Two distinct phases in the first 13 seconds of GRB110731A prompt emission

Abstract

In this work, the time-resolved BAT/GBM/LAT joint spectral analysis of GRB110731A during the prompt phase from the GBM trigger and up to 13 seconds later showed that, at the very early phase of prompt emission, the emission mechanism is closest to the standard fireball model. This model over-predicts the thermal photospheric emission and used to contradict observations. Lightcurves at different energy bands revealed two distinguishable phases that may come from different regions. First, we have an early phase, which is not detected by LAT, and is dominated by lower energies, which arises from the photospheric emissions without any emissions involved in dissipation mechanisms and characterized by low Lorentz factor and high radiation efficiency. This is followed by a later phase, having a more complex structure that remarkably follows the same track in all energy bands and is attributed to emissions from internal shocks. This burst is a good candidate to study both thermal and non-thermal emissions, since the two phases can be clearly separated in lightcurve and spectrum. The rapid variation of Lorentz factor and the values of photospheric radii, which are relatively far away from the central engine in Phase 2, are more consistent with the mechanism of collisional heating in baryonic jets. Further information can be obtained by combining more wavelengths with the help of the other detectors.