Volume 567, July 2014
|Number of page(s)||18|
|Section||Stellar structure and evolution|
|Published online||16 July 2014|
GRB 120711A: an intense INTEGRAL burst with long-lasting soft γ-ray emission and a powerful optical flash⋆
Space Science Group, School of Physics, University College
Dublin 4, Ireland
2 University of North Carolina at Chapel Hill, Campus Box 3255, Chapel Hill NC 27599-3255, USA
3 APC, Université Paris Diderot, CNRS/IN2P3, CEA/DSM, Obs. Paris, 13 rue Watt, 75205 Paris Cedex 13, France
4 Universe Cluster, Technische Universität München, Boltzmannstraße 2, 85748 Garching, Germany
5 Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
6 Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
7 Department of Physics, North Carolina A&T State University, 1601 E Market St, Greensboro NC 27411, USA
8 INAF, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
9 Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
10 European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
11 INAF-IASF Bologna, Area della Ricerca CNR, via Gobetti 101, 40129 Bologna, Italy
Accepted: 2 May 2014
A long and intense γ-ray burst (GRB) was detected by INTEGRAL on 11 July 2012 with a duration of ~115 s and fluence of 2.8 × 10-4 erg cm-2 in the 20 keV−8 MeV energy range. GRB 120711A was at z ~ 1.405 and produced soft γ-ray emission (>20 keV) for at least ~10 ks after the trigger. The GRB was observed by several ground-based telescopes that detected a powerful optical flash peaking at an R-band brightness of ~11.5 mag at ~126 s after the trigger, or ~9th magnitude when corrected for the host galaxy extinction (AV ~ 0.85). The X-ray afterglow was monitored by the Swift, XMM-Newton, and Chandra observatories from 8 ks to 7 Ms and provides evidence for a jet break at ~0.9 Ms. We present a comprehensive temporal and spectral analysis of the long-lasting soft γ-ray emission detected in the 20−200 keV band with INTEGRAL/IBIS, the Fermi/LAT post-GRB detection above 100 MeV, the soft X-ray afterglow and the optical/near-infrared detections from Watcher, Skynet/PROMPT, GROND, and REM. The prompt emission had a very hard spectrum (Epeak ~ 1 MeV) and yields an Eγ,iso ~ 1054 erg (1 keV−10 MeV rest frame), making GRB 120711A one of the most energetic GRBs detected so far. We modelled the long-lasting soft γ-ray emission using the standard afterglow scenario, which indicates a forward shock origin. The combination of data extending from the near-infrared to GeV energies suggest that the emission is produced by a broken power-law spectrum consistent with synchrotron radiation. The afterglow is well modelled using a stratified wind-like environment with a density profile k ~ 1.2, suggesting a massive star progenitor (i.e. Wolf-Rayet) with a mass-loss rate between ~10-5−10-6 M⊙ yr-1 depending on the value of the radiative efficiency (ηγ = 0.2 or 0.5). The analysis of the reverse and forward shock emission reveals an initial Lorentz factor of ~120−340, a jet half-opening angle of ~2°−5°, and a baryon load of ~10-5 − 10-6 M⊙ consistent with the expectations of the fireball model when the emission is highly relativistic. Long-lasting soft γ-ray emission from other INTEGRAL GRBs with high peak fluxes, such as GRB 041219A, was not detected, suggesting that a combination of high Lorentz factor, emission above 100 MeV, and possibly a powerful reverse shock are required. Similar long-lasting soft γ-ray emission has recently been observed from the nearby and extremely bright Fermi/LAT burst GRB 130427A.
Key words: gamma-ray burst: individual: GRB 120711A / gamma-ray burst: individual: GRB 041219A / gamma-ray burst: individual: GRB 130427A
Appendix A is available in electronic form at http://www.aanda.org
© ESO, 2014
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