Gou, LijunMcClintock, Jeffrey E.Remillard, Ronald A.Steiner, James F.Reid, Mark J.Orosz, Jerome A.Narayan, RameshHanke, ManfredGarcía, Javier2016-08-032014Gou, Lijun, Jeffrey E. McClintock, Ronald A. Remillard, James F. Steiner, Mark J. Reid, Jerome A. Orosz, Ramesh Narayan, Manfred Hanke, and Javier García. 2014. “Confirmation via the Continuum-Fitting Method That the Spin of the Black Hole in Cygnus X-1 Is Extreme.” The Astrophysical Journal 790 (1) (June 30): 29. doi:10.1088/0004-637x/790/1/29. http://dx.doi.org/10.1088/0004-637X/790/1/29.0004-637Xhttp://nrs.harvard.edu/urn-3:HUL.InstRepos:27802019In 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).en-USblack hole physicsX-rays: binariesX-rays: individual (Cygnus X-1)Journal Article2016-08-0310.1088/0004-637X/790/1/29