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GRB110721A: An extreme peak energy and signatures of the photosphere

Axelsson, M. / Baldini, L. / Barbiellini, G. / G. Baring, M. / Bellazzini, R. / Bregeon, J. / Brigida, M. / Bruel, P. / Buehler, R. / A. Caliandro, G. / A. Cameron, R. / A. Caraveo, P. / Cecchi, C. / Chaves, R.C.G. / Chekhtman, A. / Chiang, J. / Claus, R. / Conrad, J. / Cutini, S. / D'Ammando, F. / De Palma, F. / D. Dermer, C. / Do Couto E Silva, E. / S. Drell, P. / Favuzzi, C. / J. Fegan, S. / C. Ferrara, E. / B. Focke, W. / Fukazawa, Y. / Fusco, P. / Gargano, F. / Gasparrini, D. / Gehrels, N. / Germani, S. / Giglietto, N. / Giroletti, M. / Godfrey, G. / Guiriec, S. / Hadasch, D. / Hanabata, Y. / Hayashida, M. / Hou, X. / Iyyani, S. / S. Jackson, M. / Kocevski, D. / Kuss, M. / Larsson, J. / Larsson, S. / Longo, F. / Loparco, F. / Lundman, C. / N. Mazziotta, M. / E. Mcenery, J. / Mizuno, T. / E. Monzani, M. / Moretti, E. / Morselli, A. / Murgia, S. / Nuss, E. / Nymark, T. / Ohno, M. / Omodei, N. / Pesce-Rollins, M. / Piron, F. / Pivato, G. / L. Racusin, J. / Rain'O, S. / Razzano, M. / Razzaque, S. / Reimer, A. / Roth, M. / Ryde, F. / Sanchez, D.A. / Sgr'O, C. / J. Siskind, E. / Spandre, G. / Spinelli, P. / Stamatikos, M. / Tibaldo, L. / Tinivella, M. / L. Usher, T. / Vandenbroucke, J. / Vasileiou, V. / Vianello, G. / Vitale, V. / P. Waite, A. / L. Winer, B. / S. Wood, K. / M. Burgess, J. / N. Bhat, P. / Bissaldi, E. / S. Briggs, M. / Connaughton, V. / Fishman, G. / Fitzpatrick, G. / Foley, S. / Gruber, D. / M. Kippen, R. / Kouveliotou, C. / Jenke, P. / Mcbreen, S. / Mcglynn, S. / Meegan, C. / S. Pacisas, W. / Pelassa, V. / Preece, R. / Tierney, D. / Von Kienlin, A. / Wilson-Hodge, C. / Xiong, S. / Pe'Er, A. - 2015
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.

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