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Abstract: The structure of relativistic radiation mediated shocks RRMS propagatinginto a cold electron-proton plasma is calculated and analyzed. A qualitativediscussion of the physics of relativistic and non relativistic shocks,including order of magnitude estimates for the relevant temperature and lengthscales, is presented. Detailed numerical solutions are derived for shockLorentz factors $\Gamma u$ in the range $6\le\Gamma u\le30$, using a noveliteration technique solving the hydrodynamics and radiation transport equationsthe protons, electrons and positrons are argued to be coupled by collectiveplasma processes and are treated as a fluid. The shock transitiondeceleration region, where the Lorentz factor $ \Gamma $ drops from $\Gamma u $ to $ \sim 1 $, is characterized by high plasma temperatures $ T\sim\Gamma m ec^2 $ and highly anisotropic radiation, with characteristicshock-frame energy of upstream and downstream going photons of a few~$\times\,m ec^2$ and $\sim \Gamma^2 m ec^2$, respectively.Photon scattering is dominatedby e$^\pm$ pairs, with pair to proton density ratio reaching$\approx10^2\Gamma u$. The width of the deceleration region, in terms ofThomson optical depths for upstream going photons, is large,$\Delta\tau\sim\Gamma u^2$ $\Delta\tau\sim1$ neglecting the contribution ofpairs due to Klein Nishina suppression of the scattering cross section. A highenergy photon component, narrowly beamed in the downstream direction, with anearly flat power-law like spectrum, $ u I u\propto u^0$, and an energycutoff at $ \sim \Gamma u^2 m ec^2 $ carries a fair fraction of the energy fluxat the end of the deceleration region. An approximate analytic model of RRMS,reproducing the main features of the numerical results, is provided.



Author: Ran Budnik, Boaz Katz, Amir Sagiv, Eli Waxman

Source: https://arxiv.org/







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