Thermal Emission from Gamma-Ray Bursts

In recent years, there are increasing evidence for a thermal emission component that accompanies the overall nonthermal spectra of the prompt emission phase in GRBs. Both the temperature and flux of the thermal emission show a well defined temporal behaviour, a broken power law in time. The temperature is nearly constant during the first few seconds, afterwards it decays with power law index alpha similar to 0.7. The thermal flux also decays at late times as a power law with index beta similar to 2.1. This behaviour 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 behaviour 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) proportional to 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.