# Investigating subphotospheric dissipation in gamma-ray bursts using joint Fermi-Swift observations

## TL;DR

This study tests a subphotospheric dissipation model for gamma-ray burst emission using joint Fermi and Swift data, finding it explains half of the spectra and suggesting model improvements like including magnetization.

## Contribution

It introduces a Bayesian analysis of a localized subphotospheric dissipation model with joint Fermi-Swift data, providing tighter constraints and insights into GRB jet physics.

## Key findings

- 16 out of 32 spectra fit the model well
- Joint fits reduce estimates of Lorentz factor and luminosity
- Model fit quality suggests need for including magnetization

## Abstract

The jet photosphere has been proposed as the origin for the gamma-ray burst (GRB) prompt emission. In many such models, characteristic features in the spectra appear below the energy range of the $\textit{Fermi}$ GBM detectors, so joint fits with X-ray data are important in order to assess the photospheric scenario. Here we consider a particular photospheric model which assumes localized subphotospheric dissipation by internal shocks in a non-magnetized outflow. We investigate it using Bayesian inference and a sample of 8 GRBs with known redshifts which are observed simultaneously with $\textit{Fermi}$ GBM and $\textit{Swift}$ XRT. This provides us with an energy range of $0.3$~keV to $40$~MeV and much tighter parameter constraints. We analyze 32 spectra and find that 16 are well described by the model. We also find that the estimates of the bulk Lorentz factor, $\Gamma$, and the fireball luminosity, $L_{0,52}$, decrease while the fraction of dissipated energy, $\varepsilon_{\mathrm{d}}$, increase in the joint fits compared to GBM only fits. These changes are caused by a small excess of counts in the XRT data, relative to the model predictions from fits to GBM only data. The fact that our limited implementation of the physical scenario yields 50\% accepted spectra is promising, and we discuss possible model revisions in the light of the new data. Specifically, we argue that the inclusion of significant magnetization, as well as removing the assumption of internal shocks, will provide better fits at low energies.