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 nonthermal radiation on the shock structure and the resulting spectral distribution of photons. To this purpose, we assume a relativistic upstream fluid velocity (${\mathrm{\Gamma }}_u=10$) 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 ($\gtrsim 10\mathrm{GeV}$) 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 multimessenger emission from astrophysical transients.

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• Astrophysical fluid dynamics
• Collisionless shock in plasma
• Particle astrophysics