2012ApJ...758L..34Z

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, the growing trend within the community is to interpret this component as modified thermal emission from a dissipative photosphere of a GRB fireball. We analyze the time-dependent spectrum of GRB 110721A detected by the Fermi Gamma-Ray Burst Monitor and Large Area Telescope, and pay special attention to the rapid evolution of the peak energy E_p. We define a "death line" of thermally dominated dissipative 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∼ 15 MeV that is beyond the "death line." 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 is not from a dissipative photosphere, but 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 the quick discharge of magnetic energy in a magnetically dominated outflow in the optically thin region.