Fitting a photospheric prompt emission model to GRB data: The Kompaneets RMS approximation (KRA)

Abstract: Gamma-ray bursts (GRBs) are some of the most energetic events in the universe. Shocks occurring below the photosphere are likely radiation mediated shocks (RMSs) and are suspected to shape the spectra. Due to computational costs of simulating RMSs, models had not been fitted to data and a faster model was needed. The Kompaneets RMS Approximation (KRA) is an analog model of RMSs, creating spectra which are identical to full RMS simulation spectra and it is significantly faster. For a sample of short GRBs I found that spectra are very hard and close to a non-dissipative photosphere (NDP). Therefore any trace of energy dissipation is lost to thermalization and many KRA solutions are possible with statistics similar to the Band model. A sample of long GRBs have typical spectra, i.e. spectra much broader than a blackbody (BB) and the KRA can model these spectra very well. Statistically, KRA is as good as the Band model and significantly better for GRB211211. I also found two spectral shapes describing observed data equally well. First, a broadened BB for a steady-state outflow, and secondly, optically shallow shocks for a non steady-state outflow. To distinguish between these solutions, upcoming detectors with lower-energy data are important. In conclusion, the KRA can reproduce photospheric spectra altered by a RMS, and these spectra can explain observed GRB spectra. If the spectrum is not thermalized, information about the properties of the shock can be deduced from observed GRB spectra.