Spectral analysis of gamma ray bursts with thermal signature during their prompt phase

Official URL: http://risc01.sabanciuniv.edu/record=b1615045 (Table of Contents)

Gamma Ray Bursts are the most powerful physical phenomena observed in the Universe. A thermal-like photospheric emission originating from the region where relativistic outflow becomes optically thin to Thomson scattering, is expected in the Fireball Model in the gamma ray regime. Although most of the observed GRB prompt spectra have non-thermal characteristics, thermal components have been uncovered in some GRBs detected with Fermi Instrument. The shape and evolution of the thermal component, however, differs from burst to burst. To better understand how and when such photospheric emission emerges, it is crucial to identify more GRBs with this thermal signature. To this end, we performed a systematic time-resolved spectral analysis of 611 Fermi-GBM bursts which are detected between July-2008 and December-2010, with a hybrid model (thermal and non-thermal components.) We identified 11 GRBs (including four with thermal nature previously reported) with a strong statistical preference towards the hybrid model over a single non-thermal model. Here, we present time-resolved spectral analysis of the remaining 7 GRBs, the evolution of the thermal & non-thermal components within these bursts. We also discuss physical properties of the emission site deduced from the thermal component parameters.