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

TL;DR

This study analyzes the early afterglow of GRB 090618, revealing two evolving blackbody components consistent with a spine-sheath jet model, and demonstrates the applicability of a two-blackbody plus power-law model from prompt to afterglow phases.

Contribution

It introduces a combined two-blackbody plus power-law model to describe thermal emission evolution in GRB 090618, supporting a spine-sheath jet structure.

Findings

Two correlated blackbodies with decreasing temperatures are observed.

The blackbody fluxes decrease faster than the non-thermal component.

The photospheric radii increase slowly, consistent with photospheric emission models.

Abstract

GRB 090618 is a bright GRB with multiple pulses. It shows evidence of thermal emission in the initial pulses as well as in the early afterglow phase. As high resolution spectral data of \emph{Swift}/XRT is available for the early afterglow, we investigate the shape and evolution of the thermal component in this 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 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 remains constant throughout the observations, c) the black body temperatures and fluxes show a monotonic decrease with time, with the BB fluxes dropping about a factor of two faster than that 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 larger emitting radius than that 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.