The Extreme Spin of the Black Hole in Cygnus X-1

TL;DR

This study measures the spin of the black hole in Cygnus X-1, finding it to be near-extreme with a spin parameter greater than 0.95, based on relativistic modeling of the accretion disk spectrum.

Contribution

First precise measurement of the black hole's spin in Cygnus X-1 using relativistic accretion disk models and accounting for multiple uncertainties.

Findings

Black hole in Cygnus X-1 has a spin parameter a/M>0.95 (3σ).

Uncertainties in mass, inclination, distance, and flux calibration dominate error budget.

Results support a near-extreme Kerr black hole in Cygnus X-1.

Abstract

The compact primary in the X-ray binary Cygnus X-1 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 X-1 contains a near-extreme Kerr black hole with a spin parameter a/M>0.95 (3\sigma). For a less probable (synchronous) dynamical model, we find a/M>0.92 (3\sigma). 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 thin-disk model we employ are particularly small in this case given the extreme spin of the black hole and the disk's low luminosity.