Astrophysics and Space Science

, Volume 315, Issue 1, pp 335–340

On the orbital and physical parameters of the HDE 226868/Cygnus X-1 binary system

Original Article

DOI: 10.1007/s10509-008-9839-y

Cite this article as:
Iorio, L. Astrophys Space Sci (2008) 315: 335. doi:10.1007/s10509-008-9839-y


In this paper we explore the consequences of the recent determination of the mass m=(8.7±0.8)M of Cygnus X-1, obtained from the Quasi-Periodic Oscillation (QPO)-photon index correlation scaling, on the orbital and physical properties of the binary system HDE 226868/Cygnus X-1. 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 1-sigma 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 post-Newtonian (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).


Black holes: individual (Cyg X-1) Stars: individual (HDE 226868) X-rays: binaries X-rays: individual (Cyg X-1) 

Copyright information

© Springer Science+Business Media B.V. 2008

Authors and Affiliations

  1. 1.INFN, Sezione di PisaPisaItaly
  2. 2.BariItaly

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