Monte Carlo simulations of relativistic radiation-mediated shocks - I. Photon-rich regime

Hirotaka Ito, Amir Levinson, Boris E. Stern, Shigehiro Nagataki

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We explore the physics of relativistic radiation-mediated shocks (RRMSs) in the regime where photon advection dominates over photon generation. For this purpose, a novel iterative method for deriving a self-consistent steady-state structure of RRMS is developed, based on a Monte Carlo code that solves the transfer of photons subject to Compton scattering and pair production/annihilation. Systematic study is performed by imposing various upstream conditions which are characterized by the following three parameters: the photon-to-baryon inertia ratio ξ u*, the photon-to-baryon number ratio ñ, and the shock Lorentz factor γ u. We find that the properties of RRMSs vary considerably with these parameters. In particular, while a smooth decline in the velocity, accompanied by a gradual temperature increase is seen for ξ uz.ast; »1, an efficient bulk Comptonization, that leads to a heating precursor, is found for ξ u* ≲ 1. As a consequence, although particle acceleration is highly inefficient in these shocks, a broad non-thermal spectrum is produced in the latter case. The generation of high-energy photons through bulk Comptonization leads, in certain cases, to a copious production of pairs that provide the dominant opacity for Compton scattering. We also find that for certain upstream conditions a weak subshock appears within the flow. For a choice of parameters suitable to gamma-ray bursts, the radiation spectrum within the shock is found to be compatible with that of the prompt emission, suggesting that subphotospheric shocks may give rise to the observed non-thermal features despite the absence of accelerated particles.

Original languageEnglish
Pages (from-to)2828-2851
Number of pages24
JournalMonthly Notices of the Royal Astronomical Society
Issue number2
StatePublished - 21 Feb 2018


  • Gamma-ray burst: general
  • Plasmas
  • Radiation mechanisms: non-thermal
  • Radiative transfer
  • Scattering
  • Shock waves


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