Towards modelling the central engine of short GRBs

José A. Font, Luciano Rezzolla, Luca Baiotti, Bruno Giacomazzo, and David Link

Numerical relativity simulations of non-vacuum spacetimes have reached a status where a complete description of the inspiral, merger and post-merger stages of the late evolution of close binary neutron systems is possible. Determining the properties of the black-hole-torus system produced in such an event is a key aspect to understand the central engine of short-hard GRBs. Of the many properties characterizing the torus, the total rest-mass is the most important one, since it is the torus' binding energy which can be tapped to extract the large amount of energy necessary to power the GRB emission. In addition, the rest-mass density and angular momentum distribution in the torus also represent important elements which determine its secular evolution and need to be computed equally accurately for any satisfactory modelling of the GRB engine. As a first step towards modelling ab initio the central engine of SGRBs, we discuss in this talk new results from fully general-relativistic simulations of the coalescence of unequal-mass binary neutron stars. The evolution of the stars is followed through the inspiral phase, the merger and prompt collapse to a black hole, up until the appearance of a thick accretion disk, which is studied as it enters a regime of quasi-steady accretion. Although a simple ideal-fluid equation of > state is used, this work presents a systematic study within a general relativistic framework of the properties of the resulting black-hole-torus system produced by the merger of unequal-mass neutron star binaries.