Energy dissipation below gamma-ray bursts photosphere: Evidence and spectral fits

It is now established that a thermal emission component plays a major role in shaping the prompt spectra of a non-negligible fraction of gamma-ray bursts (GRBs). By studying the properties of this component in a sample of 47 GRBs, we deduce that the Lorentz factor is 10² Γ 10³, with mean value Γ 370. The acceleration radius r0 span a wide range, 10^6.5 r0 10^9.5 cm, with mean value r0 10⁸ cm. This is higher than the gravitational radius of 10 M black hole by a factor of ≈ 30. We argue that this result provides an indirect evidence for jet propagation inside a massive star, and suggests the existence of recollimation shocks that take place at this radius. We further show that sub-photospheric dissipation of the jet kinetic energy provides a self-consistent, fully physically motivated model that can fit a wide range of GRB spectra. The leading radiative process is Comptonization of the thermal component, and synchrotron emission is sub-dominant. We create a DREAM (Dissipation with Radiative Emission as a Table Model) table model for XSPEC, and show how this model can describe different types of GRB spectra.