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  • 1. Abdollahi, S.
    et al.
    Acero, F.
    Ackermann, M.
    Ajello, M.
    Atwood, W. B.
    Axelsson, M.
    Baldini, L.
    Ballet, J.
    Larsson, Stefan
    Högskolan Dalarna, Akademin Utbildning, hälsa och samhälle, Naturvetenskap. KTH Royal Inst Technol / AlbaNova, Oskar Klein Ctr Cosmoparticle Phys.
    Zaharijas, G.
    Fermi Large Area Telescope Fourth Source Catalog2020Ingår i: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 247, nr 1, artikel-id 33Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present the fourth Fermi Large Area Telescope catalog (4FGL) of gamma-ray sources. Based on the first eight years of science data from the Fermi Gamma-ray Space Telescope mission in the energy range from 50 MeV to 1 TeV, it is the deepest yet in this energy range. Relative to the 3FGL catalog, the 4FGL catalog has twice as much exposure as well as a number of analysis improvements, including an updated model for the Galactic diffuse gamma-ray emission, and two sets of light curves (one-year and two-month intervals). The 4FGL catalog includes 5064 sources above 4 sigma significance, for which we provide localization and spectral properties. Seventy-five sources are modeled explicitly as spatially extended, and overall, 358 sources are considered as identified based on angular extent, periodicity, or correlated variability observed at other wavelengths. For 1336 sources, we have not found plausible counterparts at other wavelengths. More than 3130 of the identified or associated sources are active galaxies of the blazar class, and 239 are pulsars.

  • 2. Abdollahi, S.
    et al.
    Acero, F.
    Ackermann, M.
    Baldini, L.
    Ballet, J.
    Barbiellini, G.
    Bastieri, D.
    Bellazzini, R.
    Larsson, Stefan
    Högskolan Dalarna, Institutionen för lärarutbildning, Naturvetenskap. Department of Physics, KTH Royal Institute of Technology, AlbaNova, Stockholm; The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, Stockholm.
    Zaharijas, G.
    Search for New Cosmic-Ray Acceleration Sites within the 4FGL Catalog Galactic Plane Sources2022Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 933, nr 2, artikel-id 204Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Cosmic rays are mostly composed of protons accelerated to relativistic speeds. When those protons encounter interstellar material, they produce neutral pions, which in turn decay into gamma-rays. This offers a compelling way to identify the acceleration sites of protons. A characteristic hadronic spectrum, with a low-energy break around 200 MeV, was detected in the gamma-ray spectra of four supernova remnants (SNRs), IC 443, W44, W49B, and W51C, with the Fermi Large Area Telescope. This detection provided direct evidence that cosmic-ray protons are (re-)accelerated in SNRs. Here, we present a comprehensive search for low-energy spectral breaks among 311 4FGL catalog sources located within 5° from the Galactic plane. Using 8 yr of data from the Fermi Large Area Telescope between 50 MeV and 1 GeV, we find and present the spectral characteristics of 56 sources with a spectral break confirmed by a thorough study of systematic uncertainty. Our population of sources includes 13 SNRs for which the proton-proton interaction is enhanced by the dense target material; the high-mass gamma-ray binary LS I+61 303; the colliding wind binary η Carinae; and the Cygnus star-forming region. This analysis better constrains the origin of the gamma-ray emission and enlarges our view to potential new cosmic-ray acceleration sites. © 2022. The Author(s). Published by the American Astronomical Society.

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  • 3. Abdollahi, S.
    et al.
    Acero, F.
    Baldini, L.
    Ballet, J.
    Bastieri, D.
    Bellazzini, R.
    Berenji, B.
    Berretta, A.
    Larsson, Stefan
    Högskolan Dalarna, Institutionen för lärarutbildning, Naturvetenskap.
    Zaharijas, G.
    Incremental Fermi Large Area Telescope Fourth Source Catalog2022Ingår i: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 260, nr 2, artikel-id 53Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present an incremental version (4FGL-DR3, for Data Release 3) of the fourth Fermi Large Area Telescope (LAT) catalog of 3-ray sources. Based on the first 12 years of science data in the energy range from 50 MeV to 1 TeV, it contains 6658 sources. The analysis improves on that used for the 4FGL catalog over eight years of data: more sources are fit with curved spectra, we introduce a more robust spectral parameterization for pulsars, and we extend the spectral points to 1 TeV. The spectral parameters, spectral energy distributions, and associations are updated for all sources. Light curves are rebuilt for all sources with 1 yr intervals (not 2 month intervals). Among the 5064 original 4FGL sources, 16 were deleted, 112 are formally below the detection threshold over 12 yr (but are kept in the list), while 74 are newly associated, 10 have an improved association, and seven associations were withdrawn. Pulsars are split explicitly between young and millisecond pulsars. Pulsars and binaries newly detected in LAT sources, as well as more than 100 newly classified blazars, are reported. We add three extended sources and 1607 new point sources, mostly just above the detection threshold, among which eight are considered identified, and 699 have a plausible counterpart at other wavelengths. We discuss the degree-scale residuals to the global sky model and clusters of soft unassociated point sources close to the Galactic plane, which are possibly related to limitations of the interstellar emission model and missing extended sources. © 2022. The Author(s). Published by the American Astronomical Society.

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  • 4. Ajello, M.
    et al.
    Angioni, R.
    Axelsson, M.
    Ballet, J.
    Barbiellini, G.
    Bastieri, D.
    Becerra Gonzalez, J.
    Bellazzini, R.
    Larsson, Stefan
    Högskolan Dalarna, Akademin Utbildning, hälsa och samhälle, Naturvetenskap. KTH Royal Inst Technology / AlbaNova, Oskar Klein Ctr Cosmoparticle Phys.
    Yassine, M.
    The Fourth Catalog of Active Galactic Nuclei Detected by the Fermi Large Area Telescope2020Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 892, nr 2, artikel-id 105Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The fourth catalog of active galactic nuclei (AGNs) detected by the Fermi Gamma-ray Space Telescope Large Area Telescope (4LAC) between 2008 August 4 and 2016 August 2 contains . It includes 85% more sources than the previous 3LAC catalog based on 4 yr of data. AGNs represent at least 79% of the high-latitude sources in the fourth Fermi-Large Area Telescope Source Catalog (4FGL), which covers the energy range from 50 MeV to 1 TeV. In addition, gamma-ray AGNs are found at low Galactic latitudes. Most of the 4LAC AGNs are blazars (98%), while the remainder are other types of AGNs. The blazar population consists of 24% Flat Spectrum Radio Quasars (FSRQs), 38% BL Lac-type objects, and 38% blazar candidates of unknown types (BCUs). On average, FSRQs display softer spectra and stronger variability in the gamma-ray band than BL Lacs do, confirming previous findings. All AGNs detected by ground-based atmospheric Cerenkov telescopes are also found in the 4LAC.

  • 5. Ajello, M.
    et al.
    Arimoto, M.
    Axelsson, M.
    Baldini, L.
    Barbiellini, G.
    Bastieri, D.
    Bellazzini, R.
    Berretta, A.
    Larsson, Stefan
    Högskolan Dalarna, Akademin Utbildning, hälsa och samhälle, Naturvetenskap. KTH / The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova.
    De Pasquale, M.
    Fermi and Swift Observations of GRB 190114C: Tracing the Evolution of High-energy Emission from Prompt to Afterglow2020Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 890, nr 1, artikel-id 9Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on the observations of gamma-ray burst (GRB) 190114C by the Fermi Gamma-ray Space Telescope and the Neil Gehrels Swift Observatory. The prompt gamma-ray emission was detected by the Fermi GRB Monitor (GBM), the Fermi Large Area Telescope (LAT), and the Swift Burst Alert Telescope (BAT) and the long-lived afterglow emission was subsequently observed by the GBM, LAT, Swift X-ray Telescope (XRT), and Swift UV Optical Telescope. The early-time observations reveal multiple emission components that evolve independently, with a delayed power-law component that exhibits significant spectral attenuation above 40 MeV in the first few seconds of the burst. This power-law component transitions to a harder spectrum that is consistent with the afterglow emission observed by the XRT at later times. This afterglow component is clearly identifiable in the GBM and BAT light curves as a slowly fading emission component on which the rest of the prompt emission is superimposed. As a result, we are able to observe the transition from internal-shock- to external-shock-dominated emission. We find that the temporal and spectral evolution of the broadband afterglow emission can be well modeled as synchrotron emission from a forward shock propagating into a wind-like circumstellar environment. We estimate the initial bulk Lorentz factor using the observed high-energy spectral cutoff. Considering the onset of the afterglow component, we constrain the deceleration radius at which this forward shock begins to radiate in order to estimate the maximum synchrotron energy as a function of time. We find that even in the LAT energy range, there exist high-energy photons that are in tension with the theoretical maximum energy that can be achieved through synchrotron emission from a shock. These violations of the maximum synchrotron energy are further compounded by the detection of very high-energy (VHE) emission above 300 GeV by MAGIC concurrent with our observations. We conclude that the observations of VHE photons from GRB 190114C necessitates either an additional emission mechanism at very high energies that is hidden in the synchrotron component in the LAT energy range, an acceleration mechanism that imparts energy to the particles at a rate that is faster than the electron synchrotron energy-loss rate, or revisions of the fundamental assumptions used in estimating the maximum photon energy attainable through the synchrotron process. © 2020. The American Astronomical Society..

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  • 6. Ajello, M
    et al.
    Atwood, W B
    Axelsson, M
    Bagagli, R
    Bagni, M
    Baldini, L
    Bastieri, D
    Bellardi, F
    Larsson, Stefan
    Högskolan Dalarna, Institutionen för lärarutbildning, Naturvetenskap.
    Zaharijas, G.
    Fermi Large Area Telescope Performance after 10 Years of Operation2021Ingår i: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 256, nr 1, artikel-id 12Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Large Area Telescope (LAT), the primary instrument for the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy gamma-ray telescope, covering the energy range from 30 MeV to more than 300 GeV. We describe the performance of the instrument at the 10 yr milestone. LAT performance remains well within the specifications defined during the planning phase, validating the design choices and supporting the compelling case to extend the duration of the Fermi mission. The details provided here will be useful when designing the next generation of high-energy gamma-ray observatories.

  • 7. Ajello, M.
    et al.
    Atwood, W. B.
    Axelsson, M.
    Baldini, L.
    Barbiellini, G.
    Baring, M. G.
    Bastieri, D.
    Bellazzini, R.
    Larsson, Stefan
    Högskolan Dalarna, Institutionen för lärarutbildning, Naturvetenskap. KTH Royal Inst Technol / Oskar Klein Ctr Cosmoparticle Phys.
    Zaharijas, G.
    High-energy emission from a magnetar giant flare in the Sculptor galaxy2021Ingår i: Nature Astronomy, E-ISSN 2397-3366, Vol. 5, nr 4, s. 385-391Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Magnetars are the most highly magnetized neutron stars in the cosmos (with magnetic field 10(13)-10(15) G). Giant flares from magnetars are rare, short-duration (about 0.1 s) bursts of hard X-rays and soft gamma rays(1,2). Owing to the limited sensitivity and energy coverage of previous telescopes, no magnetar giant flare has been detected at gigaelectronvolt (GeV) energies. Here, we report the discovery of GeV emission from a magnetar giant flare on 15 April 2020 (refs. (3,4) and A. J. Castro-Tirado et al., manuscript in preparation). The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope detected GeV gamma rays from 19 s until 284 s after the initial detection of a signal in the megaelectronvolt (MeV) band. Our analysis shows that these gamma rays are spatially associated with the nearby (3.5 megaparsecs) Sculptor galaxy and are unlikely to originate from a cosmological gamma-ray burst. Thus, we infer that the gamma rays originated with the magnetar giant flare in Sculptor. We suggest that the GeV signal is generated by an ultra-relativistic outflow that first radiates the prompt MeV-band photons, and then deposits its energy far from the stellar magnetosphere. After a propagation delay, the outflow interacts with environmental gas and produces shock waves that accelerate electrons to very high energies; these electrons then emit GeV gamma rays as optically thin synchrotron radiation. This observation implies that a relativistic outflow is associated with the magnetar giant flare, and suggests the possibility that magnetars can power some short gamma-ray bursts.

  • 8. Ajello, M.
    et al.
    Atwood, W. B.
    Baldini, L.
    Ballet, J.
    Barbiellini, G.
    Bastieri, D.
    Bellazzini, R.
    Berretta, A.
    Larsson, Stefan
    Högskolan Dalarna, Institutionen för lärarutbildning, Naturvetenskap.
    Zaharijas, G.
    A gamma-ray pulsar timing array constrains the nanohertz gravitational wave background2022Ingår i: Science, ISSN 0036-8075, E-ISSN 1095-9203, Vol. 376, nr 6592, s. 521-523Artikel i tidskrift (Refereegranskat)
  • 9.
    Ajello, M.
    et al.
    Clemson Univ, Dept Phys & Astron, Kinard Lab Phys, Clemson, SC 29634 USA..
    Baldini, L.
    Univ Pisa, I-56127 Pisa, Italy.;Ist Nazl Fis Nucl, Sez Pisa, I-56127 Pisa, Italy..
    Ballet, J.
    Univ Paris, Univ Paris Saclay, CNRS, AIM,CEA, F-91191 Gif Sur Yvette, France..
    Barbiellini, G.
    Ist Nazl Fis Nucl, Sez Trieste, I-34127 Trieste, Italy.;Univ Trieste, Dipartimento Fis, I-34127 Trieste, Italy..
    Bastieri, D.
    Ist Nazl Fis Nucl, Sez Padova, I-35131 Padua, Italy.;Univ Padua, Dipartimento Fis & Astron G Galilei, I-35131 Padua, Italy..
    Bellazzini, R.
    Ist Nazl Fis Nucl, Sez Pisa, I-56127 Pisa, Italy..
    Berretta, A.
    Univ Perugia, Dipartimento Fis, I-06123 Perugia, Italy..
    Bissaldi, E.
    Univ & Politecn Bari, Dipartimento Fis M Merlin, Via Amendola 173, I-70126 Bari, Italy.;Ist Nazl Fis Nucl, Sez Bari, I-70126 Bari, Italy..
    Larsson, Stefan
    Högskolan Dalarna, Institutionen för lärarutbildning, Naturvetenskap. KTH Royal Inst Technol; AlbaNova, Oskar Klein Ctr Cosmoparticle Phys.
    Zrake, J.
    Clemson Univ, Dept Phys & Astron, Kinard Lab Phys, Clemson, SC 29634 USA..
    Gamma Rays from Fast Black-hole Winds2021Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 921, nr 2, artikel-id 144Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Massive black holes at the centers of galaxies can launch powerful wide-angle winds that, if sustained over time, can unbind the gas from the stellar bulges of galaxies. These winds may be responsible for the observed scaling relation between the masses of the central black holes and the velocity dispersion of stars in galactic bulges. Propagating through the galaxy, the wind should interact with the interstellar medium creating a strong shock, similar to those observed in supernovae explosions, which is able to accelerate charged particles to high energies. In this work we use data from the Fermi Large Area Telescope to search for the gamma-ray emission from galaxies with an ultrafast outflow (UFO): a fast (v similar to 0.1 c), highly ionized outflow, detected in absorption at hard X-rays in several nearby active galactic nuclei (AGN). Adopting a sensitive stacking analysis we are able to detect the average gamma-ray emission from these galaxies and exclude that it is due to processes other than UFOs. Moreover, our analysis shows that the gamma-ray luminosity scales with the AGN bolometric luminosity and that these outflows transfer similar to 0.04% of their mechanical power to gamma-rays. Interpreting the observed gamma-ray emission as produced by cosmic rays (CRs) accelerated at the shock front, we find that the gamma-ray emission may attest to the onset of the wind-host interaction and that these outflows can energize charged particles up to the transition region between galactic and extragalactic CRs.

  • 10. Ajello, M.
    et al.
    Baldini, L.
    Bastieri, D.
    Bellazzini, R.
    Berretta, A.
    Bissaldi, E.
    Blandford, R. D.
    Bonino, R.
    Larsson, Stefan
    Högskolan Dalarna, Institutionen för lärarutbildning, Naturvetenskap. KTH Royal Institute of Technology, AlbaNova, Stockholm; The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, Stockholm.
    Zaharijas, G.
    First fermi-LAT solar flare catalog2021Ingår i: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 252, nr 2, artikel-id abd32eArtikel i tidskrift (Refereegranskat)
  • 11. Fermi-Lat Collaboration, T.
    et al.
    Baldini, L.
    Ballet, J.
    Bastieri, D.
    Becerra Gonzalez, J.
    Bellazzini, R.
    Berretta, A.
    Bissaldi, E.
    Larsson, Stefan
    Högskolan Dalarna, Institutionen för lärarutbildning, Naturvetenskap. KTH Royal Institute of Technology, Stockholm; The Oskar Klein Centre for Cosmoparticle Physics, Stockholm.
    Cavazzuti, E.
    Catalog of Long-term Transient Sources in the First 10 yr of Fermi-LAT Data2021Ingår i: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 256, nr 1, artikel-id 13Artikel i tidskrift (Refereegranskat)
  • 12. Geng, X.
    et al.
    Zeng, W.
    Rani, B.
    Britto, R. J.
    Zhang, G.
    Wen, T.
    Hu, W.
    Larsson, Stefan
    Högskolan Dalarna, Akademin Utbildning, hälsa och samhälle, Naturvetenskap. Department of Physics, KTH Royal Institute of Technology;Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, Stockholm.
    Thompson, D. J.
    Dai, B.
    Exploring high-energy emission from the bl lacertae object s5 0716+714 with the fermi large area telescope2020Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 904, nr 1, artikel-id abb603Artikel i tidskrift (Refereegranskat)
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