Abstract Details
| Name: Rupal Basak Affiliation: Nicolaus Copernicus Astronomical Center,Warsaw Conference ID: ASI2015_572 Title : The prompt emission of gamma-ray bursts: signature of the thermal component Authors and Co-Authors : A. R. Rao Tata Institute of Fundamental Research Abstract Type : Oral Abstract Category : Extragalactic astronomy Abstract : Gamma-ray bursts (GRBs) are explosive events possibly marking the collapse of massive stars or merging of compact objects which lead to the formation of compact stellar-mass objects. The compact object acting as a central engine, drives a bipolar relativistic jet. The initial burst, known as prompt emission lasts for $sim$ms (short-GRBs) to tens of seconds (long-GRBs), and shows rapid variability. This is followed by a longer and smoother afterglow phase. Understanding the radiation mechanism particularly that of the prompt phase holds the key to understand the central engine. It is believed that the radiation during the prompt emission is an optically thin synchrotron radiation. However, there are now mounting evidences of a thermal emission along with the non-thermal component. Our discussion will include the following. - Study the shape and evolution of the thermal emission in bright GRBs. Particularly, we will show that the thermal emission consists of two smoothly evolving correlated blackbodies (Basak \& Rao 2013, ApJ 768, 187; Rao, Basak \& Bhattacharya et al. 2014, RAA 14, 35). - GRB~090618 is one rare case where we find a simultaneous Swift/BAT and Swift/XRT observation. The XRT, being a focusing X-ray detector, has an order of magnitude better energy resolution than the BAT. We do identify the two evolving blackbodies in the simultaneous data (Basak \& Rao 2014a, ApJ, submitted). Our finding is reinforced by a similar observation in GRB~130925A (Basak \& Rao 2014b, ApJ, submitted). Thanks to the ultra-long duration of the prompt phase (2 hours), this burst could be studied with high resolution detectors of NuSTAR and Chandra. The data analysis again reveals the presence of the two blackbodies, and they fairly extend till the late afterglow phase. - A comparison of the spectral evolution of the two GRBs shows a remarkable similarity with different time scales. We suggest that the spectral evolution seen in both these GRBs is a tail emission of the late prompt phase rather than the early afterglow phase. The longer time scale of GRB~130925A is possibly due to a larger progenitor, and a fainter afterglow. This is supported by the finding of low density environment of the burst. We suggest that a fast spine with a slow sheath structure of GRB jet is responsible for all the spectral features we obtain in our analysis. - Finally, we shall discuss the possible future projects e.g., analysis of long-GRBs with overlapping BAT-XRT observation, exploring other physical models, detailed analysis of the prompt, afterglow and burst environment of a few representative GRBs of different classes, and finding the signature of the different environments on the spectrum. |