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GRB110721A was observed by the Fermi Gamma-ray Space Telescope using its twoinstruments the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor(GBM). The burst consisted of one major emission episode which lasted for ~24.5seconds (in the GBM) and had a peak flux of 5.7\pm0.2 x 10^{-5} erg/s/cm^2. Thetime-resolved emission spectrum is best modeled with a combination of a Bandfunction and a blackbody spectrum. The peak energy of the Band component wasinitially 15\pm2 MeV, which is the highest value ever detected in a GRB. Thismeasurement was made possible by combining GBM/BGO data with LAT Low EnergyEvents to achieve continuous 10--100 MeV coverage. The peak energy laterdecreased as a power law in time with an index of -1.89\pm0.10. The temperatureof the blackbody component also decreased, starting from ~80 keV, and the decayshowed a significant break after ~2 seconds. The spectrum provides strongconstraints on the standard synchrotron model, indicating that alternativemechanisms may give rise to the emission at these energies.

GRB110721A: An extreme peak energy and signatures of the photosphere