Confirmation via the continuum-fitting method that the spin of the black hole in Cygnus X-1 is extreme.
GOU L., McCLINTOCK J.E., REMILLARD R.A., STEINER J.F., REID M.J., OROSZ J.A., NARAYAN R., HANKE M. and GARCIA J.
Abstract (from CDS):
In Gou et al., we reported that the black hole primary in the X-ray binary Cygnus X-1 is a near-extreme Kerr black hole with a spin parameter a*> 0.95 (3σ). We confirm this result while setting a new and more stringent limit: a*> 0.983 at the 3σ (99.7%) confidence level. The earlier work, which was based on an analysis of all three useful spectra that were then available, was possibly biased by the presence in these spectra of a relatively strong Compton power-law component: the fraction of the thermal seed photons scattered into the power law was fs= 23%-31%, while the upper limit for reliable application of the continuum-fitting method is fs ≲ 25%. We have subsequently obtained six additional spectra of Cygnus X-1 suitable for the measurement of spin. Five of these spectra are of high quality with fs in the range 10%-19%, a regime where the continuum-fitting method has been shown to deliver reliable results. Individually, the six spectra give lower limits on the spin parameter that range from a*> 0.95 to a*> 0.98, allowing us to conservatively conclude that the spin of the black hole is a*> 0.983 (3σ).