Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996mnras.280..953m&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 280, Issue 3, pp. 953-962.
Astronomy and Astrophysics
Astronomy
15
Radiative Transfer, Binaries: Close, Stars: Individual: Cyg X-3, Stars: Wolf-Rayet, X-Rays: Stars
Scientific paper
By feeding information about the rapid orbital period (P) lengthening of Cygnus X-3, P_solar/P~=2.2x10^-6 yr^-1, into the binary evolution equation, we attempt to find the most likely range of the companion mass (M_2) or, alternatively, the binary mass ratio q=M_2/M_1. To achieve this, we first impose the condition (i) that the envelope created by a probable wind from the companion must not obscure the soft X-ray source. This condition immediately poses a problem for the suggestion of van Kerkwijk et al. that the companion of Cyg X-3 is a Wolf-Rayet (WR) star with a strong wind (<~10^-5 M_ yr^-1), because of the implied exceedingly large (a fact overlooked by those authors) photoionization X-ray absorption optical depth, ~300 (at 2 keV). Accordingly, the upper limit on the strength of such a wind is found to be only ~10^-7 M_ yr^-1. We also invoke some other physical constraints, such as (ii) the value of q must be reasonably stable against small variations in the parameters associated with mass loss from the binary, (iii) for wind emission, the specific angular momentum of the mass lost should be at least equal to that of the companion, and (iv) for disc-mediated accretion and emission, the specific angular momentum of the mass lost should be at least equal to that of the neutron star. For direct wind emission from the surface of the companion, such constraints result in a value of q~10^-2gamma, where gamma is the mass accretion rate on to the neutron star in units of 10^-8 M_ yr^-1 whereas, for disc emission, the upper limit is somewhat broader: q~(2-3)x10^-2gamma. If this analysis is correct, Cyg X-3 could be the immediate predecessor of a binary (radio) pulsar like PSR 1957+20, where the companion has a mass as low as 0.02 M_ and an orbital period of 9.2h.
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