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Abstract: In gamma-ray-bursts GRB, ultra-relativistic blast waves are ejected intothe circumburst medium. We analyse in unprecedented detail the deceleration ofa self-similar Blandford-McKee blast wave from a Lorentz factor 25 to thenonrelativistic Sedov phase. Our goal is to determine the stability propertiesof its frontal shock. We carried out a grid-adaptive relativistic 2Dhydro-simulation at extreme resolving power, following the GRB jet during theentire afterglow phase. We investigate the effect of the finite initial jetopening angle on the deceleration of the blast wave, and identify the growth ofvarious instabilities throughout the coasting shock front. We find that duringthe relativistic phase, the blast wave is subject to pressure-ram pressureinstabilities that ripple and fragment the frontal shock. These instabilitiesmanifest themselves in the ultra-relativistic phase alone, remain in fullagreement with causality arguments, and decay slowly to finally disappear inthe near-Newtonian phase as the shell Lorentz factor drops below 3. From thenon, the compression rate decreases to levels predicted to be stable by a linearanalysis of the Sedov phase. Our simulations confirm previous findings that theshell also spreads laterally because a rarefaction wave slowly propagates tothe jet axis, inducing a clear shell deformation from its initial sphericalshape. The blast front becomes meridionally stratified, with decreasing speedfrom axis to jet edge.



Author: Z. Meliani, R. Keppens

Source: https://arxiv.org/







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