Title: Thermal and non-thermal emission from the cocoon of a gamma-ray burst jet,
Abstract: We present hydrodynamic simulations of the hot cocoon produced when a
relativistic jet passes through the gamma-ray burst (GRB) progenitor star and
its environment, and we compute the lightcurve and spectrum of the radiation
emitted by the cocoon. The radiation from the cocoon has a nearly thermal
spectrum with a peak in the X-ray band, and it lasts for a few minutes in the
observer frame; the cocoon radiation starts at roughly the same time as when
$\gamma$-rays from a burst trigger detectors aboard GRB satellites. The
isotropic cocoon luminosity ($\sim 10^{47}$ erg s$^{-1}$) is of the same order
of magnitude as the X-ray luminosity of a typical long-GRB afterglow during the
plateau phase. This radiation should be identifiable in the Swift data because
of its nearly thermal spectrum which is distinct from the somewhat brighter
power-law component. The detection of this thermal component would provide
information regarding the size and density stratification of the GRB progenitor
star. Photons from the cocoon are also inverse-Compton (IC) scattered by
electrons in the relativistic jet. We present the IC lightcurve and spectrum,
by post-processing the results of the numerical simulations. The IC spectrum
lies in 10 keV--MeV band for typical GRB parameters. The detection of this IC
component would provide an independent measurement of GRB jet Lorentz factor
and it would also help to determine the jet magnetisation parameter.