Energy-conserving, Relativistic Corrections to Strong Shock Propagation

Abstract

Astrophysical explosions are accompanied by the propagation of a shock wave through an ambient medium. Depending on the mass and energy involved in the explosion, the shock velocity $V$ can be non-relativistic ($V \ll c$, where $c$ is the speed of light), ultra-relativistic ($V \simeq c$), or moderately relativistic ($V \sim few\times 0.1c$). While self-similar, energy-conserving solutions to the fluid equations that describe the shock propagation are known in the non-relativistic (the Sedov-Taylor blastwave) and ultra-relativistic (the Blandford-McKee blastwave) regimes, the finite speed of light violates scale invariance and self-similarity when the flow is only mildly relativistic. By treating relativistic terms as perturbations to the fluid equations, here we derive the $\mathcal{O}(V^2/c^2)$, energy-conserving corrections to the non-relativistic, Sedov-Taylor solution for the propagation of a strong shock. We show that relativistic terms modify the post-shock fluid velocity, density, pressure, and the shock speed itself, the latter being constrained by global energy conservation. We derive these corrections for a range of post-shock adiabatic indices $\gamma$ (which we set as a fixed number for the post-shock gas) and ambient power-law indices $n$, where the density of the ambient medium $\rho_{\rm a}$ into which the shock advances declines with spherical radius $r$ as $\rho_{\rm a} \propto r^{-n}$. For Sedov-Taylor blastwaves that terminate in a contact discontinuity with diverging density, we find that there is no relativistic correction to the Sedov-Taylor solution that simultaneously satisfies the fluid equations and conserves energy. These solutions have implications for relativistic supernovae, the transition from ultra- to sub-relativistic velocities in gamma-ray bursts, and other high-energy phenomena.