2021MNRAS.500.3640S

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_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_col into the eight (non-maximal) black hole spin measurements vetted by the disc continuum fitting community. In most cases, an f_col uncertainty of ±0.2-0.3 dominates the black hole spin error budget. We go on to demonstrate that plausible departures in f_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 modelling. Systematic uncertainties in f_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.