A Systematic Review of 24 Previously Measured Black Hole Spins in X-ray Binaries

We extend our recent work on black hole spin in X-ray binary systems to include an analysis of 189 archival NuSTAR observations from 24 sources. Using self-consistent data reduction pipelines, spectral models, and statistical techniques, we report an unprecedented and uniform sample of 36 stellar-mass black hole spin measurements based on relativistic reflection. This treatment suggests that prior reports of low spins in a small number of sources were generally erroneous; our comprehensive treatment finds that those sources also tend to harbor black holes with high spin values. Overall, within 1σ uncertainty, ∼86% of the sample is consistent with a ≥ 0.95, ∼94% of the sample is consistent with a≥ 0.9, and 100% is consistent with a≥ 0.7 (the theoretical maximum for neutron stars; a = cJ/GM^2). We also find that the high-mass X-ray binaries (HMXBs; those with A, B, or O-type companions) are consistent with a≥ 0.9 within 1σ errors; this is in agreement with the low-mass X-ray binary population and may be especially important for comparisons to black holes discovered in gravitational wave events. In some cases, different spectra from the same source yield similar spin measurements but conflicting values for the inclination of the inner disk; we suggest that this is due to variable disk winds obscuring the blue wing of the relativistic Fe K emission line. We discuss the implications of our measurements, the unique view of systematic uncertainties enabled by our treatment, and future efforts to characterize black hole spins with new missions.