Shock waves and gamma-ray bursts from neutron star mergers

Neutron star merger GW 170817 was accompanied by a gamma-ray burst with luminosity L∼1047 erg/s, coming with a ~2 s delay after the gravitational waves. Observations also indicate the presence of a dark, ultra-relativistic envelope ejected promptly by the merger, before the GRB emission. A plausible source of gamma-rays is a shock wave emerging at the photosphere of the envelope. I will discuss the envelope formation, the shock physics, the mechanism of gamma-ray emission, and its first-principle simulation. The simulation gives a GRB with a light curve and spectral evolution in agreement with observations of GW 170817.