MG14 - Talk detail

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Abstract

We study the properties of a significant thermal emission component that was identified in 47 GRBs observed by different instruments. Within the framework of the ``fireball'' model, we deduce the values of the Lorentz factor \Gamma, and the acceleration radius, r_0, for these bursts. We find that all the values of \Gamma in our sample are in the range 10^2 <~\Gamma <~ 10^3, with <\Gamma>= 310. We find a very weak dependence of \Gamma on the acceleration radius r_0, \Gamma ~ r_0^\alpha with \alpha = -0.10 \pm 0.09 at \sigma = 2.1 confidence level. The mean value we find for r_0 is < r_0 > ~ 10^{8.5} cm. This higher than the gravitational radius of a 10 M_sun black hole by a factor ~ 100. 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. If time permits, I will further discuss some novel approach towards fitting GRB prompt emission spectra with physically motivated model of sub-photospheric dissipation.

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