Cyg X-3: can the compact object be a black hole?

Abstract

By means of population synthesis we find that the expected Galactic number of black holes with massive helium star companions is \sim 100 and depends on an assumed threshold for M_{pre-BH}. The overwhelming majority of these systems has orbital periods in excess of 10 hr, with a maximum at \sim 100 hr, while under the Illarionov & Sunyaev (1975) disk formation criteria for accretion from the strong stellar wind of Wolf-Rayet star disk accretion is possible only for orbital periods below \sim 10 hr. However, the number of such short-period systems is vanishingly small. If the accretor in Cyg X-3 is a 10 M_sun black hole, then the accretion rate will be super-Eddington. Super-Eddington accretion may be responsible for the formation of jets in Cyg X-3 and may also support an X-ray luminosity as high as \sim 10^{39} erg/s. From the orbital period distribution for neutron stars with massive helium companions we find that if during the common envelope phase a neutron star accretes at Eddington rate and spins-up to the equilibrium period, then in most systems it acts as a ``propeller'' and accretion from the WR star wind is impossible. For the model with two massive helium stars as an immediate progenitor of Cyg X-3, requirement of accomodation of two WR stars in the post-common-envelope orbit combined with severe mass loss by them prevents formation of BH+WR systems with orbital periods less than several days.