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Cyg X-3: can the compact object be a black hole?
Ene Ergma 1, 3 and
Lev R. Yungelson 2, 3
Received 26 June 1997 / Accepted 29 January 1998
By means of population synthesis we find that the expected Galactic number of black holes with massive helium star companions is and depends on an assumed threshold for . The overwhelming majority of these systems has orbital periods in excess of 10 hr, with a maximum at hr, while under the Illarionov & Sunyaev(1975) disk formation criteria for accretion from the strong stellar wind of a Wolf-Rayet star, disk accretion is possible only for orbital periods below hr. However, the number of such short-period systems is vanishingly small. If the accretor in Cyg X-3 is a 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 erg s-1. 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 and spins-up to the equilibrium period, then in most systems the spinning neutron star 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, the 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.
Key words: binaries: close accretion, accretion disks stars: Wolf-Rayet
Send offprint requests to: E. Ergma
© European Southern Observatory (ESO) 1998
Online publication: April 15, 1998