http://arxiv.org/abs/1409.4538
GRB~090618 is a bright GRB with multiple pulses. It shows evidence of a thermal emission in the initial pulses as well as in the early afterglow phase. We investigate the shape and evolution of the thermal component in the early afterglow/ late prompt emission phase using data from Swift/BAT, Swift/XRT, and Fermi/GBM detectors. An independent fit to the BAT and the XRT data reveals two correlated blackbodies with monotonically decreasing temperatures. Hence we investigated the combined data with a model consisting of two blackbodies and a power-law (2BBPL), a model suggested for several bright GRBs. We elicit the following interesting features of the 2BBPL model: a) the same model is applicable from the peak of the last pulse in the prompt emission to the afterglow emission, b) the ratio of temperatures and the fluxes of the two black bodies remain constant throughout the observations, c) the black body temperatures and fluxes show a monotonic decrease with time, with the fluxes dropping about a factor of two faster than the flux of the power-law emission, d) attributing the blackbody emission to photospheric emissions, we find that the photospheric radii increase very slowly with time, and the lower temperature blackbody shows a radius about 65 times larger than the radius of the higher temperature black body. We find some evidence that the underlying shape of the non-thermal emission is a cut-off power-law rather than a power-law. We sketch a spine-sheath jet model to explain our observations.
R. Basak and A. Rao
Wed, 17 Sep 14
42/67
Comments: 9 pages, 6 figures, submitted to ApJ
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