Helium star/black hole mergers: A new gamma-ray burst model

We present a model for gamma-ray bursts (GRBs) 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 it, or its neutron star progenitor, first experiences a common envelope phase that carries it inward through the hydrogen envelope. Accretion of the last several solar masses of helium occurs on a timescale of roughly a minute and provides a neutrino luminosity of approximately 10(51)-10(52) ergs s(-1). Neutrino annihilation, 0.01%-0.1% efficient, along the rotational axis then gives a baryon-loaded fireball of electron-positron pairs and radiation (about 10(50) ergs total) whose beaming and relativistic interaction with the circumstellar material makes the GRB (see, e.g., Rees & Meszaros). 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 scars and black hole neutron star pairs and may be responsible for long complex GRBs but not short hard ones.