GRB110721A: An extreme peak energy and signatures of the photosphere

Abstract

GRB110721A was observed by the Fermi Gamma-ray Space Telescope using its two instruments, the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM). The burst consisted of one major emission episode which lasted for 24.5s (in the GBM) and had a peak flux of (5.7 ± 0.2) × 10 -5ergs-1cm-2. The time-resolved emission spectrum is best modeled with a combination of a Band function and a blackbody spectrum. The peak energy of the Band component was initially 15 ± 2MeV, which is the highest value ever detected in a GRB. This measurement was made possible by combining GBM/BGO data with LAT Low Energy events to achieve continuous 10-100MeV coverage. The peak energy later decreased as a power law in time with an index of -1.89 ± 0.10. The temperature of the blackbody component also decreased, starting from 80keV, and the decay showed a significant break after 2s. The spectrum provides strong constraints on the standard synchrotron model, indicating that alternative mechanisms may give rise to the emission at these energies. © © 2012. The American Astronomical Society. All rights reserved.