Gamma-Ray Bursts with Thermal Signature Observed with FERMI/GBM

In the fireball model of GRBs, 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 and of GRB 110721A, 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 thermal-candidate GRBs and GRB 110721A, and show the evolution of the thermal and non-thermal components within these bursts.