Non-dissipative photospheres in GRBs: spectral appearance in the Fermi/GBM catalogue

Abstract

We find that the low-energy photon indices obtain values ranging −0.4 < α < 0.0, peaking at around −0.1. This is significantly softer than the asymptotic value of the theoretical spectrum of α ∼ 0.4. The reason for the α-values to be much softer than expected, is the limitation of the empirical functions to capture the true curvature of the theoretical spectrum. We conclude that more than a 1/3 of the bursts in the GBM catalogue have α-values that are consistent with spectra from a non-dissipative outflow, releasing its thermal energy at the photosphere. The fraction of spectra consistent with emission from the photosphere will increase even more if dissipation of kinetic energy in the flow occurs below the photosphere.