Hard X-ray spectrum of Cyg X-1

TLDR: In this paper, the authors considered only the X-ray spectrum and the source luminosity of Cyg X-1 and provided a new basis for a comparison between observational results and theoretical predictions.

Abstract: THE three most likely black hole candidates known in X-ray astronomy are Cyg X-1, Circ X-1 and V 861 Scorpii1. Cyg X-1 has a key role as it is the best studied of these objects. Unfortunately, the evidence for its being a black hole is entirely circumstantial; namely, the mass of the compact object and its observed short time variability. The spectrum of Cyg X-1 is also very different from those of accreting neutron stars, but it is similar to spectra of extragalactic sources which may contain massive black holes. Eardley et al.2 have recently reviewed the existing evidence. Here we consider only the X-ray spectrum and the source luminosity. This work was motivated by the spectral measurements with very high statistical accuracy of Cyg X-1 made in the hard X-ray range by the AIT/MPI group3. Progress has been made in Moscow in calculating source spectra expected from hot accretion disks. Thus, there is a new basis for a comparison between observational results and theoretical predictions.