Thermal Emission from Gamma-Ray Bursts

Abstract

In recent years, there are increasing evidence for a thermal emission component that accompanies the overall non-thermal spectra of the prompt emission phase in GRBs. Both the temperature and flux of the thermal emission show a well defined temporal behavior, a broken power law in time. The temperature is nearly constant during the first few seconds, afterwards it decays with power law index alpha ~0.7. The thermal flux also decays at late times as a power law with index beta ~2.1. This behavior is very ubiquitous, and was observed in a sample currently containing 32 BATSE bursts. These results are naturally explained by considering emission from the photosphere. The photosphere of a relativistically expanding plasma wind strongly depends on the angle to the line of sight, theta. As a result, thermal emission can be seen after tens of seconds. By introducing probability density function P(r,theta) of a thermal photon to escape the plasma at radius r and angle theta, the late time behavior of the flux can be reproduced analytically. During the propagation below the photosphere, thermal photons lose energy as a result of the slight misalignment of the scattering electrons velocity vectors, which leads to photon comoving energy decay epsilon'(r)~r^{-2/3}. This in turn can explain the decay of the temperature observed at late times. Finally, I show that understanding the thermal emission is essential in understanding the high energy, non-thermal spectra. Moreover, thermal emission can be used to directly measure the Lorentz factor of the flow and the initial jet radius.