A very-high-energy component deep in the $\gamma$-ray burst afterglow

H. Abdalla , R. Adam (1) , F. Aharonian , F. Ait Benkhali , E.O. Angüner (2) , M. Arakawa , C. Arcaro , C. Armand (3) , H. Ashkar (4) , M. Backes , V. Barbosa Martins , M. Barnard , Y. Becherini , D. Berge , K. Bernlöhr , E. Bissaldi , R. Blackwell , M. Böttcher , C. Boisson (5) , J. Bolmont (1) , S. Bonnefoy , J. Bregeon (6) , M. Breuhaus , F. Brun (4) , P. Brun (4) , M. Bryan , M. Büchele , T. Bulik , T. Bylund , M. Capasso , S. Caroff (7) , A. Carosi (3) , S. Casanova , M. Cerruti (7) , T. Chand , S. Chandra , A. Chen , S. Colafrancesco , M. Curyło , I.D. Davids , C. Deil , J. Devin (8) , P. Dewilt , L. Dirson , A. Djannati-Atai (9) , A. Dmytriiev (5) , A. Donath , V. Doroshenko , J. Dyks , K. Egberts , G. Emery (7) , J.-P. Ernenwein (2) , S. Eschbach , K. Feijen , S. Fegan (1) , A. Fiasson (3) , G. Fontaine (1) , S. Funk , M. Füssling , S. Gabici (9) , Y.A. Gallant (6) , F. Gaté (3) , G. Giavitto , L. Giunti (9) , D. Glawion , J.F. Glicenstein (4) , D. Gottschall , M.-H. Grondin (8) , J. Hahn , M. Haupt , G. Heinzelmann , G. Henri (10) , G. Hermann , J.A. Hinton , W. Hofmann , C. Hoischen , T.L. Holch , M. Holler , D. Horns , D. Huber , H. Iwasaki , M. Jamrozy , D. Jankowsky , F. Jankowsky , A. Jardin-Blicq , I. Jung-Richardt , M.A. Kastendieck , K. Katarzyński , M. Katsuragawa , U. Katz , D. Khangulyan , B. Khélifi (9) , J. King , S. Klepser , W. Kluźniak , Nu. Komin , K. Kosack (4) , D. Kostunin , M. Kreter , G. Lamanna (3) , A. Lemière (9) , M. Lemoine-Goumard (8) , J.-P. Lenain (7) , E. Leser , C. Levy (7) , T. Lohse , I. Lypova , J. Mackey , J. Majumdar , D. Malyshev , V. Marandon , Alexandre Marcowith (6) , A. Mares (8) , C. Mariaud (1) , G. Martí-Devesa , R. Marx , G. Maurin (3) , P.J. Meintjes , A.M.W. Mitchell , R. Moderski , M. Mohamed , L. Mohrmann , C. Moore , E. Moulin (4) , J. Muller (1) , T. Murach , S. Nakashima , M. de Naurois (1) , H. Ndiyavala , F. Niederwanger , J. Niemiec , L. Oakes , P. O'Brien , H. Odaka , S. Ohm , E. de Ona Wilhelmi , M. Ostrowski , I. Oya , M. Panter , R.D. Parsons , C. Perennes (7) , P.-O. Petrucci (10) , B. Peyaud (4) , Q. Piel (3) , S. Pita (9) , V. Poireau (3) , A. Priyana Noel , D.A. Prokhorov , H. Prokoph , G. Pühlhofer , M. Punch (9) , A. Quirrenbach , S. Raab , R. Rauth , A. Reimer , O. Reimer , Q. Remy (6) , Matthieu Renaud (6) , F. Rieger , L. Rinchiuso (4) , C. Romoli , G. Rowell , B. Rudak , E. Ruiz-Velasco , V. Sahakian , S. Sailer , S. Saito , D.A. Sanchez (3) , A. Santangelo , M. Sasaki , R. Schlickeiser , F. Schüssler (4) , A. Schulz , H.M. Schutte , U. Schwanke , S. Schwemmer , M. Seglar-Arroyo (4) , M. Senniappan , A.S. Seyffert , N. Shafi , K. Shiningayamwe , R. Simoni , A. Sinha (9) , H. Sol (5) , A. Specovius , M. Spir-Jacob (9) , R. Steenkamp , C. Stegmann , C. Steppa , T. Takahashi , T. Tavernier (4) , A.M. Taylor , R. Terrier (9) , D. Tiziani , M. Tluczykont , C. Trichard (1) , M. Tsirou (6) , N. Tsuji , R. Tuffs , Y. Uchiyama , D.J. van Der Walt , C. van Eldik , C. van Rensburg , B. van Soelen , George Vasileiadis (6) , J. Veh , C. Venter , P. Vincent (7) , J. Vink , H.J. Völk , T. Vuillaume (3) , Z. Wadiasingh , S.J. Wagner , R. White , A. Wierzcholska , R. Yang , H. Yoneda , M. Zacharias , R. Zanin , A.A. Zdziarski , A. Zech (5) , A. Ziegler , J. Zorn , N. Żywucka , F. de Palma , M. Axelsson , O.J. Roberts

1 LLR - Laboratoire Leprince-Ringuet
2 CPPM - Centre de Physique des Particules de Marseille
3 LAPP - Laboratoire d'Annecy de Physique des Particules
4 IRFU - Institut de Recherches sur les lois Fondamentales de l'Univers
5 LUTH (UMR_8102) - Laboratoire Univers et Théories
6 LUPM - Laboratoire Univers et Particules de Montpellier
7 LPNHE (UMR_7585) - Laboratoire de Physique Nucléaire et de Hautes Énergies
8 CENBG - Centre d'Etudes Nucléaires de Bordeaux Gradignan
9 APC (UMR_7164) - AstroParticule et Cosmologie
10 IPAG - Institut de Planétologie et d'Astrophysique de Grenoble

Gamma-ray bursts (GRBs) are brief flashes of γ-rays and are considered to be the most energetic explosive phenomena in the Universe 1 . The emission from GRBs comprises a short (typically tens of seconds) and bright prompt emission, followed by a much longer afterglow phase. During the afterglow phase, the shocked outflow—produced by the interaction between the ejected matter and the circumburst medium—slows down, and a gradual decrease in brightness is observed 2 . GRBs typically emit most of their energy via γ-rays with energies in the kiloelectronvolt-to-megaelectronvolt range, but a few photons with energies of tens of gigaelectronvolts have been detected by space-based instruments 3 . However, the origins of such high-energy (above one gigaelectronvolt) photons and the presence of very-high-energy (more than 100 gigaelectronvolts) emission have remained elusive 4 . Here we report observations of very-high-energy emission in the bright GRB 180720B deep in the GRB afterglow—ten hours after the end of the prompt emission phase, when the X-ray flux had already decayed by four orders of magnitude. Two possible explanations exist for the observed radiation: inverse Compton emission and synchrotron emission of ultrarelativistic electrons. Our observations show that the energy fluxes in the X-ray and γ-ray range and their photon indices remain comparable to each other throughout the afterglow. This discovery places distinct constraints on the GRB environment for both emission mechanisms, with the inverse Compton explanation alleviating the particle energy requirements for the emission observed at late times. The late timing of this detection has consequences for the future observations of GRBs at the highest energies.

Mots clés: gamma ray: burst; particle: energy; X-ray: flux; gamma ray: UHE; photon; VHE; synchrotron; brightness; electron

Domaines: Astrophysique [astro-ph]

H. Abdalla, R. Adam, F. Aharonian, F. Ait Benkhali, E.O. Angüner, et al.. A very-high-energy component deep in the $\gamma$-ray burst afterglow. Nature, 2019, 575 (7783), pp.464-467. ⟨10.1038/s41586-019-1743-9⟩. ⟨hal-02381934⟩