Black Hole Spin in X-ray Binaries: Giving Uncertainties an $f$

TL;DR

This paper highlights the significant impact of uncertainties in the colour correction factor on black hole spin measurements in X-ray binaries, demonstrating that accounting for these uncertainties can reconcile conflicting measurement techniques.

Contribution

It introduces a systematic approach to incorporate uncertainties in the colour correction factor into existing spin measurements, revealing their dominant role in the error budget.

Findings

Uncertainties in $f_{col}$ often dominate spin measurement errors.

Adjusting $f_{col}$ can reconcile discrepancies between different measurement methods.

Systematic effects like magnetization need further study for accurate spin estimates.

Abstract

The two established techniques for measuring black hole spin in X-ray binaries often yield conflicting results, which must be resolved before either method may be deemed robust. In practice, black hole spin measurements based on fitting the accretion disc continuum effectively do not marginalize over the colour correction factor $f_{\mathrm{col}}$. This factor parametrizes spectral hardening of the disc continuum by the disc atmosphere, whose true properties are poorly constrained. We incorporate reasonable systematic uncertainties in $f_{\mathrm{col}}$ into the eight (non-maximal) black hole spin measurements vetted by the disc continuum fitting community. In most cases, an $f_{\mathrm{col}}$ uncertainty of $\pm$0.2-0.3 dominates the black hole spin error budget. We go on to demonstrate that plausible departures in $f_{\mathrm{col}}$ values from those adopted by the disc continuum fitting practitioners can bring the discrepant black hole spins into agreement with those from iron line modeling. Systematic uncertainties in $f_{\mathrm{col}}$, such as the effects of strong magnetization, should be better understood before dismissing their potentially dominant impact on the black hole spin error budget.