Fermi Gamma-ray Space Telescope

Spectral Analysis of Gamma Ray Bursts with Thermal Signature During their Prompt Phase

A. M. Unsal
Y. Kaneko, S. Guiriec, E. Gogus

Abstract:

?In the fireball model of GRB, a thermal-like photospheric emission originating from the region where relativistic outflow becomes optically thin to Thomson scattering is expected in the gamma ray regime. Although most of the observed GRB prompt emission?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 10 GRBs (including three 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 Here, we present time-resolved spectral analysis of the 7 unpublished thermal-candidate?GRBs,?and show?the evolution? ?of the thermal and non-thermal components within these bursts, as well as the statistical significance of the thermal component?s?. We also estimate physical properties of the emission site deduced from the thermal component parameters.