THERE has been much discussion concerning the X-ray object Cygnus X-1 and its optical counterpart, HDE226868, a highly reddened star of type O9.7 Iab (ref. 1). The O star is a single-lined spectroscopic binary, and various assignments of the minimum mass of the secondary, taken to be the X-ray object, suggest that the latter is a black hole. These conclusions are dependent on the mass of the O star being 2030 M, as would be the case if the star were normal for its luminosity class. But the possibility of heating of the primary by the nearby X-ray object, and the spectroscopic indications of mass loss2, indicate that such an assumption is unsafe. In order to determine the mass of HDE226868, a more complete spectroscopic analysis, in which the effective temperature and surface gravity are deduced, and a determination of luminosity (from the distance to the object) are necessary. The distance to the object is crucial to the problem, because once the effective temperature and gravity of the O star are known, the mass will vary as the square of the distance. An error in the distance by a factor of about two means that the model for the X-ray source need not include a black hole.
- 1 - general ; 2 - general science & technology ; 3 - general science & technology
- 1 - sciences appliquees, technologies et medecines ; 2 - sciences biologiques et medicales ; 3 - sciences biologiques fondamentales et appliquees. psychologie