On the orbital and physical parameters of the HDE 226868/Cygnus X1 binary system
Abstract
In this paper we explore the consequences of the recent determination of the mass m=(8.7±0.8) M _{☉} of Cygnus X1, obtained from the QuasiPeriodic Oscillation (QPO)photon index correlation scaling, on the orbital and physical properties of the binary system HDE 226868/Cygnus X1. By using such a result and the latest spectroscopic optical data of the HDE 226868 supergiant star we get M=(24±5) M _{☉} for its mass. It turns out that deviations from the third Kepler law significant at more than 1sigma level would occur if the inclination i of the system’s orbital plane to the plane of the sky falls outside the range ≈41 56 deg: such deviations cannot be due to the first postNewtonian (1PN) correction to the orbital period because of its smallness; interpreted in the framework of the Newtonian theory of gravitation as due to the stellar quadrupole mass moment Q, they are unphysical because Q would take unreasonably large values. By conservatively assuming that the third Kepler law is an adequate model for the orbital period we obtain i=(48±7) deg which yields for the relative semimajor axis a=(42±9) R _{☉} (≈0.2 AU).
 Publication:

Astrophysics and Space Science
 Pub Date:
 June 2008
 DOI:
 10.1007/s105090089839y
 arXiv:
 arXiv:0707.3525
 Bibcode:
 2008Ap&SS.315..335I
 Keywords:

 Black holes: individual (Cyg X1);
 Stars: individual (HDE 226868);
 Xrays: binaries;
 Xrays: individual (Cyg X1);
 General Relativity and Quantum Cosmology;
 Astrophysics
 EPrint:
 Latex2e, 7 pages, 1 table, 4 figures. To appear in ApSS (Astrophysics and Space Science)