Towards modelling the central engine of short GRBs

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 gamma-ray bursts (sGRBs). 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 sGRB 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 sGRB engine. In this paper we summarize our recent results from fully general-relativistic simulations of the coalescence of unequal-mass binary neutron stars, whose evolution 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. Our simulations show that large-scale, quasi-Keplerian tori with masses as large as ∼ 0.2M can be produced as the result of the inspiral and merger of binary neutron stars with unequal masses.

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