# New paper: Radiation mediated shocks in gamma-ray bursts: Pair creation

Published Aug 30, 2017

Sub-photospheric shock dissipation is one of the main proposed mechanisms for producing the prompt gamma-ray burst (GRB) emission. Such shocks are mediated by scattering of radiation. We introduce a time dependent, special relativistic code which dynamically couples Monte Carlo radiative transfer to the flow hydrodynamics. The code also self-consistently implements electron-positron pair production and annihilation. We simulate shocks with properties relevant for GRBs and study the steady-state solutions, which are accurate deep below the jet photosphere. The shock generates a power-law photon spectrum through the first-order Fermi mechanism, extending upwards from the typical upstream photon energy. Strong shocks (for which the downstream pressure is much larger than the upstream pressure) have rising $$\nu F_\nu$$ shock spectra. The spectrum extends up to $$\epsilon_\mathrm{max}\equiv E_\mathrm{max}/m_ec^2\sim v^2$$ for non-relativistic shocks, where $$m_e$$ is the electron rest mass and $$v$$ is the relative speed between the upstream and downstream in units of the speed of light $$c$$. For mildly relativistic shocks the power law softens at $$\epsilon\geq10^{-1}$$ due to Klein-Nishina effects, and shocks with $$v\gamma\ge1$$, where $$\gamma\equiv(1-v^2)^{-1/2}$$, produce electron-positron pairs. As an example, a strong shock with $$v\gamma=3$$ and a photon-to-proton ratio of $$n_\gamma/n_p=2\times10^5$$ has a peak pair-to-proton ratio of $$Z_\pm\approx225$$. The main effect of pairs in a steady-state shock is to decrease its spatial width by a factor of $$\sim Z_\pm$$. The post-shock spectrum thermalizes in the downstream. In absence of emission and absorption processes, kinetic equilibrium at temperature $$\theta_d\equiv kT_d/m_ec^2\approx \epsilon_d/3$$ is reached at an optical depth of $$\tau\gg\theta_d^{-1}$$ behind the shock, where $$\epsilon_d$$ is the average downstream photon energy. The paper on arXiv.