Title: Relativistic shock wave hydrodynamics and breakout

Abstract: Relativistic shock waves exist in various astrophysical environments, such as gamma-ray burst afterglows and the outflows of binary neutron star mergers. Detailed theoretical models for shock wave hydrodynamics and the radiation that accompanies shock breakout are crucial for a correct interpretation of observations, and can help uncover the properties of the exploding systems. In this talk I will discuss our hydrodynamic solution for the interiors of ultra-relativistic blast waves (Faran & Sari 2021).  Blandford & McKee (1976) find a very narrow region behind the shock where the flow is relativistic, while most of the volume is Newtonian. Therefore, the Blandford-McKee solution becomes invalid close to the shock. We obtain a self-similar solution that describes the fluid dynamics at the vicinity of the shock wave, at ultra-relativistic velocities, and all the way to the slow interiors of the explosion. We show that beyond the first scale height behind the shock, the velocity of the outflow does not depend on the shock Lorentz factor. I will then discuss our results on the breakout relativistic shock waves. The physics of shock breakout becomes much more complicated when temperatures behind the shock exceed 50 keV, allowing for the production of electron-positron pairs. When pairs dominate the plasma number density, they highly affect the shock structure, the luminosity and the spectrum of the observed emission when the shock breaks out. I will highlight applications of this work to low luminosity gamma ray bursts and to the gamma-ray flare that followed GW170817.