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Astron. Astrophys. 333, 151-158 (1998)


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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

1 Physics Department, Tartu University, Ülikooli 18, EE2400 Tartu, Estonia (ene@physic.ut.ee)
2 Institute of Astronomy of the Russian Academy of Sciences, 48 Pyatnitskaya Str., 109017 Moscow, Russia (lry@inasan.rssi.ru)
3 Astronomical Institute "Anton Pannekoek", University of Amsterdam, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received 26 June 1997 / Accepted 29 January 1998

Abstract

By means of population synthesis we find that the expected Galactic number of black holes with massive helium star companions is [FORMULA] and depends on an assumed threshold for [FORMULA].  The overwhelming majority of these systems has orbital periods in excess of 10 hr, with a maximum at [FORMULA] 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 [FORMULA] hr. However, the number of such short-period systems is vanishingly small. If the accretor in Cyg X-3 is a [FORMULA] 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 [FORMULA]  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 [FORMULA] 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
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