A cocoon shock breakout as the origin of the γ -ray emission in GW170817

The short Gamma-Ray Burst, GRB 170817A, that followed the binary neutron star merger gravitational waves signal, GW170817, is not a usual short gamma-ray burst (sGRB). It is weaker by three orders of magnitude than the weakest sGRB seen before and its spectrum, showing a hard early signal followed by a softer thermal spectrum, is unique. We show, first, that the γ -rays must have emerged from at least mildly relativistic outflow, implying that a relativistic jet was launched following the merger. We then show that the observations are consistent with the predictions of a mildly relativistic shock breakout: a minute γ -ray energy as compared with the total energy and a rather smooth light curve with a hard to soft evolution.We present here a novel analytic study and detailed numerical 2D and 3D relativistic hydrodynamic and radiation simulations that support the picture in which the observed γ -rays arose from a shock breakout of a cocoon from the merger's ejecta. The cocoon can be formed either by a choked jet which does not generate an sGRB (in any direction) or by a successful jet which generates an undetected regular sGRB along the system's axis pointing away from us. Remarkably, for the choked jet model, the macronova signal produced by the ejecta (which is partially boosted to high velocities by the cocoon's shock) and the radio that is produced by the interaction of the shocked cocoon material with the surrounding matter agree with the observed ultraviolet/optical/infrared emission and with current radio observations. Finally, we discuss the possibility that the jet propagation within the ejecta may photodissociate some of the heavy elements and may affect the composition of a fraction of ejecta and, in turn, the opacity and the early macronova light.