Re-estimating the Spin Parameter of the Black Hole in Cygnus X-1

TL;DR

This paper re-estimates the spin of the black hole in Cygnus X-1 using updated system parameters and the continuum-fitting method, confirming its near-maximal spin with high confidence.

Contribution

It applies the continuum-fitting technique with new parameters and Monte Carlo simulations to refine the black hole spin estimate in Cygnus X-1.

Findings

Black hole spin in Cygnus X-1 is extremely high, >0.9985 at 3σ confidence.

Updated system parameters lead to a more precise spin measurement.

The spin is confirmed to be near maximal, supporting theories of black hole formation.

Abstract

Cygnus X-1 is a well-studied persistent black hole X-ray binary. Recently, the three parameters needed to estimate the black hole spin of this system, namely the black hole mass $M$, the orbital inclination $i$ and the source distance $D$, have been updated. In this work we redetermine the spin parameter using the continuum-fitting technique for those updated parameter values. Based on the assumption that the spin axis of the black hole is aligned with the orbital plane, we fit the thermal disk component to a fully relativistic thin accretion disk model. The error in the spin estimate arising from the combined observational uncertainties is obtained via Monte Carlo (MC) simulations. We demonstrate that, without considering the counteracting torque effect, the new spin parameter is constrained to be a$_* > 0.9985$ (3$\sigma$), which confirms that the spin of the black hole in Cygnus X-1 is extreme.