Photospheric emission from gamma-ray bursts altered by radiation-mediated shocks

QC 2026-01-14

Abstract

This thesis explores gamma-ray bursts (GRBs), and more specifically the prompt emission phase, which is the first ∼ 10 seconds of gamma-rays. GRBs come from the launching of a relativistic jet in connection with core-collapse supernovae or compact object (neutron star or black hole) mergers. The relativistic jet accelerates and eventually most of the energy is kinetic, and that energy is then somehow converted into internal energy that is emitted in gamma-rays, and is what we observe. The mechanism responsible for this conversion in the prompt phase is not fully understood and this thesis deals with one possible such mechanism, radiation-mediated shocks (RMSs). Such shocks occurring below the photosphere alters the photon spectral energy distribution, which is then released at the photosphere towards an observer. An analogue model of RMSs, called the Kompaneets RMS approximation (KRA) is discussed and then later applied in Paper I & II. In Paper I we generalize a method to measure the bulk outflow Lorentz factor based on the properties of photospheric emission and the evolution of the photon energy distribution in the jet. We find that depending on the quality of the data, either a value or an upper limit can be found for the Lorentz factor. In Paper II we do a time-resolved spectral analysis of GRB 211211A, a GRB with a broad spectrum containing two breaks, one in the tens of keV and one around a few MeV. Using the method presented in Paper I we find typical Lorentz factor values of ∼ 300. From the Lorentz factors and the KRA model we find the time evolution of the RMS parameters, here a strong shock occurring at moderate optical depths. We also show that the KRA model can fit these broad spectra with two breaks very well.

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