On the Spin of the Black Hole in IC 10 X-1

TL;DR

This study confirms IC 10 X-1 hosts a black hole with a likely high spin, using X-ray spectral analysis, despite previous uncertainties about its mass.

Contribution

The paper provides the first spin measurement of the black hole in IC 10 X-1 through X-ray continuum fitting, clarifying its nature and properties.

Findings

IC 10 X-1 contains a black hole, not a neutron star.

The black hole likely has a high spin parameter.

The spectrum is dominated by accretion disk emission.

Abstract

The compact X-ray source in the eclipsing X-ray binary IC 10 X-1 has reigned for years as ostensibly the most massive stellar-mass black hole, with a mass estimated to be about twice that of its closest rival. However, striking results presented recently by Laycock et al. reveal that the mass estimate, based on emission-line velocities, is unreliable and that the mass of the X-ray source is essentially unconstrained. Using Chandra and NuSTAR data, we rule against a neutron-star model and conclude that IC 10 X-1 contains a black hole. The eclipse duration of IC 10 X-1 is shorter and its depth shallower at higher energies, an effect consistent with the X-ray emission being obscured during eclipse by a Compton-thick core of a dense wind. The spectrum is strongly disk-dominated, which allows us to constrain the spin of the black hole via X-ray continuum fitting. Three other wind-fed black-hole systems are known; the masses and spins of their black holes are high: M ~ 10-15 Msun and a*>0.8. If the mass of IC 10 X-1's black hole is comparable, then its spin is likewise high.