Mergers of Neutron Star-Black Hole Binaries with Small Mass Ratios: Nucleosynthesis, Gamma-Ray Bursts, and Electromagnetic Transients

We discuss simulations of the coalescence of black hole-neutron star binary systems with black hole masses between 14 and 20 M_☉. The calculations use a three-dimensional smoothed particle hydrodynamics code, a temperature-dependent, nuclear equation of state, and a multiflavor neutrino scheme. General relativistic effects are mimicked using the Paczyński-Wiita pseudo-potential and gravitational radiation reaction forces. Unlike previous, purely Newtonian calculations, in none of the explored cases does episodic mass transfer occur. The neutron star is always completely disrupted after most of its mass has been transferred directly into the hole. For black hole masses between 14 and 16 M_☉ an accretion disk forms; large parts of it, however, are inside the last stable orbit and therefore falling with large radial velocities into the hole. These disks are (opposite to the neutron star merger case) thin and—except for a spiral shock—essentially cold. For higher mass black holes (M_BH ≥ 18 M_☉) almost the entire neutron star disappears in the hole without forming an accretion disk. In these cases the surviving material is spun up by tidal torques and ejected as a half-ring of neutron-rich matter. None of the investigated systems is a promising gamma-ray burst (GRB) central engine. We find between 0.01 and 0.2 M_☉ of the neutron star to be dynamically ejected. Like in a Type Ia supernova, the radioactive decay of this material powers a light curve with a peak luminosity of a few times 10⁴⁴ ergs s^-1. The maximum is reached about 3 days after the coalescence and is mainly visible in the optical/near-infrared band. The coalescence itself may produce a precursor pulse with a thermal spectrum of ~10 ms duration.