Helium Star/Black Hole Mergers: A New Gamma-Ray Burst Model

We present a model for gamma-ray bursts (GRB's) in which a stellar mass black hole acquires a massive accretion disk by merging with the helium core of its red giant companion. The black hole enters the helium core after first experiencing a common envelope phase that carries it inwards through the hydrogen envelope. Accretion of the last several solar masses of helium occurs on a time scale of roughly a minute and gives rise to an enormous neutrino luminosity, approximately 10⁵1 - 10⁵2 erg/s. Neutrino annihilation, 0. 01% to 0. 1% efficient, along the rotational axis then gives a baryon loaded fireball of electron-positron pairs and radiation (about 10⁵0 erg total) whose beaming and relativistic interaction with circumstellar material makes the GRB. The useful energy can be greatly increased if energy can be extracted from the rotational energy of the black hole by magnetic interaction with the disk. Such events should occur at a rate comparable to that of merging neutron stars and black hole neutron star pairs and may be responsible for long complex GRB's (but not short hard ones).