Stability of Relativistic Blast Waves
  A spherical blast wave with relativistic velocity can be described by a similarity solution, that is used for theoretical models of gamma-ray bursts. We consider the linear stability of such a relativistic blast wave propagating into a medium with density gradient. The perturbation can also be expressed by a self-similar form. We show that the shock front is unstable in general, and we evaluate the growth rate. 
Jun Ogura, Yasufumi Kojima