A REANALYSIS OF THE ORBITAL INCLINATION OF CYGNUS XR-1 DETERMINED POLARIMETRICALLY

The orbital inclincation of the Cyg XR-1/HDE 226868 system was determined in 1989 by Dolan and Tapia to be 62-degrees (+ 5-degrees, -37-degrees) from polarimetric observations. Together with the spectroscopic mass function of the binary, this corresponds to a mass of 6.3 M. for the X-ray source. The observations were not corrected for any component of the polarization caused by tidal deformation of the primary. Numerical simulations show that there is no straightforward way of removing tidal deformation polarization (TDP) from polarimetric observations of close binaries. A reanalysis of the data shows that the existence of TDP in the Stokes parameter light curves observed from Cyg XR-1 does not significantly bias the mass derived to larger values. The mass of Cyg XR-1 must be significantly larger than 5 M.. This fact alone does not require Cyg XR-1 to be a black hole, however. Recent theoretical investigations of possible equations of state of matter at nuclear density show that the maximum mass of a compact object which is not a black hole may be much greater than 5 M..