Thermalisation of radiation-mediated-shock emission in gamma-ray bursts

Sammanfattning

Gamma-ray bursts (GRBs) are among the most luminous events in the Universe. Their prompt emission is thought to be shaped by dissipation via radiation-mediated shocks (RMS) occurring below the photosphere. In the RMS scenario, whether the emerging photospheric spectrum appears thermal or non-thermal depends on whether the radiation has time to thermalise before reaching the photosphere. In this project, I define the thermalisation depth τth as the depth at which the observed spectrum becomes indistinguishable from that of a non-dissipative photosphere (NDP). This depth is obtained from the thermalisation parameter Π∗ , which varies with the properties of the shocks. Photospheric emission spectra have been simulated using the Kompaneets RMS Approximation (KRA) code, developed to fit GRB spectra significantly faster than previous RMS models. The resulting parameter-space mapping provides a global understanding of how GRB spectral shapes relate to the thermalisation parameter. The method uses likelihood-ratio tests to compare NDP and KRA models, and also studies how the signal-to-noise ratio affects the inferred thermalisation parameter. The main result is a practical method to infer Π∗ for observed GRB prompt-emission spectra and to compare the corresponding thermalisation depth with the shock depth. The thermalisation parameter Π∗ can be constrained to a precision of ±5 for typical values in the range 5-50, within the validity domain of the KRA parameters. This validity domain defines the conditions under which the developed method can be applied.