Cyg X-1 - A massive neutron star

The expected X-ray emission from Cyg X-1, considered a massive neutron star (8-15 solar masses) according to some gravity theories, is studied within the framework of Rosen's bimetric gravity theory (1973, 1974). It is shown that in such massive neutron stars, the innermost stable orbit lies far outside the star surface, and therefore the X-ray spectrum consists of two components: a soft one emitted from a cold accretion disk and a hard one emitted by the matter striking the neutron star surface after spiraling down freely from the disk. The proposed model is shown to be in good agreement with the observed luminosities. The model predicts a surface gravitational redshift of 3.16 which could be tested by the future X- and gamma-ray detectors.