Formation of low-mass X-ray novae with black holes from triple systems

We apply a population synthesis technique to study the formation and evolut ion of low-mass X-ray binaries with black holes, observed as X-ray novae, f rom hierarchical triple systems. A scenario is suggested in which an inner close binary system evolves into an X-ray system with a large mass ratio. T he high rate of accretion onto the neutron star leads to a common envelope stage, which may result in the formation of a Thorne-Zytkow (TZ) object. Du ring its evolution, the envelope of the TZ object expands, encompassing the third star. The recurrent common-envelope stage decreases the size of the orbit of the third star, leading to the formation of a low-mass X-ray nova with a black hole. The dynamical stability of triple systems automatically ensures that only low-mass X-ray novae form. We also consider the possible formation of an X-ray nova from a binary in the case of asymmetrical core c ollapse during a supernova explosion. (C) 2001 MAIK "Nauka/Interperiodica".