RELATIVISTIC RADIATION MEDIATED SHOCKS

The structure of relativistic radiation mediated shocks (RRMSs) propagating into a cold electron-proton plasma is calculated and analyzed. A qualitative discussion of the physics of relativistic and non-relativistic shocks, including order of magnitude estimates for the relevant temperature and length scales, is presented. Detailed numerical solutions are derived for shock Lorentz factors ? u in the range 6 ? ? u ? 30, using a novel iteration technique solving the hydrodynamics and radiation transport equations (the protons, electrons, and positrons are argued to be coupled by collective plasma processes and are treated as a fluid). The shock transition (deceleration) region, where the Lorentz factor ? drops from ? u to ~1, is characterized by high plasma temperatures T ~ ?mec 2 and highly anisotropic radiation, with characteristic shock-frame energy of upstream (US) and downstream (DS) going photons of a few ? mec 2 and ~?2 mec 2, respectively. Photon scattering is dominated by e ? pairs, with the pair-to-proton density ratio reaching 102? u . The width of the deceleration region, in terms of Thomson optical depths for US-going photons, is large, ?? ~ ?2 u (?? ~ 1 neglecting the contribution of pairs) due to Klein-Nishina suppression of the scattering cross section. A high-energy photon component, narrowly beamed in the DS direction, with a nearly flat power-law-like spectrum, ?I ? ?0, and an energy cutoff at ~?2 u mec 2 carries a fair fraction of the energy flux at the end of the deceleration region. An approximate analytic model of RRMS, reproducing the main features of the numerical results, is provided.