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GRB Fermi-LAT Afterglows: Explaining Flares, Breaks, and Energetic Photons

Fraija, N. and Laskar, T. and Dichiara, S. and Beniamini, P. and Barniol Duran, R. and Dainotti, M. G. and Becerra, R. L. (2020) GRB Fermi-LAT Afterglows: Explaining Flares, Breaks, and Energetic Photons. Astrophysical Journal, 905 (2). Art. No. 112. ISSN 0004-637X. doi:10.3847/1538-4357/abc41a.

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The Fermi-LAT collaboration presented the second gamma-ray burst (GRB) catalog covering its first 10 years of operations. A significant fraction of afterglow-phase light curves in this catalog cannot be explained by the closure relations of the standard synchrotron forward-shock model, suggesting that there could be an important contribution from another process. In view of the above, we derive the synchrotron self-Compton (SSC) light curves from the reverse shock in the thick- and thin-shell regime for a uniform-density medium. We show that this emission could explain the GeV flares exhibited in some LAT light curves. Additionally, we demonstrate that the passage of the forward shock synchrotron cooling break through the LAT band from jets expanding in a uniform-density environment may be responsible for the late time (≈10² s) steepening of LAT GRB afterglow light curves. As a particular case, we model the LAT light curve of GRB 160509A that exhibited a GeV flare together with a break in the long-lasting emission, and also two very high energy photons with energies of 51.9 and 41.5 GeV observed 76.5 and 242 s after the onset of the burst, respectively. Constraining the microphysical parameters and the circumburst density from the afterglow observations, we show that the GeV flare is consistent with an SSC reverse-shock model, the break in the long-lasting emission with the passage of the synchrotron cooling break through the Fermi-LAT band, and the very energetic photons with SSC emission from the forward shock, when the outflow carries a significant magnetic field (R_B ≃ 30) and it decelerates in a uniform-density medium with a very low density (n = 4.554_(-1.121)^(+1.128) x 10⁻⁴ cm⁻³.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Laskar, T.0000-0003-1792-2338
Dichiara, S.0000-0001-6849-1270
Beniamini, P.0000-0001-7833-1043
Barniol Duran, R.0000-0002-5565-4824
Dainotti, M. G.0000-0002-7957-7931
Additional Information:© 2020. The American Astronomical Society. Received 2020 June 15; revised 2020 October 12; accepted 2020 October 19; published 2020 December 18. We thank Xiang-Yu Wang, Alan Watson and B. B. Zhang for useful discussions and comments. N. F. acknowledges support from UNAM-DGAPA-PAPIIT through grants IA102019 and IN10751. R. B. D. acknowledges support from the National Science Foundation (NSF) under grant 1816694. M. G. D. acknowledges support from the American Astronomical Society Chretienne Fellowship and from MINIATURA2 grant 2018/02/X/ST9/03673.
Funding AgencyGrant Number
Dirección General Asuntos del Personal Académico (DGAPA)IA102019
Dirección General Asuntos del Personal Académico (DGAPA)IN10751
American Astronomical SocietyUNSPECIFIED
National Science Centre (Poland)2018/02/X/ST9/03673
Subject Keywords:Gamma-ray bursts ; Radiative processes ; Magnetic fields ; Non-thermal radiation sources ; Interstellar medium
Issue or Number:2
Classification Code:Unified Astronomy Thesaurus concepts: Gamma-ray bursts (629); Radiative processes (2055); Magnetic fields (994); Non-thermal radiation sources (1119); Interstellar medium (847)
Record Number:CaltechAUTHORS:20201218-090150288
Persistent URL:
Official Citation:N. Fraija et al 2020 ApJ 905 112
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107187
Deposited By: George Porter
Deposited On:18 Dec 2020 17:52
Last Modified:16 Nov 2021 19:00

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