GRB110721A: An extreme peak energy and signatures of the photosphere
  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.
