Xygnus X1 was the first X-ray source widely accepted to be related to a black hole. Now a team of astronomers have measured its mass, diameter and rotation, and has found it to be rotating extremely fast

The compact primary in the X-ray binary Cygnus X-1 discovered during a rocket flight in 1964, was the first black hole to be established via dynamical observations. The black hole is rotating in a doubble-star system, with the giant star HDE 226868 600 lightyears from Earth

Astronomers have now 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 an asynchronous dynamical model, they 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, they have determined that Cygnus X-1 contains a near-extreme Kerr black hole with a spin parameter a/M>0.97.

This means that thay . if their measurement of it's mass to 8 Solar masses is corect, it rotates around its axis in less than a second!

Their results take into account all significant sources of observational and model-parameter uncertainties, which are dominated by the uncertainties in black hole mass, orbital inclination angle and distance. The uncertainties introduced by the thin-disk model we employ are particularly small in this case, given the disk's low luminosity

Source: arXiv