Prompt Mergers of Neutron Stars with Black Holes

Mergers of neutron stars with black holes have been suggested as candidates for short gamma-ray bursts. They have also been studied for their potential as gravitational wave sources observable with ground-based detectors. For these purposes, it is important to know under what circumstances such a merger could leave an accretion disk or result in a period of stable mass transfer. We show that, consistent with recent numerical simulations, it is expected that mergers between neutron stars and black holes will be prompt, with no accretion disk and no stable mass transfer, if the black hole has a mass greater than that of the neutron star and is spinning slowly. The reason is that for comparable masses, angular momentum loss to gravitational radiation starts a plunge orbit well outside the innermost stable circular orbit, causing direct merging rather than extended mass transfer. Even when the black hole is spinning rapidly and exactly prograde with respect to the orbit, we show that it is possible within current understanding that no accretion disk will form under any circumstances, but resolution of this will require full general relativistic numerical simulations with no approximations.