The Multi-Dimensional Structure of Radiative Shocks: Suppressed Thermal X-rays and Relativistic Ion Acceleration

TL;DR

This paper investigates radiative shocks in astrophysical phenomena, revealing suppressed X-ray emission due to thin-shell instabilities and demonstrating how shock corrugation facilitates relativistic ion acceleration, with implications for gamma-ray observations.

Contribution

It presents detailed hydrodynamical simulations of radiative shocks, quantifies X-ray suppression, and combines particle-in-cell results to predict ion acceleration efficiency in astrophysical shocks.

Findings

X-ray luminosity is suppressed by a factor of approximately 4.5/M^{4/3} for Mach numbers 4-36.

Weak shocks driven into cold filaments drain thermal energy faster than it radiates.

Shock corrugation enables efficient relativistic ion acceleration in certain regions.

Abstract

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 interacting with circumstellar material. The dense layer behind high Mach number $\mathcal{M} \gg 1$ radiative shocks is susceptible to thin-shell instabilities, creating a "corrugated" shock interface. We present two and three-dimensional hydrodynamical simulations of optically-thin radiative shocks to study their thermal radiation and acceleration of non-thermal relativistic ions. We employ a moving-mesh code and a specialized numerical technique to eliminate artificial heat conduction across grid cells. The fraction of the shock's luminosity $L_{\rm tot}$ radiated at X-ray temperatures $kT_{\rm sh} \approx (3/16)\mu m_p v_{\rm sh}^{2}$ expected from a one-dimensional analysis is suppressed by a factor $L(>T_{\rm sh}/3)/L_{\rm tot} \approx 4.5/\mathcal{M}^{4/3}$ for $\mathcal{M} \approx 4-36$. This suppression results in part from weak shocks driven into under-pressured 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 $\epsilon_{\rm nth} \sim 0.005-0.02$ for $\mathcal{M} \approx 12-36$, in agreement with modeling of the gamma-ray nova ASASSN-16ma.