Statistical Investigations ofthe Emission Processes in Gamma-ray Bursts

Abstract: Physical emission mechanisms responsible for gamma-ray bursts (GRBs) remainelusive to this day, 50 years after their discovery. Although there are well studiedphysical models, their power to explain the observed data is a matter of debate. Inthis thesis, the main focus is the statistical studies of the dierent physical modelsgiven the available data from the Fermi Gamma-Ray Space Observatory to makebetter comparisons between these models as well as ascertaining how well they canexplain the available observations so far. To this end, theoretically predicted thermaland non-thermal GRB spectra are investigated. This investigation entails bothnding groupings in the catalog data (clustering) and then simulating the expectedphysical emission processes to test how they would look like in the current dataacquiry, processing and tting procedures. Finally, a Bayesian model comparisonis performed in a sub-sample of these bursts to quantify the preference of dierentmodels by the data. In conclusion, it is found that around one third of all burstsinclude intervals where the emission is from a photosphere which is non-dissipative.This means that during these intervals, the emission is either emitted close to thesaturation radius or in a flow which is laminar. The results further indicate thatdissipation below the photosphere is responsible for the spectral shape in a majorityof GRB spectra. It is consequently argued that the dominant emission mechanismduring the prompt emission phase in GRBs is thermal emission from the jet photosphereat distance of around 1012 cm from the central engine. A small percentageof the bursts are better explained with a non-thermal generating process such asthe synchrotron emission.