The Extreme Spin of the Black Hole in Cygnus X1
Abstract
The compact primary in the Xray binary Cygnus X1 was the first black hole to be established via dynamical observations. We have recently determined accurate values for its mass and distance, and for the orbital inclination angle of the binary. Building on these results, which are based on our favored (asynchronous) dynamical model, we have measured the radius of the inner edge of the black hole's accretion disk by fitting its thermal continuum spectrum to a fully relativistic model of a thin accretion disk. Assuming that the spin axis of the black hole is aligned with the orbital angular momentum vector, we have determined that Cygnus X1 contains a nearextreme Kerr black hole with a spin parameter a _{*} > 0.95 (3σ). For a less probable (synchronous) dynamical model, we find a _{*} > 0.92 (3σ). In our analysis, we include the uncertainties in black hole mass, orbital inclination angle, and distance, and we also include the uncertainty in the calibration of the absolute flux via the Crab. These four sources of uncertainty totally dominate the error budget. The uncertainties introduced by the thindisk model we employ are particularly small in this case given the extreme spin of the black hole and the disk's low luminosity.
 Publication:

The Astrophysical Journal
 Pub Date:
 December 2011
 DOI:
 10.1088/0004637X/742/2/85
 arXiv:
 arXiv:1106.3690
 Bibcode:
 2011ApJ...742...85G
 Keywords:

 accretion;
 accretion disks;
 black hole physics;
 stars: individual: Cygnus X1;
 Xrays: binaries;
 Astrophysics  High Energy Astrophysical Phenomena;
 General Relativity and Quantum Cosmology;
 High Energy Physics  Phenomenology
 EPrint:
 Paper III of three papers on Cygnus X1