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 blac k 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, o r its neutron star progenitor, first experiences a common envelope phase th at carries it inward through the hydrogen envelope. Accretion of the last s everal 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 (a bout 10(50) ergs total) whose beaming and relativistic interaction with the circumstellar material makes the GRB (see, e.g., Rees & Meszaros). The use ful energy can be greatly increased if energy can be extracted from the rot ational energy of the black hole by magnetic interaction with the disk. Suc h 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.