Thermal Emissions Spanning the Prompt and the Afterglow Phase of the Ultra-long GRB 130925A

Abstract

GRB 130925A is an ultra-long GRB, and it shows clear evidences for a thermal emission in the soft X-ray data of \emph{Swift}/XRT ($\sim0.5$\,keV), lasting till the X-ray afterglow phase. Due to the long duration of the GRB, the burst could be studied in hard X-rays with high-resolution focusing detectors (\emph{NuSTAR}). The blackbody temperature, as measured by the \emph{Swift}/XRT, shows a decreasing trend till the late phase (Piro et al. 2014) whereas the high-energy data reveals a significant blackbody component during the late epochs at an order of magnitude higher temperature ($\sim5$\,keV), as compared to the contemporaneous low energy data (Bellm et al. 2014). We resolve this apparent contradiction by demonstrating that a model with two black bodies and a power-law (2BBPL) is consistent with the data right from the late prompt emission to the afterglow phase. Both the blackbodies show a similar cooling behaviour upto the late time. We invoke a structured jet, having a fast spine and a slower sheath layer, to identify the location of these blackbodies. Independent of the physical interpretation, we propose that the 2BBPL model is a generic feature of the prompt emission of all long GRBs, and the thermal emission found in the afterglow phase of different GRBs reflects the lingering thermal component of the prompt emission with diverse time-scales. We strengthen this proposal by pointing out a close similarity between the spectral evolutions of this GRB and GRB~090618, a source with significant wide band data during the early afterglow phase.