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 Gamma(u) in the range 6 <= Gamma(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 Gamma drops from Gamma(u) to similar to 1, is characterized by high plasma temperatures T similar to Gamma m(e)c(2) and highly anisotropic radiation, with characteristic shock-frame energy of upstream (US) and downstream (DS) going photons of a few x m(e)c(2) and similar to Gamma(2)m(e)c(2), respectively. Photon scattering is dominated by e(perpendicular to) pairs, with the pair-to-proton density ratio reaching approximate to 10(2)Gamma(u). The width of the deceleration region, in terms of Thomson optical depths for US-going photons, is large, Delta tau similar to Gamma(2)(u) (Delta tau similar to 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,nu I-nu proportional to nu(0), and an energy cutoff at similar to Gamma(2)(u)m(e)c(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.