Simulations of coalescing neutron star and black hole binaries

Abstract

We investigate the dynamics and evolution of merging neutron stars, of neutron stars coalescing with stellar-mass black holes, and the formation and properties of accretion tori around the (remnant) black holes. The three-dimensional Newtonian hydrodynamics equations are integrated by a Eulerian PPM code on four nested Cartesian grids. The code includes the emission and backreaction of gravitational waves as well as a physical nuclear equation-of-state [Lattimer & Swesty 1991] and the neutrino emission from the hot matter. Lepton number and energy losses of the gas due to neutrino emission are treated by an elaborate neutrino leakage scheme which takes into account neutrinos and antineutrinos of all flavors. Neutrino-antineutrino annihilation in the vicinity of the merger is evaluated in a post-processing step. The gravity of the black hole is described with a Newtonian or, alternatively, with a Paczyński-Wiita potential. The hydrodynamic effect of the black hole is simulated by extracting all matter that flows into a sphere with radius equal to the Schwarzschild radius of the black hole or, in accretion simulations, twice the Schwarzschild radius. The NS/NS and BH/NS merging models yield information about gravitational wave and neutrino emission. Also, they allow us to determine the physical properties (masses, densities, temperatures, estimated life times) of the remnants of the mergers: a black hole surrounded by an accretion torus. In the NS/NS case, a thick disk with a mass around 0.1 M⊙ forms after the massive and very compact central object that contains most of the mass of the merged neutron stars, has collapsed to a black hole, presumably on a dynamical time scale. The dynamics of BH/NS mergers is very sensitive to the neutron star to black hole mass ratio. For low ratios the neutron star transfers mass to the black hole during a few cycles of orbital decay and subsequent widening before finally being disrupted, whereas for ratios near unity the neutron star is already destroyed during its first approach. A gas mass of about 0.5 M⊙ is left in an accretion torus around the black hole. The accretion tori radiate neutrinos at luminosities of up to several 1053 erg/s. Higher luminosities are found for larger disk masses and smaller black holes. The emitted neutrinos and antineutrinos annihilate into e± pairs with efficiencies of a few percent and rates of up to ̃ 2 × 1052 erg/s, releasing an estimated energy of up to ̃ 1051 erg in a pair-plasma fireball. The torus geometry favors relativistic expansion of the pair plasma along the baryon depleted system axis. The occurrence of moderately beamed jets with opening angles of several ten degrees is therefore likely. The jet energies and short accretion times of the tori of about 0.1 s are in the right range to account for the subclass of short and less energetic gamma-ray bursts.