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  • 1. Ackermann, M.
    et al.
    Ajello, M.
    Baldini, L.
    Ballet, J.
    Barbiellini, G.
    Bastieri, D.
    Bellazzini, R.
    Bissaldi, E.
    Blandford, R. D.
    Bloom, E. D.
    Bonino, R.
    Bottacini, E.
    Brandt, T. J.
    Bregeon, J.
    Bruel, P.
    Buehler, R.
    Cameron, R. A.
    Caputo, R.
    Caraveo, P. A.
    Castro, D.
    Cavazzuti, E.
    Charles, E.
    Cheung, C. C.
    Chiaro, G.
    Ciprini, S.
    Cohen-Tanugi, J.
    Costantin, D.
    Cutini, S.
    D'Ammando, F.
    de Palma, F.
    Desai, A.
    Di Lalla, N.
    Di Mauro, M.
    Di Venere, L.
    Favuzzi, C.
    Finke, J.
    Franckowiak, A.
    Fukazawa, Y.
    Funk, S.
    Fusco, P.
    Gargano, F.
    Gasparrini, D.
    Giglietto, N.
    Giordano, F.
    Giroletti, M.
    Green, D.
    Grenier, I. A.
    Guillemot, L.
    Guiriec, S.
    Hays, E.
    Hewitt, J. W.
    Horan, D.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Kensei, S.
    Kuss, M.
    Larsson, S.
    Latronico, L.
    Lemoine-Goumard, M.
    Li, J.
    Longo, F.
    Loparco, F.
    Lovellette, M. N.
    Lubrano, P.
    Magill, J. D.
    Maldera, S.
    Manfreda, A.
    Mazziotta, M. N.
    McEnery, J. E.
    Meyer, M.
    Mizuno, T.
    Monzani, M. E.
    Morselli, A.
    Moskalenko, I. V.
    Negro, M.
    Nuss, E.
    Omodei, N.
    Orienti, M.
    Orlando, E.
    Ormes, J. F.
    Palatiello, M.
    Paliya, V. S.
    Paneque, D.
    Perkins, J. S.
    Persic, M.
    Pesce-Rollins, M.
    Piron, F.
    Porter, T. A.
    Principe, G.
    Raino, S.
    Rando, R.
    Rani, B.
    Razzaque, S.
    Reimer, A.
    Reimer, O.
    Reposeur, T.
    Sgro, C.
    Siskind, E. J.
    Spandre, G.
    Spinelli, P.
    Suson, D. J.
    Tajima, H.
    Thayer, J. B.
    Tibaldo, L.
    Torres, D. F.
    Tosti, G.
    Valverde, J.
    Venters, T. M.
    Vogel, M.
    Wood, K.
    Wood, M.
    Zaharijas, G.
    Biteau, J.
    The Search for Spatial Extension in High-latitude Sources Detected by the Fermi Large Area Telescope2018In: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 237, no 2, article id 32Article in journal (Refereed)
    Abstract [en]

    We present a search for spatial extension in high-latitude (vertical bar b vertical bar > 5 degrees) sources in recent Fermi point source catalogs. The result is the Fermi High-Latitude Extended Sources Catalog, which provides source extensions (or upper limits thereof) and likelihood profiles for a suite of tested source morphologies. We find 24. extended sources, 19 of which were not previously characterized as extended. These include sources that are potentially associated with supernova remnants and star-forming regions. We also found extended.-ray emission in the vicinity of the Cen. A radio lobes and-at GeV energies for the first time-spatially coincident with the radio emission of the SNR CTA 1, as well as from the Crab Nebula. We also searched for halos around active galactic nuclei, which are predicted from electromagnetic cascades induced by the e(+)e(-) pairs that are deflected in intergalactic magnetic fields. These pairs are produced when gamma-rays interact with background radiation fields. We do not find evidence for extension in individual sources or in stacked source samples. This enables us to place limits on the flux of the extended source components, which are then used to constrain the intergalactic magnetic field to be stronger than 3 x 10(-16) G for a coherence length lambda greater than or similar to 10 kpc, even when conservative assumptions on the source duty cycle are made. This improves previous limits by several orders of magnitude.

  • 2. Ackermann, M.
    et al.
    Ajello, M.
    Baldini, L.
    Ballet, J.
    Barbiellini, G.
    Bastieri, D.
    Bellazzini, R.
    Bissaldi, E.
    Blandford, R. D.
    Bonino, R.
    Bottacini, E.
    Bregeon, J.
    Bruel, P.
    Buehler, R.
    Burns, E.
    Buson, S.
    Cameron, R. A.
    Caputo, R.
    Caraveo, P. A.
    Cavazzuti, E.
    Chen, S.
    Chiaro, G.
    Ciprini, S.
    Costantin, D.
    Cuoco, A.
    Cutini, S.
    D'Ammando, F.
    Luque, P. de la Torre
    de Palma, F.
    Desai, A.
    Digel, S. W.
    Di Lalla, N.
    Di Mauro, M.
    Di Venere, L.
    Dirirsa, F. Fana
    Favuzzi, C.
    Franckowiak, A.
    Fukazawa, Y.
    Funk, S.
    Fusco, P.
    Gargano, F.
    Gasparrini, D.
    Giglietto, N.
    Giordano, F.
    Giroletti, M.
    Green, D.
    Grenier, I. A.
    Guillemot, L.
    Guiriec, S.
    Horan, D.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Kuss, M.
    Larsson, S.
    Latronico, L.
    Li, J.
    Liodakis, I.
    Longo, F.
    Loparco, F.
    Lubrano, P.
    Magill, J. D.
    Maldera, S.
    Malyshev, D.
    Manfreda, A.
    Mazziotta, M. N.
    Mereu, I.
    Michelson, P. F.
    Mitthumsiri, W.
    Mizuno, T.
    Monzani, M. E.
    Morselli, A.
    Moskalenko, I. V.
    Negro, M.
    Nuss, E.
    Orienti, M.
    Orlando, E.
    Palatiello, M.
    Paliya, V. S.
    Paneque, D.
    Persic, M.
    Pesce-Rollins, M.
    Petrosian, V.
    Piron, F.
    Porter, T. A.
    Principe, G.
    Raino, S.
    Rando, R.
    Razzano, M.
    Razzaque, S.
    Reimer, A.
    Reimer, O.
    Serini, D.
    Sgro, C.
    Siskind, E. J.
    Spandre, G.
    Spinelli, P.
    Suson, D. J.
    Tajima, H.
    Takahashi, M.
    Thayer, J. B.
    Tibaldo, L.
    Torres, D. F.
    Troja, E.
    Venters, T. M.
    Vianello, G.
    Wood, K.
    Yassine, M.
    Zaharijas, G.
    Ammazzalorso, S.
    Fornengo, N.
    Regis, M.
    Unresolved Gamma-Ray Sky through its Angular Power Spectrum2018In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 121, no 24, article id 241101Article in journal (Refereed)
    Abstract [en]

    The gamma-ray sky has been observed with unprecedented accuracy in the last decade by the Fermi-large area telescope (LAT), allowing us to resolve and understand the high-energy Universe. The nature of the remaining unresolved emission [unresolved gamma-ray background (UGRB)] below the LAT source detection threshold can be uncovered by characterizing the amplitude and angular scale of the UGRB fluctuation field. This Letter presents a measurement of the UGRB autocorrelation angular power spectrum based on eight years of Fermi-LAT Pass 8 data products. The analysis is designed to be robust against contamination from resolved sources and noise systematics. The sensitivity to subthreshold sources is greatly enhanced with respect to previous measurements. We find evidence (with similar to 3.7 sigma significance) that the scenario in which two classes of sources contribute to the UGRB signal is favored over a single class. A double power law with exponential cutoff can explain the anisotropy energy spectrum well, with photon indices of the two populations being 2.55 +/- 0.23 and 1.86 +/- 0.15.

  • 3.
    Ahrens, Maryon
    et al.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Bohm, Christian
    Stockholm University, Faculty of Science, Department of Physics.
    Conrad, Jan
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Dumm, Jonathan P.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Finley, Chad
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Flis, Samuel
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Hultqvist, Klas
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    O'Sullivan, Erin
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Walck, Christian
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Wagner, Robert M.
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).
    Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A2018In: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 361, no 6398, article id 1378Article in journal (Refereed)
  • 4. Ajello, M.
    et al.
    Arimoto, M.
    Axelsson, Magnus
    Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). KTH Royal Institute of Technology, Sweden.
    Baldini, L.
    Barbiellini, G.
    Bastieri, D.
    Bellazzini, R.
    Bhat, P. N.
    Bissaldi, E.
    Blandford, R. D.
    Bonino, R.
    Bonnell, J.
    Bottacini, E.
    Bregeon, J.
    Bruel, P.
    Buehler, R.
    Cameron, R. A.
    Caputo, R.
    Caraveo, P. A.
    Cavazzuti, E.
    Chen, S.
    Cheung, C. C.
    Chiaro, G.
    Ciprini, S.
    Costantin, D.
    Crnogorcevic, M.
    Cutini, S.
    Dainotti, M.
    D'Ammando, F.
    Luque, P. de la Torre
    de Palma, F.
    Desai, A.
    Desiante, R.
    Di Lalla, N.
    Di Venere, L.
    Dirirsa, F. Fana
    Fegan, S. J.
    Franckowiak, A.
    Fukazawa, Y.
    Funk, S.
    Fusco, P.
    Gargano, F.
    Gasparrini, D.
    Giglietto, N.
    Giordano, F.
    Giroletti, M.
    Green, D.
    Grenier, I. A.
    Grove, J. E.
    Guiriec, S.
    Hays, E.
    Hewitt, J. W.
    Horan, D.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Kocevski, D.
    Kuss, M.
    Latronico, L.
    Li, J.
    Longo, F.
    Loparco, F.
    Lovellette, M. N.
    Lubrano, P.
    Maldera, S.
    Manfreda, A.
    Marti-Devesa, G.
    Mazziotta, M. N.
    Mereu, I
    Meyer, M.
    Michelson, P. F.
    Mirabal, N.
    Mitthumsiri, W.
    Mizuno, T.
    Monzani, M. E.
    Moretti, E.
    Morselli, A.
    Moskalenko, I.
    Negro, M.
    Nuss, E.
    Ohno, M.
    Omodei, N.
    Orienti, M.
    Orlando, E.
    Palatiello, M.
    Paliya, V. S.
    Paneque, D.
    Persic, M.
    Pesce-Rollins, M.
    Petrosian, V
    Piron, F.
    Poolakkil, S.
    Poon, H.
    Porter, T. A.
    Principe, G.
    Racusin, J. L.
    Raino, S.
    Rando, R.
    Razzano, M.
    Razzaque, S.
    Reimer, A.
    Reimer, O.
    Reposeur, T.
    Ryde, F.
    Serini, D.
    Sgro, C.
    Siskind, E. J.
    Sonbas, E.
    Spandre, G.
    Spinelli, P.
    Suson, D. J.
    Tajima, H.
    Takahashi, M.
    Tak, D.
    Thayer, J. B.
    Torres, D. F.
    Troja, E.
    Valverde, J.
    Veres, P.
    Vianello, G.
    von Kienlin, A.
    Wood, K.
    Yassine, M.
    Zhu, S.
    Zimmer, S.
    A Decade of Gamma-Ray Bursts Observed by Fermi-LAT: The Second GRB Catalog2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 878, no 1, article id 52Article in journal (Refereed)
    Abstract [en]

    The Large Area Telescope (LAT) aboard the Fermi spacecraft routinely observes high-energy emission from gamma-ray bursts (GRBs). Here we present the second catalog of LAT-detected GRBs, covering the first 10 yr of operations, from 2008 to 2018 August 4. A total of 186 GRBs are found; of these, 91 show emission in the range 30-100 MeV (17 of which are seen only in this band) and 169 are detected above 100 MeV. Most of these sources were discovered by other instruments (Fermi/GBM, Swift/BAT, AGILE, INTEGRAL) or reported by the Interplanetary Network (IPN); the LAT has independently triggered on four GRBs. This catalog presents the results for all 186 GRBs. We study onset, duration, and temporal properties of each GRB, as well as spectral characteristics in the 100 MeV-100 GeV energy range. Particular attention is given to the photons with the highest energy. Compared with the first LAT GRB catalog, our rate of detection is significantly improved. The results generally confirm the main findings of the first catalog: the LAT primarily detects the brightest GBM bursts, and the high-energy emission shows delayed onset as well as longer duration. However, in this work we find delays exceeding 1 ks and several GRBs with durations over 10 ks. Furthermore, the larger number of LAT detections shows that these GRBs not only cover the high-fluence range of GBM-detected GRBs but also sample lower fluences. In addition, the greater number of detected GRBs with redshift estimates allows us to study their properties in both the observer and rest frames. Comparison of the observational results with theoretical predictions reveals that no model is currently able to explain all results, highlighting the role of LAT observations in driving theoretical models.

  • 5. Boschini, M. J.
    et al.
    Della Torre, S.
    Gervasi, M.
    Grandi, D.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Kachelriess, M.
    La Vacca, G.
    Masi, N.
    Moskalenko, I. V.
    Orlando, E.
    Ostapchenko, S. S.
    Pensotti, S.
    Porter, T. A.
    Quadrani, L.
    Rancoita, P. G.
    Rozza, D.
    Tacconi, M.
    Solution of Heliospheric Propagation: Unveiling the Local Interstellar Spectra of Cosmic-ray Species2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 840, no 2, article id 115Article in journal (Refereed)
    Abstract [en]

    Local interstellar spectra (LIS) for protons, helium, and antiprotons are built using the most recent experimental results combined with state-of-the-art models for propagation in the Galaxy and heliosphere. Two propagation packages, GALPROP and HelMod, are combined to provide a single framework that is run to reproduce direct measurements of cosmic-ray (CR) species at different modulation levels and at both polarities of the solar magnetic field. To do so in a self-consistent way, an iterative procedure was developed, where the GALPROP LIS output is fed into HelMod, providing modulated spectra for specific time periods of selected experiments to compare with the data; the HelMod parameter optimization is performed at this stage and looped back to adjust the LIS using the new GALPROP run. The parameters were tuned with the maximum likelihood procedure using an extensive data set of proton spectra from 1997 to 2015. The proposed LIS accommodate both the low-energy interstellar CR spectra measured by Voyager 1 and the high-energy observations by BESS, Pamela, AMS-01, and AMS-02 made from the balloons and near-Earth payloads; it also accounts for Ulysses counting rate features measured out of the ecliptic plane. The found solution is in a good agreement with proton, helium, and antiproton data by AMS-02, BESS, and PAMELA in the whole energy range.

  • 6. Boschini, M. J.
    et al.
    Della Torre, S.
    Gervasi, M.
    Grandi, D.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    La Vacca, G.
    Masi, N.
    Moskalenko, I. V.
    Pensotti, S.
    Porter, T. A.
    Quadrani, L.
    Rancoita, P. G.
    Rozza, D.
    Tacconi, M.
    Deciphering the Local Interstellar Spectra of Primary Cosmic-Ray Species with HELMOD2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 858, no 1, article id 61Article in journal (Refereed)
    Abstract [en]

    Local interstellar spectra (LIS) of primary cosmic ray (CR) nuclei, such as helium, oxygen, and mostly primary carbon are derived for the rigidity range from 10 MV to similar to 200 TV using the most recent experimental results combined with the state-of-the-art models for CR propagation in the Galaxy and in the heliosphere. Two propagation packages, GALPROP and HELMOD, are combined into a single framework that is used to reproduce direct measurements of CR species at different modulation levels, and at both polarities of the solar magnetic field. The developed iterative maximum-likelihood method uses GALPROP-predicted LIS as input to HELMOD, which provides the modulated spectra for specific time periods of the selected experiments for model-data comparison. The interstellar and heliospheric propagation parameters derived in this study are consistent with our prior analyses using the same methodology for propagation of CR protons, helium, antiprotons, and electrons. The resulting LIS accommodate a variety of measurements made in the local interstellar space (Voyager 1) and deep inside the heliosphere at low (ACE/CRIS, HEAO-3) and high energies (PAMELA, AMS-02).

  • 7. Boschini, M. J.
    et al.
    Della Torre, S.
    Gervasi, M.
    Grandi, D.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    La Vacca, G.
    Masi, N.
    Moskalenko, I. V.
    Pensotti, S.
    Porter, T. A.
    Quadrani, L.
    Rancoita, P. G.
    Rozza, D.
    Tacconi, M.
    HelMod in the Works: From Direct Observations to the Local Interstellar Spectrum of Cosmic-Ray Electrons2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 854, no 2, article id 94Article in journal (Refereed)
    Abstract [en]

    The local interstellar spectrum (LIS) of cosmic-ray (CR) electrons for the energy range 1 MeV to 1 TeV is derived using the most recent experimental results combined with the state-of-the-art models for CR propagation in the Galaxy and in the heliosphere. Two propagation packages, GALPROP and HELMOD, are combined to provide a single framework that is run to reproduce direct measurements of CR species at different modulation levels, and at both polarities of the solar magnetic field. An iterative maximum-likelihood method is developed that uses GALPROP-predicted LIS as input to HELMOD, which provides the modulated spectra for specific time periods of the selected experiments for model-data comparison. The optimized HelMod parameters are then used to adjust GALPROP parameters to predict a refined LIS with the procedure repeated subject to a convergence criterion. The parameter optimization uses an extensive data set of proton spectra from 1997 to 2015. The proposed CR electron LIS accommodates both the low-energy interstellar spectra measured by Voyager 1 as well as the high-energy observations by PAMELA and AMS-02 that are made deep in the heliosphere; it also accounts for Ulysses counting rate features measured out of the ecliptic plane. The interstellar and heliospheric propagation parameters derived in this study agree well with our earlier results for CR protons, helium nuclei, and anti-protons propagation and LIS obtained in the same framework.

  • 8.
    Jóhannesson, Guðlaugur
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Larsson, S.
    MAGIC and Fermi-LAT gamma-ray results on unassociated HAWC sources2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 485, no 1, p. 356-366Article in journal (Refereed)
    Abstract [en]

    The HAWC Collaboration released the 2HWC catalogue of TeV sources, in which 19 show no association with any known high-energy (HE; E greater than or similar to 10 GeV) or very-high-energy (VHE; E greater than or similar to 300 GeV) sources. This catalogue motivated follow-up studies by both the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) and Fermi-LAT (Large Area Telescope) observatories with the aim of investigating gamma-ray emission over a broad energy band. In this paper, we report the results from the first joint work between High Altitude Water Cherenkov (HAWC), MAGIC, and Fermi-LAT on three unassociated HAWC sources: 2HWC J2006+341, 2HWC J1907+084*, and 2HWC J1852+013*. Although no significant detection was found in the HE and VHE regimes, this investigation shows that a minimum 1 degrees extension (at 95 per cent confidence level) and harder spectrum in the GeV than the one extrapolated from HAWC results are required in the case of 2HWC J1852+013*, whilst a simply minimum extension of 0.16 degrees (at 95 per cent confidence level) can already explain the scenario proposed by HAWC for the remaining sources. Moreover, the hypothesis that these sources are pulsar wind nebulae is also investigated in detail.

  • 9.
    Jóhannesson, Guðlaugur
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Larsson, S.
    VERITAS and Fermi-LAT Observations of TeV Gamma-Ray Sources Discovered by HAWC in the 2HWC Catalog2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 866, no 1, article id 24Article in journal (Refereed)
    Abstract [en]

    The High Altitude Water Cherenkov (HAWC) collaboration recently published their 2HWC catalog, listing 39 very high energy (VHE; >100 GeV) gamma-ray sources based on 507 days of observation. Among these, 19 sources are not associated with previously known teraelectronvolt (TeV) gamma-ray sources. We have studied 14 of these sources without known counterparts with VERITAS and Fermi-LAT. VERITAS detected weak gamma-ray emission in the 1 TeV-30 TeV band in the region of DA 495, a pulsar wind nebula coinciding with 2HWC J1953+294, confirming the discovery of the source by HAWC. We did not find any counterpart for the selected 14 new HAWC sources from our analysis of Fermi-LAT data for energies higher than 10 GeV. During the search, we detected gigaelectronvolt (GeV) gamma-ray emission coincident with a known TeV pulsar wind nebula, SNR G54.1+0.3 (VER J1930+188), and a 2HWC source, 2HWC J1930+188. The fluxes for isolated, steady sources in the 2HWC catalog are generally in good agreement with those measured by imaging atmospheric Cherenkov telescopes. However, the VERITAS fluxes for SNR G54.1+0.3, DA 495, and TeV J2032+4130 are lower than those measured by HAWC, and several new HAWC sources are not detected by VERITAS. This is likely due to a change in spectral shape, source extension, or the influence of diffuse emission in the source region.

  • 10.
    Jóhannesson, Guðlaugur
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Porter, Troy A.
    Moskalenko, Igor V.
    The Three-dimensional Spatial Distribution of Interstellar Gas in the Milky Way: Implications for Cosmic Rays and High-energy Gamma-ray Emissions2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 856, no 1, article id 45Article in journal (Refereed)
    Abstract [en]

    Direct measurements of cosmic ray (CR) species combined with observations of their associated gamma-ray emissions can be used to constrain models of CR propagation, trace the structure of the Galaxy, and search for signatures of new physics. The spatial density distribution of interstellar gas is a vital element for all these studies. So far, models have employed the 2D cylindrically symmetric geometry, but their accuracy is well behind that of the available data. In this paper, 3D spatial density models for neutral and molecular hydrogen are constructed based on empirical model fitting to gas line-survey data. The developed density models incorporate spiral arms and account for the warping of the disk, and the increasing gas scale height with radial distance from the Galactic center. They are employed together with the GALPROP CR propagation code to investigate how the new 3D gas models affect calculations of CR propagation and high-energy gamma-ray intensity maps. The calculations reveal non-trivial features that are directly related to the new gas models. The best-fit values for propagation model parameters employing 3D gas models are presented and they differ significantly from those derived with the 2D gas density models that have been widely used. The combination of 3D CR and gas density models provide a more realistic basis for the interpretation of non-thermal emissions from the Galaxy.

  • 11.
    Jóhannesson, Guđlaugur
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Björnsson, Gunnlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
    Low-energy Electrons in Gamma-Ray Burst Afterglow Models2018In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 859, no 1, article id LllArticle in journal (Refereed)
    Abstract [en]

    Observations of gamma-ray burst (GRB) afterglows have long provided the most detailed information about the origin of this spectacular phenomenon. The model that is most commonly used to extract physical properties of the event from the observations is the relativistic fireball model, where ejected material moving at relativistic speeds creates a shock wave when it interacts with the surrounding medium. Electrons are accelerated in the shock wave, generating the observed synchrotron emission through interactions with the magnetic field in the downstream medium. It is usually assumed that the accelerated electrons follow a simple power-law distribution in energy between specific energy boundaries, and that no electron exists outside these boundaries. This Letter explores the consequences of adding a low-energy power-law segment to the electron distribution with energy that contributes insignificantly to the total energy budget of the distribution. The low-energy electrons have a significant impact on the radio emission, providing synchrotron absorption and emission at these long wavelengths. Shorter wavelengths are affected through the normalization of the distribution. The new model is used to analyze the light curves of GRB 990510, and the resulting parameters are compared to a model without the extra electrons. The quality of the fit and the best-fit parameters are significantly affected by the additional model component. The new component is in one case found to strongly affect the X-ray light curves, showing how changes to the model at radio frequencies can affect light curves at other frequencies through changes in best-fit model parameters.

  • 12.
    Jóhannesson, Guđlaugur
    et al.
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Porter, Troy A.
    Moskalenko, Igor
    Cosmic-Ray Propagation in Light of the Recent Observation of Geminga2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 879, no 2, article id 91Article in journal (Refereed)
    Abstract [en]

    The High Altitude Water Cherenkov (HAWC) telescope recently observed extended emission around the Geminga and PSR. B0656+14 pulsar wind nebulae (PWNe). These observations have been used to estimate cosmic-ray (CR) diffusion coefficients near the PWNe that appear to be more than two orders of magnitude smaller than the diffusion coefficients typically derived for the interstellar medium from measured abundances of secondary species in CRs. Two-zone diffusion models have been proposed as a solution to this discrepancy, where the slower diffusion zone (SDZ) is confined to a small region around the PWN. Such models are shown to successfully reproduce the HAWC observations of the Geminga PWN while retaining consistency with other CR data. It is found that the size of the SDZ influences the predicted positron flux and the spectral shape of the extended gamma-ray emission at lower energies that can be observed with the Fermi Large Area Telescope. If the two observed PWNe are not unique, then it is likely that there are similar pockets of slow diffusion around many CR sources elsewhere in the Milky Way. The consequences of such a picture for Galactic CR propagation is explored.

  • 13. Porter, T. A.
    et al.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Moskalenko, I. V.
    High-energy Gamma Rays from the Milky Way: Three-dimensional Spatial Models for the Cosmic-Ray and Radiation Field Densities in the Interstellar Medium2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 846, no 1, article id 67Article in journal (Refereed)
    Abstract [en]

    High-energy gamma-rays of interstellar origin are produced by the interaction of cosmic-ray (CR) particles with the diffuse gas and radiation fields in the Galaxy. The main features of this emission are well understood and are reproduced by existing CR propagation models employing 2D galactocentric cylindrically symmetrical geometry. However, the high-quality data from instruments like the Fermi Large Area Telescope reveal significant deviations from the model predictions on few to tens of degrees scales, indicating the need to include the details of the Galactic spiral structure and thus requiring 3D spatial modeling. In this paper, the high-energy interstellar emissions from the Galaxy are calculated using the new release of the GALPROP code employing 3D spatial models for the CR source and interstellar radiation field (ISRF) densities. Three models for the spatial distribution of CR sources are used that are differentiated by their relative proportion of input luminosity attributed to the smooth disk or spiral arms. Two ISRF models are developed based on stellar and dust spatial density distributions taken from the literature that reproduce local near-to far-infrared observations. The interstellar emission models that include arms and bulges for the CR source and ISRF densities provide plausible physical interpretations for features found in the residual maps from high-energy gamma-ray data analysis. The 3D models for CR and ISRF densities provide a more realistic basis that can be used for the interpretation of the nonthermal interstellar emissions from the Galaxy.

  • 14. Porter, T. A.
    et al.
    Rowell, G. P.
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Moskalenko, I. V.
    Galactic PeVatrons and helping to find them: Effects of galactic absorption on the observed spectra of very high energy gamma-ray sources2018In: Physical Review D: covering particles, fields, gravitation, and cosmology, ISSN 2470-0010, E-ISSN 2470-0029, Vol. 98, no 4, article id 041302Article in journal (Refereed)
    Abstract [en]

    Identification of the cosmic-ray (CR) PeVatrons, which are sources capable of accelerating particles to similar to 10(15) eV energies and higher, may lead to resolving the long-standing question of the origin of the spectral feature in the all-particle CR spectrum known as the knee. Because CRs with these energies are deflected by interstellar magnetic fields identification of individual sources and determination of their spectral characteristics is more likely via very high energy gamma-ray emissions, which provide the necessary directional information. However, pair production on the interstellar radiation field (ISRF) and cosmic microwave background (CMB) leads to steepening of the high energy tails of gamma-ray spectra, and should be corrected for to enable true properties of the spectrum at the source to be recovered. Employing recently developed three-dimensional ISRF models this paper quantifies the pair-absorption effect on spectra for sources in the Galactic center (GC) direction at 8.5 and 23.5 kpc distances, with the latter corresponding to the far side of the Galactic stellar disc where it is expected that discrimination of spectral features >10 TeV is possible by the forthcoming Cherenkov Telescope Array (CTA). The estimates made suggest spectral cutoffs could be underestimated by factors of a few in the energy range so far sampled by TeV gamma-ray telescopes. As an example to illustrate this, the recent HESS measurements of diffuse gamma-ray emissions possibly associated with injection of CRs nearby Sgr A* are ISRF corrected, and estimates of the spectral cutoff are reevaluated. It is found that it could be higher by up to a factor of similar to 2, indicating that these emissions may be consistent with a CR accelerator with a spectral cutoff of at least 1 PeV at the 95% confidence level.

  • 15. Riley, Alexander H.
    et al.
    Strigari, Louis E.
    Porter, Troy A.
    Blandford, Roger D.
    Murgia, Simona
    Kerr, Matthew
    Jóhannesson, Guðlaugur
    Stockholm University, Nordic Institute for Theoretical Physics (Nordita). University of Iceland, Iceland.
    Possible Detection of Gamma-Rays from Epsilon Eridani2019In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 878, no 1, article id 8Article in journal (Refereed)
    Abstract [en]

    We use the Fermi Large Area Telescope gamma-ray observatory to search for gamma-ray emission from four nearby, debris disk-hosting main-sequence stars: tau Ceti, epsilon Eridani, Fomalhaut, and Vega. For three stars (tau Ceti, Fomalhaut, and Vega), we establish upper limits that are consistent with theoretical expectations. For epsilon Eridani, we find a possible spatially coincident source with a soft energy spectrum of dN/dE similar to E-3.6. However, at this stage we are unable to rule out that this emission is due to a more extended feature in the diffuse background. In the interpretation that the emission is due to e Eridani, the >100 MeV gamma-ray luminosity is similar to 10(27) erg s(-1)similar or equal to 3 x 10(-7) L-circle dot, which is similar to 10(10 )times the gamma-ray luminosity from the disk of the quiet Sun. We find less than or similar to 2 sigma evidence of source variability over a similar to 7 yr timescale. In the interpretation that the gamma-ray emission is from e Eridani itself, we consider two possible models: (1) cosmic-ray collisions with solid bodies in the debris disk, which extends out similar to 60 au from the host star, and (2) emission from the stellar activity. For the former model, assuming a total disk mass consistent with infrared measurements, we find that the size distribution of bodies is steeper than expected for a collisional cascade state. If confirmed as being associated with epsilon Eridani, this would be the first indication of gamma-ray emission from the vicinity of a main-sequence star other than the Sun.

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