Abstract
The nature of the prompt emission mechanism in gamma-ray bursts (GRBs) remains uncertain to date. This question is also tied to the composition of the jet: thermal, Poynting flux dominated or hybrid with both types of components. In this work, we aim to study these questions in the context of GRB 130518A. By analysing its prompt emission data from the Fermi Gamma-ray Burst Monitor (GBM) and Large Area Telescope (LAT), we find that the time-integrated spectrum has a nonthermal component along with a subdominant blackbody component. We use these results to study the properties of the jet in various scenarios through the frameworks developed by Hascoët et al. and Gao & Zhang. Both frameworks exclude the pure fireball model for this GRB. In all other cases, the initial magnetic fraction is greater than the thermal fraction. For small launching radii of the jet, the favored nonthermal emission process is internal shocks. Magnetic reconnection seems likely only for very large initial radii.
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