Magnetized Shocks Mediated by Radiation from Leptonic and Hadronic Processes

TL;DR

This study models radiation-mediated shocks in astrophysical transients, examining how magnetic fields and non-thermal radiation influence shock structure and photon spectra, with implications for multi-messenger astrophysics.

Contribution

It introduces a comprehensive model coupling hydrodynamics with radiative transfer, accounting for particle acceleration and magnetic effects in relativistic shocks.

Findings

Synchrotron self-absorption significantly alters shock profiles for certain magnetizations.

A prominent subshock forms at higher magnetizations ($\sigma_u angle 0.1$).

High-energy photon tails are produced by proton-related radiative processes.

Abstract

Shocks in astrophysical transients are key sites of particle acceleration. If the shock upstream is optically thick, radiation smoothens the velocity discontinuity at the shock (radiation-mediated shocks). However, in mildly magnetized outflows, a collisionless subshock can form, enhancing the efficiency of particle acceleration. We solve the hydrodynamic equations of a steady-state, radiation-mediated shock together with the radiative transfer equations accounting for electron and proton acceleration. Our goal is to explore the impact of the magnetic field and non-thermal radiation on the shock structure and the resulting spectral distribution of photons. To this purpose, we assume a relativistic upstream fluid velocity ($\Gamma_u = 10$) and investigate shock configurations with variable upstream magnetization ($\sigma_u = 0$, $10^{-8}$, $10^{-4}$, $0.1$, and $0.3$). We find that synchrotron self-absorption alters the shock profile for $\sigma_u \gtrsim 10^{-8}$, with resulting changes up to $100\%$ in the bulk Lorentz factor at the shock; for $\sigma_u \gtrsim 0.1$, a prominent subshock forms. The spectral energy distributions of upstream- and downstream-going photons are also altered. Radiative processes linked to accelerated protons are responsible for a high-energy ($\gtrsim 10$ GeV) tail in the photon spectrum; however, the radiation flux and pressure are negligibly affected with consequent minor impact on the shock structure. Our work highlights the importance of coupling the shock hydrodynamics to the transport of photons, electrons, protons, and intermediate particles to forecast the multi-messenger emission from astrophysical transients.