Photospheric signatures imprinted on the γ-ray burst spectra

Abstract

A solution is presented for the spectrum of high-energy γ -ray burst (GRB) photons confined to a quasi-thermal baryonic photosphere. The solution is valid in the steady-state limit assuming the region under consideration is optically thick to the continuously injected photons. It is shown that for a high-luminosity photosphere, the non-thermal electrons resulting from γ -ray Compton cooling lose their energy by up-scattering the soft thermalized radiation. The resulting spectral modifications offer the possibility of diagnosing not only the burst comoving luminosity but also the baryon load of the ejecta. This model leads to a simple physical interpretation of X-ray rich bursts and anomalous low-energy slopes. © 2005 RAS.