Observational test for the existence of a rotating black hole in Cyg X-1

Abstract

X-RAY satellite experiments^1,13,14 are now investigating the degree and plane of linear polarisation of the radiation from Cyg X-1. This radiation can be explained as coming from an accretion disk around a black hole, the polarisation of the X rays being due to electron scattering in the hotter, inner regions of the disk. Novick reported² observing 3% linear polarisation (marginally significant at the 2.5σ level) at 2.6 keV from Cyg X-1. Existing predictions of the polarisation properties, as a function of energy, have been based on a Newtonian approximation^3–5, thus neglecting gravitational effects on the rays as they propagate from the disk's surface to an observer at infinity. We present here preliminary results of a full general-relativistic calculation which show that gravitational effects completely alter the polarisation properties, and provide a sensitive test of the existence of a black hole.