Thermal Emission from short GRB jets breaking out of binary neutron star merger environments: Relativistic hydrodynamic simulations with M1 radiation transport

In August 2017, the combined detection of the gravitational wave signal GW170817 and its electromagnetic (EM) counterparts, including in particular the high-energy burst GRB 170817A, confirmed the canonical scenario according to which short-hard gamma-ray bursts (SGRBs) are produced as a consequence of the merger of two neutron stars in a binary system. This also proved that the remnant object formed in a binary neutron star (BNS) merger is able to launch a powerful relativistic jet, which is a necessary ingredient to produce a SGRB. Moreover, this event was observed from a viewing angle between about 15 and 30 degrees away from the main jet propagation (or remnant spin) axis, resulting in the very first observation of a SGRB signal along a direction outside the narrow cone of the jet core. At such viewing angles, the so-called prompt SGRB emission is likely dominated by the flash of radiation that accompanies the shock breakout induced by the incipient jet piercing through the surrounding BNS merger material. A proper modelling of shock breakout signals in such context, which is still missing, represents then a key ingredient for the interpretation of the August 2017 event. This Thesis presents a theoretical study of SGRB jets breaking out of the environment surrounding a BNS merger and the corresponding burst of radiation as observed at generic viewing angles. The work, based on the open-source numerical code PLUTO, combines for the first time special relativistic hydrodynamic simulations of SGRB jets emerging from a realistic BNS merger environment with a recently developed two-moment scheme for treating photon radiation transport. The scope of the Thesis is to demonstrate the approach and provide a first set of results based on a fiducial SGRB jet model. The emerging shock-breakout signals obtained here implicitly assume that radiation is essentially thermal, which represents a very first step towards a more detailed investigation where non-thermal radiation processes are also consistently included.