Merging white dwarf/black hole binaries and gamma-ray bursts

The merger of compact binaries, especially black holes and neutron stars, is frequently invoked to explain gamma-ray bursts (GRBs). In this paper, we present three-dimensional hydrodynamical simulations of the relatively neglected mergers of white dwarfs and black holes. During the merger, the white dwarf is tidally disrupted and sheared into an accretion disk. Nuclear reactions are followed, and the energy release is negligible. Peak accretion rates are similar to 0.05 M. s(-1) (less for lower mass white dwarfs) and last for approximately a minute. Many of the disk parameters can be explained by a simple analytic model that we derive and compare to our simulations. This model can be used to predict accretion rates for white dwarf and black hole (or neutron star) masses that are not simulated here. Although the mergers studied here create disks with larger radii and longer accretion times than those from the merger of double neutron stars, a larger fraction of the white dwarfs mass becomes part of the disk. Thus the merger of a white dwarf and a black hole could produce a long-duration GRB. The event rate of these mergers may be as high as 10(-6) yr(-1) per galaxy.