An observed correlation between thermal and non-thermal emission in gamma-ray bursts

Michael, Burgess J. and Preece, R. D. and Ryde, F. and Veres, Péter and Mészáros, P. and Connaughton, V. and Briggs, M. and Pe'Er, A. and Iyyani, S. and Goldstein, A. and Axelsson, M. and Baring, M. G. and Bhat, P. N. and Byrne, D. and Fitzpatrick, G. and Foley, S. and Kocevski, D. and Omodei, N. and Paciesas, W. S. and Pelassa, V. and Kouveliotou, C. and Xiong, S. and Yu, H. -F. and Zhang, B. and Zhu, S. (2014) An observed correlation between thermal and non-thermal emission in gamma-ray bursts. ASTROPHYSICAL JOURNAL LETTERS, 784 (2). ISSN 2041-8205

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Abstract

Recent observations by the Fermi Gamma-ray Space Telescope have confirmed the existence of ther- mal and non-thermal components in the prompt photon spectra of some Gamma-ray bursts (GRBs). Through an analysis of six bright Fermi GRBs, we have discovered a correlation between the observed photospheric and non-thermal γ-ray emission components of several GRBs using a physical model that has previously been shown to be a good fit to the Fermi data. From the spectral parameters of these fits we find that the characteristic energies, Ep and kT , of these two components are correlated via the relation Ep ∝ T α which varies from GRB to GRB. We present an interpretation in which the value of index α indicates whether the jet is dominated by kinetic or magnetic energy. To date, this jet composition parameter has been assumed in the modeling of GRB outflows rather than derived from the data.

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