paper

Surprise in simplicity: an unusual spectral evolution of a single pulse GRB 151006A

arXiv:1707.09924 · doi:10.1093/mnras/stx1970

Abstract

We present a detailed analysis of GRB 151006A, the first GRB detected by Astrosat CZT Imager (CZTI). We study the long term spectral evolution by exploiting the capabilities of \emph{Fermi} and \emph{Swift} satellites at different phases, which is complemented by the polarization measurement with the CZTI. While the light curve of the GRB in different energy bands show a simple pulse profile, the spectrum shows an unusual evolution. The first phase exhibits a hard-to-soft (HTS) evolution until \,s, followed by a sudden increase in the spectral peak reaching a few MeV. Such a dramatic change in the spectral evolution in case of a single pulse burst is reported for the first time. This is captured by all models we used namely, Band function, Blackbody+Band and two blackbodies+power law. Interestingly, the \emph{Fermi} Large Area Telescope (LAT) also detects its first photon (\,MeV) during this time. This new injection of energy may be associated with either the beginning of afterglow phase, or a second hard pulse of the prompt emission itself which, however, is not seen in the otherwise smooth pulse profile. By constructing Bayesian blocks and studying the hardness evolution we find a good evidence for a second hard pulse. The \emph{Swift} data at late epochs ( of the GRB) also shows a significant spectral evolution consistent with the early second phase. The CZTI data (100--350\,keV), though having low significance (), show high values of polarization in the two epochs ( to ), in agreement with our interpretation.

arXiv comment13 pages, accepted for publication in MNRAS

References in corpus (67)

Cited by in corpus (5)