GRB110721A: An extreme peak energy and signatures of the photosphere

TL;DR

This paper reports the observation of an exceptionally high peak energy in GRB110721A, combining data from Fermi's instruments to analyze its spectral evolution and challenge existing emission models.

Contribution

It presents the first detection of a peak energy as high as 15 MeV in a GRB and demonstrates the importance of combined instrument data for spectral analysis.

Findings

Peak energy reached 15 MeV, the highest in a GRB.

Spectral evolution follows a power-law decay with index -1.89.

Blackbody component temperature decreased from 80 keV with a break.

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 ~24.5 seconds (in the GBM) and had a peak flux of 5.7\pm0.2 x 10^{-5} erg/s/cm^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\pm2 MeV, 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--100 MeV coverage. The peak energy later decreased as a power law in time with an index of -1.89\pm0.10. The temperature of the blackbody component also decreased, starting from ~80 keV, and the decay showed a significant break after ~2 seconds. The spectrum provides strong constraints on the standard synchrotron model, indicating that alternative mechanisms may give rise to the emission at these energies.