Dynamics and stability of relativistic GRB blast waves

TL;DR

This study uses high-resolution 2D relativistic hydro-simulations to analyze the stability and deceleration of GRB blast waves, revealing transient instabilities during the relativistic phase that fade in the nonrelativistic stage.

Contribution

It provides detailed insights into the stability properties and instability growth of relativistic GRB blast waves during their entire afterglow phase.

Findings

Relativistic phase exhibits pressure-ram pressure instabilities.

Instabilities decay as the blast wave transitions to nonrelativistic phase.

Shell spreading and stratification occur due to rarefaction waves.

Abstract

In gamma-ray-bursts (GRB), ultra-relativistic blast waves are ejected into the circumburst medium. We analyse in unprecedented detail the deceleration of a self-similar Blandford-McKee blast wave from a Lorentz factor 25 to the nonrelativistic Sedov phase. Our goal is to determine the stability properties of its frontal shock. We carried out a grid-adaptive relativistic 2D hydro-simulation at extreme resolving power, following the GRB jet during the entire afterglow phase. We investigate the effect of the finite initial jet opening angle on the deceleration of the blast wave, and identify the growth of various instabilities throughout the coasting shock front. We find that during the relativistic phase, the blast wave is subject to pressure-ram pressure instabilities that ripple and fragment the frontal shock. These instabilities manifest themselves in the ultra-relativistic phase alone, remain in full agreement with causality arguments, and decay slowly to finally disappear in the near-Newtonian phase as the shell Lorentz factor drops below 3. From then on, the compression rate decreases to levels predicted to be stable by a linear analysis of the Sedov phase. Our simulations confirm previous findings that the shell also spreads laterally because a rarefaction wave slowly propagates to the jet axis, inducing a clear shell deformation from its initial spherical shape. The blast front becomes meridionally stratified, with decreasing speed from axis to jet edge.