Bing Zhang, Rui-Jing Lu, En-Wei Liang, Xue-Feng Wu
The prompt emission spectra of gamma-ray bursts (GRBs) usually have a dominant component that is well described by a phenomenological "Band" function. The physical origin of this spectral component is debated. Although the traditional interpretation is synchrotron radiation of non-thermal electrons accelerated in internal shocks or magnetic dissipation regions, a growing trend in the community is to interpret this component as emission from the dissipative photosphere of a GRB fireball. We analyze the time dependent spectrum of GRB 110721A detected by Fermi GBM and LAT, and pay special attention to the rapid evolution of the peak energy $E_p$. We define a "death line" of baryonic photospheric emission in the $E_p - L$ plane, and show that $E_p$ of GRB 110721A at the earliest epoch has a very high $E_p \sim 15$ MeV that is beyond the "death line". This rules out the baryonic photosphere model for the "Band" component for this burst. Together with the finding that an additional "shoulder" component exists in this burst that is consistent with a photospheric origin, we suggest that at least for some bursts, the "Band" component must invoke a non-thermal origin (e.g. synchrotron or inverse Compton) in the optically thin region of a GRB outflow. We also suggest that the rapid "hard-to-soft" spectral evolution is consistent with quick discharge of magnetic energy in a magnetically-dominated outflow.
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http://arxiv.org/abs/1208.1812
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