Dissipative photosphere models of gamma-ray bursts and X-ray flashes

We consider dissipative effects occurring in the optically thick inner parts of the relativistic outflows producing gamma-ray bursts and X-ray flashes, emphasizing in particular the Comptonization of the thermal radiation flux that is advected from the base of the outflow. Such dissipative effects—e.g., from magnetic reconnection, neutron decay, or shocks would boost the energy density of the thermal radiation. The dissipation can lead to pair production, in which case the pairs create an effective photosphere farther out than the usual baryonic one. In a slow dissipation scenario, pair creation can be suppressed, and the effects are most important when dissipation occurs below the baryonic photosphere. In both cases an increased photospheric luminosity is obtained. We suggest that the spectral peak in gamma-ray bursts is essentially due to the Comptonized thermal component from the photosphere, where the comoving optical depth in the outflow falls to unity. Typical peak photon energies range between those of classical bursts and X-ray flashes. The relationship between the observed photon peak energy and the luminosity depends on the details of the dissipation, but under plausible assumptions can resemble the observed correlations.