Confirmation Via the Continuum-Fitting Method that the Spin of the Black Hole in Cygnus X-1 is Extreme

TL;DR

This study confirms that the black hole in Cygnus X-1 has an extremely high spin, exceeding 0.983 at 99.7% confidence, using an improved continuum-fitting method with new high-quality spectra.

Contribution

The paper refines the spin measurement of Cygnus X-1's black hole, setting a more stringent lower limit using additional spectra and addressing previous potential biases.

Findings

Black hole spin in Cygnus X-1 exceeds 0.983 at 3σ confidence.

New spectra with lower scattering fractions improve measurement reliability.

Previous estimates of near-extreme spin are confirmed and refined.

Abstract

In Gou et al. (2011), 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(3{\sigma}). We confirm this result while setting a new and more stringent limit: a*>0.983 at the 3{\sigma}(99.7%) level of confidence. 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 f_s=23-31%, while the upper limit for reliable application of the continuum-fitting method is f_s<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 f_s in the range 10% to 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 (3{\sigma}).