Mergers of neutron star black hole binaries with small mass ratios: nucleosynthesis, gamma-ray bursts and electromagnetic transients

Abstract

I discuss simulations of the coalescence of black hole neutron star binary systems with black hole masses between 14 and 20 \msun. The calculations use a three-dimensional smoothed particle hydrodynamics code, a temperature-dependent, nuclear equation of state and a multi-flavor neutrino scheme. General relativistic effects are mimicked using the \Pacz-Wiita pseudo-potential and gravitational radiation reaction forces. Opposite to previous, purely Newtonian calculations, in none of the explored cases episodic mass transfer occurs. 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 \Msun 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 -apart from a spiral shock- essentially cold. For higher mass black holes ($M_{\rm BH} \ge 18$ \msun) almost the complete 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 GRB central engine. We find between 0.01 and 0.2 \Msun of the neutron star to be dynamically ejected. Like in a type Ia supernova, the radioactive decay of this material will power a light curve with a peak luminosity of a few times $10^{44}$ erg/s. The maximum will be reached about three days after the coalescence and will be mainly visible in the optical/near infrared band. The coalescence itself may produce a precursor pulse with a thermal spectrum of $\sim 10$ ms duration.