Investigating the Nature of Late-time High-energy GRB Emission through Joint Fermi/Swift Observations

Ajello M, Baldini L, Barbiellini G, Bastieri D, Bellazzini R, Bissaldi E, Blandford RD, Bonino R, Bottacini E, Bregeon J, Bruel P, Buehler R, Cameron RA, Caputo R, Caraveo PA, Chiaro G, Ciprini S, Cohen-Tanugi J, Costantin D, D'Ammando F, De Palma F, Di Lalla N, Di Mauro M, Di Venere L, Dominguez A, Favuzzi C, Franckowiak A, Fukazawa Y, Funk S, Fusco P, Gargano F, Gasparrini D, Giglietto N, Giordano F, Giroletti M, Green D, Grenier IA, Guiriec S, Holt C, Horan D, Johannesson G, Kocevski D, Kuss M, La Mura G, Larsson S, Li J, Longo F, Loparco F, Lubrano P, Magill JD, Maldera S, Manfreda A, Mazziotta MN, Michelson PF, Mizuno T, Monzani ME, Morselli A, Negro M, Nuss E, Omodei N, Orienti M, Orlando E, Paliya VS, Perkins JS, Persic M, Pesce-Rollins M, Piron F, Principe G, Racusin JL, Raino S, Rando R, Razzano M, Razzaque S, Reimer A, Reimer O, Sgro C, Siskind EJ, Spandre G, Spinelli P, Tak D, Thayer JB, Torres DF, Tosti G, Valverde J, Vogel M, Wood K (2018)

Publication Status: Published

Publication Type: Journal article

Publication year: 2018



Book Volume: 863

Article Number: ARTN 138

Journal Issue: 2

DOI: 10.3847/1538-4357/aad000


We use joint observations by the Swift X-ray Telescope (XRT) and the Fermi Large Area Telescope (LAT) of gamma-ray burst (GRB) afterglows to investigate the nature of the long-lived high-energy emission observed by Fermi LAT. Joint broadband spectral modeling of XRT and LAT data reveals that LAT nondetections of bright X-ray afterglows are consistent with a cooling break in the inferred electron synchrotron spectrum below the LAT and/or XRT energy ranges. Such a break is sufficient to suppress the high-energy emission so as to be below the LAT detection threshold. By contrast, LAT-detected bursts are best fit by a synchrotron spectrum with a cooling break that lies either between or above the XRT and LAT energy ranges. We speculate that the primary difference between GRBs with LAT afterglow detections and the nondetected population may be in the type of circumstellar environment in which these bursts occur, with late-time LAT detections preferentially selecting GRBs that occur in low wind-like circumburst density profiles. Furthermore, we find no evidence of high-energy emission in the LAT-detected population significantly in excess of the flux expected from the electron synchrotron spectrum fit to the observed X-ray emission. The lack of excess emission at high energies could be due to a shocked external medium in which the energy density in the magnetic field is stronger than or comparable to that of the relativistic electrons behind the shock, precluding the production of a dominant synchrotron self-Compton (SSC) component in the LAT energy range. Alternatively, the peak of the SSC emission could be beyond the 0.1-100 GeV energy range considered for this analysis.

Authors with CRIS profile

Involved external institutions

National Institute for Nuclear Physics / Istituto Nazionale di Fisica Nucleare (INFN) IT Italy (IT) Stanford University US United States (USA) (US) Politecnico di Bari IT Italy (IT) National Aeronautics and Space Administration (NASA) US United States (USA) (US) University of Padua / Universita degli Studi di Padova IT Italy (IT) University of Montpellier / Université Montpellier FR France (FR) Praxis, Inc. US United States (USA) (US) California State University, Los Angeles US United States (USA) (US) University of Maryland US United States (USA) (US) Institute of Space Sciences (IEEC-CSIC) ES Spain (ES) Università degli Studi di Perugia IT Italy (IT) Institut national de physique nucléaire et de physique des particules (IN2P3) FR France (FR) University of Pisa / Università di Pisa (UniPi) IT Italy (IT) Hiroshima University / 広島大学 JP Japan (JP) Clemson University US United States (USA) (US) Universidad Complutense de Madrid (UCM) ES Spain (ES) Deutsches Elektronen-Synchrotron DESY DE Germany (DE) National Institute for Astrophysics / Istituto Nazionale Astrofisica (INAF) IT Italy (IT) Institució Catalana de Recerca i Estudis Avançats (ICREA) / Catalan Institution for Research and Advanced Studies ES Spain (ES) AlbaNova University Centre / Kungliga Tekniska Högskolan AlbaNova Universitetscentrum SE Sweden (SE) Italian Space Agency / Agenzia Spaziale Italiana (ASI) IT Italy (IT) University of Johannesburg ZA South Africa (ZA) NYCB Real-Time Computing, Inc. US United States (USA) (US) University of Iceland (UI) / Háskóli Íslands IS Iceland (IS) University of Maryland, Baltimore County (UMBC) US United States (USA) (US) University of Paris 7 - Denis Diderot / Université Paris VII Denis Diderot FR France (FR)

How to cite


Ajello, M., Baldini, L., Barbiellini, G., Bastieri, D., Bellazzini, R., Bissaldi, E.,... Wood, K. (2018). Investigating the Nature of Late-time High-energy GRB Emission through Joint Fermi/Swift Observations. Astrophysical Journal, 863(2).


Ajello, Marco, et al. "Investigating the Nature of Late-time High-energy GRB Emission through Joint Fermi/Swift Observations." Astrophysical Journal 863.2 (2018).

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