Monte Carlo simulations of Photospheric emission in relativistic outflows

Abstract

We study the spectra of photospheric emission from highly relativistic gamma-ray burst outflows using a Monte Carlo (MC) code. We consider the Comptonization of photons with a fast cooled synchrotron spectrum in a relativistic jet with photon to electron number ratio $N_{\gamma}/N_e = 10^5$. For all our simulations, we use mono-energetic protons which interact with thermalised electrons through the Coulomb interaction. The photons, electrons and protons are cooled adiabatically as the jet expands outwards. We find that the initial energy distribution of the protons and electrons do not have any appreciable effect on the photon peak energy and the power-law spectrum above the peak energy. We also find that the Coulomb interaction between the electrons and the protons does not affect the output photon spectrum significantly as the energy of the electrons is elevated only marginally. The peak energy and the spectral indices for the low and high energy power-law tails of the photon spectrum remain practically unchanged even in the presence of electron-proton coupling. Increasing the initial optical depth $\tau_{in}$ results in shallower photon spectrum below the peak energy ($f_{\nu} \propto \nu^{1.1}$ for $\tau_{in} = 2$ to $f_{\nu} \propto \nu^{0.3}$ for $\tau_{in} = 16$) and fewer photons at the high-energy tail, although $f_{\nu} \propto \nu^{-0.5}$ above the peak energy up to $\sim 1$ MeV, independent of $\tau_{in}$. The peak energy of the seed photon spectrum $E_{\gamma,peak}$ determines the peak energy and the shape of the output photon spectrum. Lastly, we find that our simulation results are quite sensitive to $N_{\gamma}/N_e$, for $N_{e} = 10^3$. For almost all our simulations, we obtain an output photon spectrum with power-law tail above $E_{\gamma,peak}$ extending up to $\sim 1$ MeV.