The multidimensional structure of radiative shocks: Suppressed thermal X-rays and relativistic ion acceleration

Radiative shocks, behind which gas cools faster than the dynamical time, play a key role in many astrophysical transients, including classical novae and young supernovae interactingwith circumstellar material. The dense layer behind high Mach number M ≫ 1 radiative shocks is susceptible to thin-shell instabilities, creating a 'corrugated' shock interface. We present 2D and 3D hydrodynamical simulations of optically thin radiative shocks to study their thermal radiation and acceleration of non-thermal relativistic ions.We employ amoving-mesh code and a specialized numerical technique to eliminate artificial heat conduction across grid cells. The fraction of the shock's luminosity Ltot radiated at X-ray temperatures kT_sh ≈ (3/16)μm_pv²_sh expected from a 1D analysis is suppressed by a factor L(> T_sh/3)/L_tot ≈ 4.5/M^4/3 for M ≈ 4-36. This suppression results in part from weak shocks driven into underpressured cold filaments by hot shocked gas, which sap thermal energy from the latter faster than it is radiated. Combining particle-in-cell simulation results for diffusive shock acceleration with the inclination angle distribution across the shock (relative to an upstream magnetic field in the shock plane - the expected geometry for transient outflows), we predict the efficiency and energy spectrum of ion acceleration. Though negligible acceleration is predicted for adiabatic shocks, the corrugated shock front enables local regions to satisfy the quasi-parallel magnetic field geometry required for efficient acceleration, resulting in an average acceleration efficiency of ε_nth ~ 0.005-0.02 for M ≈ 12-36, in agreement with modelling of the gamma-ray nova ASASSN-16ma.