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GRB 090618 is a bright GRB with multiple pulses. It shows evidence of thermalemission in the initial pulses as well as in the early afterglow phase. As highresolution spectral data of \emph{Swift}/XRT is available for the earlyafterglow, we investigate the shape and evolution of the thermal component inthis phase using data from the \emph{Swift}/BAT, the \emph{Swift}/XRT, and the\emph{Fermi}/GBM detectors. An independent fit to the BAT and XRT data revealstwo correlated blackbodies with monotonically decreasing temperatures. Hence weinvestigated the combined data with a model consisting of two blackbodies and apower-law (2BBPL), a model suggested for several bright GRBs. We elicit thefollowing interesting features of the 2BBPL model: a) the same model isapplicable from the peak of the last pulse in the prompt emission to theafterglow emission, b) the ratio of temperatures and the fluxes of the twoblack bodies remains constant throughout the observations, c) the black bodytemperatures and fluxes show a monotonic decrease with time, with the BB fluxesdropping about a factor of two faster than that of the power-law emission, d)attributing the blackbody emission to photospheric emissions, we find that thephotospheric radii increase very slowly with time, and the lower temperatureblackbody shows a larger emitting radius than that of the higher temperatureblack body. We find some evidence that the underlying shape of the non-thermalemission is a cut-off power-law rather than a power-law. We sketch aspine-sheath jet model to explain our observations.

Discovery of Smoothly Evolving Blackbodies in the Early Afterglow of GRB 090618 : Evidence for a Spine-Sheath Jet?