Hard X-ray spectrum of Cyg X-1

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.