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American Astronomical Society, Astrophysical Journal Letters, 2(757), p. L31, 2012

DOI: 10.1088/2041-8205/757/2/l31

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GRB110721A: An extreme peak energy and signatures of the photosphere

Journal article published in 2012 by P. S. Drell, Baring Mg, M. S. Jackson, S. Rain'O, C. Sgr'O, Caliandro Ga, Cameron Ra, Caraveo Pa, R. C. G., R. A. Cameron, Chaves Rcg, P. A. Caraveo, C. Cecchi, R. C. G. Chaves, K. S. Wood and other authors.
This paper is made freely available by the publisher.
This paper is made freely available by the publisher.

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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±0.2 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±2 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±0.10. 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. ; Comment: 5 pages, 4 figures, submitted 120721 to ApJL