Evidence for a Photospheric Component in the Prompt Emission of the Short GRB120323A and its Effects on the GRB Hardness-Luminosity Relation

Author: Sylvain Guiriec

Abstract: GRB 120323A, is a very bright, short, Gamma-Ray Burst (GRB) with unusually soft prompt emission spectra when fitted with the empirical Band function alone. However, when the spectra are fitted with a Band function and a thermal-like component, we find that the Band function parameters are more typical to what is expected for such an intense short event. While the single component fits render values of peak energies (Epeak) and low energy power law indices (alpha), which exhibit a somehow erratic behavior difficult to interpret with the prevalent theoretical models, the two component fits shape are consistent with an interpretation of a simultaneous detection of photospheric (thermal-like) emission in addition to the non-thermal synchrotron emission. Both emissions are predicted in the popular fireball model. Here we compare the single- and two-component scenarios for GRB 120323A as well as the evolution of the parameters of both the thermal and non-thermal contributions. Using a larger sample of GRBs for which a thermal and a non-thermal component are also simultaneously identified, we show the existence, within each GRB, of a strong correlation between the flux of the Band function (i.e., non-thermal emission) and the nuFnu peak energy (Epeak) of this function (i.e., hardness of the non-thermal spectrum). This flux-hardness relation, which seems to be intrinsic to the non-thermal emission mechanism, is adequately fitted with a PL, whose slope is similar for the various bursts tested. Finally, this flux-hardness relation may represent a universal luminosity-hardness relation predicting GRB distances, and thus making GRBs possible cosmological probes.