageable number of TDE candidates for complete spectroscopic follow-up and real-time classification in the Zwicky Transient Facility era. We measure a TDE per galaxy rate of 1.7(-1.3)(+2.9) 10(-4) gal yr(-1) (90% CL in Poisson statistics). This does not account for TDEs outside our selection criteria, and thus may not reflect the total TDE population, which is yet to be fully mapped.

  • 12. Irani, Ido
    et al.
    Schulze, Steve
    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).
    Brink, Thomas G.
    Zheng, WeiKang
    Filippenko, Alexei V.
    Yang, Yi
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    de Jaeger, Thomas
    Nugent, Peter E.
    Kasliwal, Mansi M.
    Fremling, Christoffer
    Neill, James Don
    Rebbapragada, Umaa
    Masci, Frank J.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Yaron, Ofer
    On the Origin of SN 2016hil-A Type II Supernova in the Remote Outskirts of an Elliptical Host2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 887, no 2, article id 127Article in journal (Refereed)
    Abstract [en]

    Type II supernovae (SNe) stem from the core collapse of massive (>8 M-circle dot) stars. Due to their short lifespan, we expect a very low rate of such events in elliptical hosts, where the star formation rate is low, and which are mostly comprised of an old stellar population. SN 2016hil (iPTF16hil) is an SN II located in the extreme outskirts of an elliptical galaxy at z.=.0.0608 (projected distance 27.2 kpc). It was detected near peak (M-r similar to -17 mag) 9 days after the last non-detection. The event has some potentially peculiar properties: it presented an apparently double-peaked light curve, and its spectra suggest low metallicity content (Z < 0.4 Z(circle dot)). We place a tentative upper limit on the mass of a potential faint host at log M/M-circle dot = 7.27(-0.24)(+0.43) using deep optical imaging from Keck/LRIS. In light of this, we discuss the possibility of the progenitor forming locally and other more exotic formation scenarios such as a merger or common-envelope evolution causing a time-delayed explosion. Further observations of the explosion site in the UV are needed in order to distinguish between the cases. Regardless of the origin of the transient, observing a population of such seemingly hostless SNe II could have many uses, including an estimate the amount of faint galaxies in a given volume, and tests of the prediction of a time-delayed population of core-collapse SNe in locations otherwise unfavorable for the detection of such events.

  • 13. Katebi, Reza
    et al.
    Chornock, Ryan
    Berger, Edo
    Jones, David O.
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Margutti, Raffaella
    Rest, Armin
    Scolnic, Daniel M.
    Burgett, William S.
    Kaiser, Nick
    Kudritzki, Rolf-Peter
    Magnier, Eugene A.
    Wainscoat, Richard J.
    Waters, Christopher
    PS1-13cbe: the rapid transition of a Seyfert 2 to a Seyfert 12019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 487, no 3, p. 4057-4070Article in journal (Refereed)
    Abstract [en]

    We present a nuclear transient event, PS1-13cbe, that was first discovered in the Pan-STARRS1 survey in 2013. The outburst occurred in the nucleus of the galaxy SDSS J222153.87+003054.2 at z = 0.123 55, which was classified as a Seyfert 2 in a pre-outburst archival Sloan Digital Sky Survey (SDSS) spectrum. PS1-13cbe showed the appearance of strong broad H alpha and H beta emission lines and a non-stellar continuum in a Magellan spectrum taken 57 d after the peak of the outburst that resembled the characteristics of a Seyfert 1. These broad lines were not present in the SDSS spectrum taken a decade earlier and faded away within 2 yr, as observed in several late-time MDM spectra. We argue that the dramatic appearance and disappearance of the broad lines and a factor of similar to 8 increase in the optical continuum are most likely caused by variability in the pre-existing accretion disc than a tidal disruption event, supernova, or variable obscuration. The time-scale for the turn-on of the optical emission of similar to 70 d observed in this transient is among the shortest observed in a 'changing-look' active galactic nucleus,

  • 14. Kupfer, T.
    et al.
    Ramsay, G.
    van Roestel, J.
    Brooks, J.
    MacFarlane, S. A.
    Toma, R.
    Groot, P. J.
    Woudt, P. A.
    Bildsten, L.
    Marsh, T. R.
    Green, M. J.
    Breedt, E.
    Kilkenny, D.
    Freudenthal, J.
    Geier, S.
    Heber, U.
    Bagnulo, S.
    Blagorodnova, N.
    Buckley, D. A. H.
    Dhillon, V. S.
    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). California Institute of Technology, USA.
    Prince, T. A.
    The OmegaWhite Survey for Short-period Variable Stars. V. Discovery of an Ultracompact Hot Subdwarf Binary with a Compact Companion in a 44-minute Orbit2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 851, no 1, article id 28Article in journal (Refereed)
    Abstract [en]

    We report the discovery of the ultracompact hot subdwarf (sdOB) binary OW J074106.0-294811.0 with an orbital period of P-orb = 44.66279 +/- 1.16 x 10(-4) minutes, making it the most compact hot subdwarf binary known. Spectroscopic observations using the VLT, Gemini and Keck telescopes revealed a He-sdOB primary with an intermediate helium abundance, T-eff = 39 400 +/- 500 K and log g = 5.74 +/- 0.09. High signal-to-noise ratio light curves show strong ellipsoidal modulation resulting in a derived sdOB mass M-sdOB= 0.23 +/- 0.12 M-circle dot with a WD companion (M-WD = 0.72 +/- 0.17 M-circle dot). The mass ratio was found to be q = M-sdOB/M-WD = 0.32 +/- 0.10. The derived mass for the He-sdOB is inconsistent with the canonical mass for hot subdwarfs of approximate to 0.47 M-circle dot. To put constraints on the structure and evolutionary history of the sdOB star we compared the derived T-eff, log g, and sdOB mass to evolutionary tracks of helium stars and helium white dwarfs calculated with Modules for Experiments in Stellar Astrophysics (MESA). We find that the best-fitting model is a helium white dwarf with a mass of 0.320 M-circle dot, which left the common envelope approximate to 1.1 Myr ago, which is consistent with the observations. As a helium white dwarf with a massive white dwarf companion, the object will reach contact in 17.6 Myr at an orbital period of 5 minutes. Depending on the spin-orbit synchronization timescale the object will either merge to form an R CrB star or end up as a stably accreting AM CVn-type system with a helium white dwarf donor.

  • 15. Laskar, Tanmoy
    et al.
    Alexander, Kate D.
    Berger, Edo
    Guidorzi, Cristiano
    Margutti, Raffaella
    Fong, Wen-fai
    Kilpatrick, Charles D.
    Milne, Peter
    Drout, Maria R.
    Mundell, C. G.
    Kobayashi, Shiho
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). California Institute of Technology, USA.
    Duran, Rodolfo Barniol
    Menten, Karl M.
    Ioka, Kunihito
    Williams, Peter K. G.
    First ALMA Light Curve Constrains Refreshed Reverse Shocks and Jet Magnetization in GRB 161219B2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 862, no 2, article id 94Article in journal (Refereed)
    Abstract [en]

    We present detailed multiwavelength observations of GRB 161219B at z = 0.1475, spanning the radio to X-ray regimes, and the first Atacama Large Millimeter/submillimeter Array (ALMA) light curve of a gamma-ray burst (GRB) afterglow. The centimeter- and millimeter-band observations before 8.5 days require emission in excess of that produced by the afterglow forward shock (FS). These data are consistent with radiation from a refreshed reverse shock (RS) produced by the injection of energy into the FS, signatures of which are also present in the X-ray and optical light curves. We infer a constant-density circumburst environment with an extremely low density, n(0) approximate to 3 x 10(-4 )cm(-3), and show that this is a characteristic of all strong RS detections to date. The Karl G. Lansky Very Large Array (VLA) observations exhibit unexpected rapid variability on roughly minute timescales, indicative of strong interstellar scintillation. The X-ray, ALMA, and VLA observations together constrain the jet break time, t(jet) approximate to 32 days, yielding a wide jet opening angle of theta(jet) approximate to 13 degrees, implying beaming-corrected gamma-ray and kinetic energies of E-gamma approximate to 4.9 x 10(48) erg and E-K approximate to 1.3 x 10(50) erg, respectively. Comparing the RS and FS emission, we show that the ejecta are only weakly magnetized, with relative magnetization, R-B approximate to 1, compared to the FS. These direct, multifrequency measurements of a refreshed RS spanning the optical to radio bands highlight the impact of radio and millimeter data in probing the production and nature of GRB jets.

  • 16.
    Lunnan, Ragnhild
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Harvard-Smithsonian Center for Astrophysics, USA.
    Chornock, R.
    Berger, E.
    Jones, D. O.
    Rest, A.
    Czekala, I.
    Dittmann, J.
    Drout, M. R.
    Foley, R. J.
    Fong, W.
    Kirshner, R. P.
    Laskar, T.
    Leibler, C. N.
    Margutti, R.
    Milisavljevic, D.
    Narayan, G.
    Pan, Y. -C.
    Riess, A. G.
    Roth, K. C.
    Sanders, N. E.
    Scolnic, D.
    Smartt, S. J.
    Smith, K. W.
    Chambers, K. C.
    Draper, P. W.
    Flewelling, H.
    Huber, M. E.
    Kaiser, N.
    Kudritzki, R. P.
    Magnier, E. A.
    Metcalfe, N.
    Wainscoat, R. J.
    Waters, C.
    Willman, M.
    Hydrogen-poor Superluminous Supernovae from the Pan-STARRS1 Medium Deep Survey2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 852, no 2, article id 81Article in journal (Refereed)
    Abstract [en]

    We present light curves and classification spectra of 17 hydrogen-poor superluminous supernovae (SLSNe) from the Pan-STARRS1 Medium Deep Survey (PS1 MDS). Our sample contains all objects from the PS1. MDS sample with spectroscopic classification that are similar to either of the prototypes SN 2005ap or SN 2007bi, without an explicit limit on luminosity. With a redshift range 0.3 < z < 1.6, PS1. MDS is the first SLSN sample primarily probing the high-redshift population; our multifilter PS1 light curves probe the rest-frame UV emission, and hence the peak of the spectral energy distribution. We measure the temperature evolution and construct bolometric light curves, and find peak luminosities of (0.5-5) x 10(44) erg s(-1) and lower limits on the total radiated energies of (0.3-2) x 10(51) erg. The light curve shapes are diverse, with both rise and decline times spanning a factor of similar to 5 and several examples of double-peaked light curves. When correcting for the flux-limited nature of our survey, we find a median peak luminosity at 4000 angstrom of M-4000 = -21.1 mag and a spread of sigma = 0.7 mag.

  • 17.
    Lunnan, Ragnhild
    et al.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). California Institute of Technology, USA.
    Fransson, Claes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Vreeswijk, P. M.
    Woosley, S. E.
    Leloudas, G.
    Perley, D. A.
    Quimby, R. M.
    Yan, Lin
    Blagorodnova, N.
    Bue, B. D.
    Cenko, S. B.
    Cia, A. De
    Cook, D. O.
    Fremling, C. U.
    Gatkine, P.
    Gal-Yam, A.
    Kasliwal, M. M.
    Kulkarni, S. R.
    Masci, F. J.
    Nugent, P. E.
    Nyholm, Anders
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Rubin, A.
    Suzuki, N.
    Wozniak, P.
    A UV resonance line echo from a shell around a hydrogen-poor superluminous supernova2018In: Nature astronomy, ISSN 2397-3366, Vol. 2, no 11, p. 887-895Article in journal (Refereed)
    Abstract [en]

    Hydrogen-poor superluminous supernovae (SLSN-I) are a class of rare and energetic explosions that have been discovered in untargeted transient surveys in the past decade(1,2). The progenitor stars and the physical mechanism behind their large radiated energies (about 1O(51) erg or 1O(44) J) are both debated, with one class of models primarily requiring a large rotational energy(3,4) and the other requiring very massive progenitors that either convert kinetic energy into radiation through interaction with circumstellar material (CSM)(5-8 )or engender an explosion caused by pair-instability (loss of photon pressure due to particle-antiparticle production)(9,10). Observing the structure of the CSM around SLSN-I offers a powerful test of some scenarios, although direct observations are scarce(11,)(12). Here, we present a series of spectroscopic observations of the SLSN-I iPTF16eh, which reveal both absorption and time- and frequency-variable emission in the Mg n resonance doublet. We show that these observations are naturally explained as a resonance scattering light echo from a circumstellar shell. Modelling the evolution of the emission, we infer a shell radius of 0.1 pc and velocity of 3,300 km s(-1), implying that the shell was ejected three decades before the supernova explosion. These properties match theoretical predictions of shell ejections occurring because of pulsational pair-instability and imply that the progenitor had a helium core mass of about 50-55 M-circle dot, corresponding to an initial mass of about 115 M-circle dot.

  • 18. Mahabal, Ashish
    et al.
    Rebbapragada, Umaa
    Walters, Richard
    Masci, Frank J.
    Blagorodnova, Nadejda
    van Roestel, Jan
    Ye, Quan-Zhi
    Biswas, Rahul
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Burdge, Kevin
    Chang, Chan-Kao
    Duev, Dmitry A.
    Golkhou, V. Zach
    Miller, Adam A.
    Nordin, Jakob
    Ward, Charlotte
    Adams, Scott
    Bellm, Eric C.
    Branton, Doug
    Bue, Brian
    Cannella, Chris
    Connolly, Andrew
    Dekany, Richard
    Feindt, Ulrich
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hung, Tiara
    Fortson, Lucy
    Frederick, Sara
    Fremling, C.
    Gezari, Suvi
    Graham, Matthew
    Groom, Steven
    Kasliwal, Mansi M.
    Kulkarni, Shrinivas
    Kupfer, Thomas
    Lin, Hsing Wen
    Lintott, Chris
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Parejko, John
    Prince, Thomas A.
    Riddle, Reed
    Rusholme, Ben
    Saunders, Nicholas
    Sedaghat, Nima
    Shupe, David L.
    Singer, Leo P.
    Soumagnac, Maayane T.
    Szkody, Paula
    Tachibana, Yutaro
    Tirumala, Kushal
    van Velzen, Sjoert
    Wright, Darryl
    Machine Learning for the Zwicky Transient Facility2019In: Publications of the Astronomical Society of the Pacific, ISSN 0004-6280, E-ISSN 1538-3873, Vol. 131, no 997, article id 038002Article in journal (Refereed)
    Abstract [en]

    The Zwicky Transient Facility is a large optical survey in multiple filters producing hundreds of thousands of transient alerts per night. We describe here various machine learning (ML) implementations and plans to make the maximal use of the large data set by taking advantage of the temporal nature of the data, and further combining it with other data sets. We start with the initial steps of separating bogus candidates from real ones, separating stars and galaxies, and go on to the classification of real objects into various classes. Besides the usual methods (e.g., based on features extracted from light curves) we also describe early plans for alternate methods including the use of domain adaptation, and deep learning. In a similar fashion we describe efforts to detect fast moving asteroids. We also describe the use of the Zooniverse platform for helping with classifications through the creation of training samples, and active learning. Finally we mention the synergistic aspects of ZTF and LSST from the ML perspective.

  • 19. Quimby, Robert M.
    et al.
    De Cia, Annalisa
    Gal-Yam, Avishay
    Leloudas, Giorgos
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Stockholm University, Faculty of Science, Department of Astronomy.
    Perley, Daniel A.
    Vreeswijk, Paul M.
    Yan, Lin
    Bloom, Joshua S.
    Cenko, S. Bradley
    Cooke, Jeff
    Ellis, Richard
    Filippenko, Alexei V.
    Kasliwal, Mansi M.
    Kleiser, Io K. W.
    Kulkarni, Shrinivas R.
    Matheson, Thomas
    Nugent, Peter E.
    Pan, Yen-Chen
    Silverman, Jeffrey M.
    Sternberg, Assaf
    Sullivan, Mark
    Yaron, Ofer
    Spectra of Hydrogen-poor Superluminous Supernovae from the Palomar Transient Factory2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 855, no 1, p. 1-57, article id 2Article in journal (Refereed)
    Abstract [en]

    Most Type I superluminous supernovae (SLSNe-I) reported to date have been identified by their high peak luminosities and spectra lacking obvious signs of hydrogen. We demonstrate that these events can be distinguished from normal-luminosity SNe (including Type Ic events) solely from their spectra over a wide range of light-curve phases. We use this distinction to select 19 SLSNe-I and four possible SLSNe-I from the Palomar Transient Factory archive (including seven previously published objects). We present 127 new spectra of these objects and combine these with 39 previously published spectra, and we use these to discuss the average spectral properties of SLSNe-I at different spectral phases. We find that Mn II most probably contributes to the ultraviolet spectral features after maximum light, and we give a detailed study of the O II features that often characterize the early-time optical spectra of SLSNe-I. We discuss the velocity distribution of O II, finding that for some SLSNe-I this can be confined to a narrow range compared to relatively large systematic velocity shifts. Mg II and Fe II favor higher velocities than O II and C II, and we briefly discuss how this may constrain power-source models. We tentatively group objects by how well they match either SN 2011ke or PTF12dam and discuss the possibility that physically distinct events may have been previously grouped together under the SLSN-I label.

  • 20. Scolnic, D. M.
    et al.
    Jones, D. O.
    Rest, A.
    Pan, Y. C.
    Chornock, R.
    Foley, R. J.
    Huber, M. E.
    Kessler, R.
    Narayan, G.
    Riess, A. G.
    Rodney, S.
    Berger, E.
    Brout, D. J.
    Challis, P. J.
    Drout, M.
    Finkbeiner, D.
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy.
    Kirshner, R. P.
    Sanders, N. E.
    Schlafly, E.
    Smartt, S.
    Stubbs, C. W.
    Tonry, J.
    Wood-Vasey, W. M.
    Foley, M.
    Hand, J.
    Johnson, E.
    Burgett, W. S.
    Chambers, K. C.
    Draper, P. W.
    Hodapp, K. W.
    Kaiser, N.
    Kudritzki, R. P.
    Magnier, E. A.
    Metcalfe, N.
    Bresolin, F.
    Gall, E.
    Kotak, R.
    McCrum, M.
    Smith, K. W.
    The Complete Light-curve Sample of Spectroscopically Confirmed SNe Ia from Pan-STARRS1 and Cosmological Constraints from the Combined Pantheon Sample2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 859, no 2, article id 101Article in journal (Refereed)
    Abstract [en]

    We present optical light curves, redshifts, and classifications for 365 spectroscopically confirmed Type Ia supernovae (SNe Ia) discovered by the Pan-STARRS1 (PS1) Medium Deep Survey. We detail improvements to the PS1 SN photometry, astrometry, and calibration that reduce the systematic uncertainties in the PS1 SN Ia distances. We combine the subset of 279 PS1 SNe Ia (0.03 < z < 0.68) with useful distance estimates of SNe Ia from the Sloan Digital Sky Survey (SDSS), SNLS, and various low-z and Hubble Space Telescope samples to form the largest combined sample of SNe Ia, consisting of a total of 1048 SNe Ia in the range of 0.01 < z < 2.3, which we call the Pantheon Sample. When combining Planck 2015 cosmic microwave background (CMB) measurements with the Pantheon SN sample, we find Omega(m) = 0.307 +/- 0.012 and w = -1.026 +/- 0.041 for the wCDM model. When the SN and CMB constraints are combined with constraints from BAO and local H-0 measurements, the analysis yields the most precise measurement of dark energy to date: w(0) = -1.007 +/- 0.089 and w(a) = -0.222 +/- 0.407 for the w(0)w(a) CDM model. Tension with a cosmological constant previously seen in an analysis of PS1 and low-z SNe has diminished after an increase of 2x in the statistics of the PS1 sample, improved calibration and photometry, and stricter light-curve quality cuts. We find that the systematic uncertainties in our measurements of dark energy are almost as large as the statistical uncertainties, primarily due to limitations of modeling the low-redshift sample. This must be addressed for future progress in using SNe Ia to measure dark energy.

  • 21.
    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).
    Taddia, Francesco
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Arcavi, I.
    Fremling, Christoffer
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). California Institute of Technology, USA.
    Fransson, Claes
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Burke, J.
    Cenko, S. B.
    Andersen, O.
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Andreoni, I.
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Blagorodova, N.
    Brink, T. G.
    Filippenko, A. V.
    Gal-Yam, A.
    Hiramatsu, D.
    Hosseinzadeh, G.
    Howell, D. A.
    de Jaeger, T.
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    McCully, C.
    Perley, D. A.
    Tartaglia, Leonardo
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Terreran, G.
    Valenti, S.
    Wang, X.
    Late-time observations of the extraordinary Type II supernova iPTF14hls2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, article id A30Article in journal (Refereed)
    Abstract [en]

    Aims. We study iPTF14hls, a luminous and extraordinary long-lived Type II supernova, which lately has attracted much attention and disparate interpretation.

    Methods. We have presented new optical photometry that extends the light curves up to more than three years past discovery. We also obtained optical spectroscopy over this period, and furthermore present additional space-based observations using Swift and HST.

    Results. After an almost constant luminosity for hundreds of days, the later light curve of iPTF14hls finally fades and then displays a dramatic drop after about 1000 d, but the supernova is still visible at the latest epochs presented. The spectra have finally turned nebular, and our very last optical spectrum likely displays signatures from the deep and dense interior of the explosion. A high-resolution HST image highlights the complex environment of the explosion in this low-luminosity galaxy.

    Conclusions. We provide a large number of additional late-time observations of iPTF14hls, which are (and will continue to be) used to assess the many different interpretations for this intriguing object. In particular, the very late (+1000 d) steep decline of the optical light curve is difficult to reconcile with the proposed central engine models. The lack of very strong X-ray emission, and the emergence of intermediate-width emission lines including [S II] that we propose originate from dense, processed material in the core of the supernova ejecta, are also key observational tests for both existing and future models.

  • 22.
    Taddia, Francesco
    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).
    Fremling, C.
    Barbarino, Cristina
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Karamehmetoglu, Emir
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Arcavi, I.
    Cenko, S. B.
    Filippenko, A.
    Gal-Yam, A.
    Hiramatsu, D.
    Hosseinzadeh, G.
    Howell, D. A.
    Kulkarni, S. R.
    Laher, 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). California Institute of Technology, USA.
    Masci, F.
    Nugent, P. E.
    Nyholm, Anders
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Perley, D. A.
    Quimby, R.
    Silverman, J. M.
    Analysis of broad-lined Type Ic supernovae from the (intermediate) Palomar Transient Factory2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, article id A71Article in journal (Refereed)
    Abstract [en]

    We study 34 Type Ic supernovae that have broad spectral features (SNe Ic-BL). This is the only SN type found in association with long-duration gamma-ray bursts (GRBs). We obtained our photometric data with the Palomar Transient Factory (PTF) and its continuation, the intermediate PTF (iPTF). This is the first large, homogeneous sample of SNe Ic-BL from an untargeted survey. Furthermore, given the high observational cadence of iPTF, most of these SNe Ic-BL were discovered soon after explosion. We present K-corrected Bgriz light curves of these SNe, obtained through photometry on template-subtracted images. We analyzed the shape of the r-band light curves, finding a correlation between the decline parameter Delta m(15) and the rise parameter Delta m-(10 ). We studied the SN colors and, based on g - r, we estimated the host-galaxy extinction for each event. Peak r-band absolute magnitudes have an average of -18.6 +/- 0.5 mag. We fit each r-band light curve with that of SN 1998bw (scaled and stretched) to derive the explosion epochs. We computed the bolometric light curves using bolometric corrections, r-band data, and g - r colors. Expansion velocities from Fen were obtained by fitting spectral templates of SNe Ic. Bolometric light curves and velocities at peak were fitted using the semianalytic Arnett model to estimate ejecta mass M-ej , explosion energy E-K and Ni-56 mass M( Ni-56) for each SN. We find average values of M-ej = 4 +/- 3 M-circle dot, E-K = (7 +/- 6) x 10(51) erg, and M( Ni-56) = 0.31 +/- 0.16 M-circle dot . The parameter distributions were compared to those presented in the literature and are overall in agreement with them. We also estimated the degree of Ni-56 mixing using scaling relations derived from hydrodynamical models and we find that all the SNe are strongly mixed. The derived explosion parameters imply that at least 21% of the progenitors of SNe Ic-BL are compatible with massive (>28 M-circle dot), possibly single stars, whereas at least 64% might come from less massive stars in close binary systems.

  • 23.
    Taddia, Francesco
    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).
    Fremling, C.
    Karamehmetoglu, Emir
    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).
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    West, Stuart
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Gal-Yam, A.
    The luminous late-time emission of the type-Ic supernova iPTF15dtg-evidence for powering from a magnetar?2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 621, article id A64Article in journal (Refereed)
    Abstract [en]

    Context. The transient iPTF15dtg is a type-Ic supernova (SN) showing a broad light curve around maximum light, consistent with massive ejecta if we assume a radioactive-powering scenario.

    Aims. We aim to study the late-time light curve of iPTF15dtg, which turned out to be extraordinarily luminous for a stripped-envelope (SE) SN, and investigate possible powering mechanisms.

    Methods. We compare the observed light curves to those of other SE SNe and also to models for the (56) Co decay. We analyze and compare the spectra to nebular spectra of other SE SNe. We build a bolometric light curve and fit it with different models, including powering by radioactivity, magnetar powering, and a combination of the two.

    Results. Between 150 and 750 d post-explosion, the luminosity of iPTF15dtg declined by merely two magnitudes instead of the six magnitudes expected from Co-56 decay. This is the first spectroscopically regular SE SN found to show this behavior. The model with both radioactivity and magnetar powering provides the best fit to the light curve and appears to be the most realistic powering mechanism. An alternative mechanism might be circumstellar-medium (CSM) interaction. However, the spectra of iPTF15dtg are very similar to those of other SE SNe, and do not show signs of strong CSM interaction.

    Conclusions. The object iPTF15dtg is the first spectroscopically regular SE SN whose light curve displays such clear signs of a magnetar contributing to its late-time powering. Given this result, the mass of the ejecta needs to be revised to a lower value, and therefore the progenitor mass could be significantly lower than the previously estimated >35 M-circle dot.

  • 24. Villar, V. A.
    et al.
    Berger, E.
    Miller, G.
    Chornock, R.
    Rest, A.
    Jones, D. O.
    Drout, M. R.
    Foley, R. J.
    Kirshner, 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).
    Magnier, E.
    Milisavljevic, D.
    Sanders, N.
    Scolnic, D.
    Supernova Photometric Classification Pipelines Trained on Spectroscopically Classified Supernovae from the Pan-STARRS1 Medium-deep Survey2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 884, no 1, article id 83Article in journal (Refereed)
    Abstract [en]

    Photometric classification of supernovae (SNe) is imperative as recent and upcoming optical time-domain surveys, such as the Large Synoptic Survey Telescope (LSST), overwhelm the available resources for spectrosopic follow-up. Here we develop a range of light curve (LC) classification pipelines, trained on 513 spectroscopically classified SNe from the Pan-STARRS1 Medium-Deep Survey (PS1-MDS): 357 Type Ia, 93 Type II, 25 Type IIn, 21 Type Ibc, and 17 Type I superluminous SNe (SLSNe). We present a new parametric analytical model that can accommodate a broad range of SN LC morphologies, including those with a plateau, and fit this model to data in four PS1 filters (g(P1)r(P1)i(P1)z(P1)). We test a number of feature extraction methods, data augmentation strategies, and machine-learning algorithms to predict the class of each SN. Our best pipelines result in approximate to 90% average accuracy, approximate to 70% average purity, and approximate to 80% average completeness for all SN classes, with the highest success rates for SNe Ia and SLSNe and the lowest for SNe Ibc. Despite the greater complexity of our classification scheme, the purity of our SN Ia classification, approximate to 95%, is on par with methods developed specifically for Type Ia versus non-Type Ia binary classification. As the first of its kind, this study serves as a guide to developing and training classification algorithms for a wide range of SN types with a purely empirical training set, particularly one that is similar in its characteristics to the expected LSST main survey strategy. Future work will implement this classification pipeline on approximate to 3000 PS1/MDS LCs that lack spectroscopic classification.

  • 25. Whitesides, L.
    et al.
    Lunnan, Ragnhild
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). California Institute of Technology, USA.
    Kasliwal, M. M.
    Perley, D. A.
    Corsi, A.
    Cenko, S. B.
    Blagorodnova, N.
    Cao, Y.
    Cook, D. O.
    Doran, G. B.
    Frederiks, D. D.
    Fremling, Christoffer
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hurley, K.
    Karamehmetoglu, Emir
    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.
    Leloudas, G.
    Masci, F.
    Nugent, P. E.
    Ritter, A.
    Rubin, A.
    Savchenko, V.
    Sollerman, Jesper
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Svinkin, D. S.
    Taddia, Francesco
    Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Vreeswijk, P.
    Wozniak, P.
    iPTF 16asu: A Luminous, Rapidly Evolving, and High-velocity Supernova2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 851, no 2, article id 107Article in journal (Refereed)
    Abstract [en]

    Wide-field surveys are discovering a growing number of rare transients whose physical origin is not yet well understood. Here we present optical and UV data and analysis of intermediate Palomar Transient Factory (iPTF) 16asu, a luminous, rapidly evolving, high-velocity, stripped-envelope supernova ( SN). With a rest-frame rise time of just four. days and a peak absolute magnitude of M-g = -20.4 mag, the light curve of iPTF 16asu is faster and more luminous than that of previous rapid transients. The spectra of iPTF 16asu show a featureless blue continuum near peak that develops into an SN Ic-BL spectrum on the decline. We show that while the late-time light curve could plausibly be powered by Ni-56 decay, the early emission requires a different energy source. Nondetections in the X-ray and radio strongly constrain the energy coupled to relativistic ejecta to be at most comparable to the class of low-luminosity gamma-ray bursts (GRBs). We suggest that the early emission may have been powered by either a rapidly spinning-down magnetar or by shock breakout in an extended envelope of a very energetic explosion. In either scenario a central engine is required, making iPTF 16asu an intriguing transition object between superluminous SNe, SNe Ic-BL, and low-luminosity GRBs.

  • 26. Yan, Lin
    et al.
    Perley, D. A.
    De Cia, A.
    Quimby, 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).
    Rubin, Kate H. R.
    Brown, P. J.
    Far-UV HST Spectroscopy of an Unusual Hydrogen-poor Superluminous Supernova: SN2017egm2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 858, no 2, article id 91Article in journal (Refereed)
    Abstract [en]

    SN2017egm is the closest (z = 0.03) H-poor superluminous supernova (SLSN-I) detected to date, and a rare example of an SLSN-I in a massive, metal-rich galaxy. We present the HST UV and optical spectra covering 1000-5500 angstrom, taken at + 3 day relative to the peak. Our data reveal two absorption systems at redshifts matching the host galaxy NGC 3191 (z = 0.0307) and its companion galaxy (z = 0.0299) 73 '' apart. Weakly damped Ly alpha absorption lines are detected at these two redshifts, with H I column densities of (3.0 +/- 0.8) x 10(19) and (3.7 +/- 0.9) x 10(19) cm(-2), respectively. This is an order of magnitude smaller than the H I column densities in the disks of nearby galaxies (> 10(10) M-circle dot) and suggests that SN2017egm is on the near side of NGC 3191 and has a low host extinction (E(B - V) similar to 0.007). Using unsaturated metal absorption lines, we find that the host of SN2017egm probably has a solar or higher metallicity and is unlikely to be a dwarf companion to NGC 3191. Comparison of early-time UV spectra of SN2017egm, Gaia16apd, iPTF13ajg, and PTF12dam finds that the continuum at lambda > 2800 angstrom is well fit by a blackbody, whereas the continuum at lambda < 2800 angstrom is considerably below the model. The degree of UV suppression varies from source to source, with the 1400-2800 angstrom continuum flux ratio of 1.5 for Gaia16apd and 0.4 for iPTF13ajg. This cannot be explained by the differences in magnetar power or blackbody temperature. Finally, the UV spectra reveal a common set of seven broad absorption features and their equivalent widths are similar (within a factor of 2) among the four events.

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