Classification of Photospheric Emission in Short Gamma-ray Bursts

  Hüsne Dereli-Bégué  ,  Asaf Pe’er [1]  ,  Felix Ryde[3]  
[1] Department of Physics, Bar-Ilan University, Ramat-Gan 52900, Israel
[2] Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, D-85748 Garching, Germany
[3] Department of Physics, KTH Royal Institute of Technology, and The Oskar Klein Centre, SE-106 91 Stockholm, Sweden

In this talk, I will briefly introduce the classification of emission mechanisms observed in the gamma-ray bursts (GRBs). Then I will focus on the photospheric emission observed in the short GRBs. In our work, we find that about 1/3 of the short GRBs are consistent with a pure, non-dissipative photospheric model, at least around the peak of the pulse. For these bursts, we find (i) a bimodal distribution in the values of the Lorentz factors and the hardness ratios and (ii) an anti-correlation between burst duration,T_90 and the peak energy, E_pk. Our results thus imply that the short GRB population may in fact be composed of two separate populations: one that is a continuation of the long GRB population to shorter durations, and another that is distinctly separate with different physical properties. Our results also show that, thermal emission is initially ubiquitous, but is accompanied at longer times by additional radiation (likely synchrotron).