Why do X-ray binary black holes spin fast, merging ones slowly?

X-ray continuum-fitting and reflection methods give spins $a* > \sim 0.8$ for many black holes in X-ray binaries, while gravitational-wave populations show $\chi_{\mathrm{eff}}$ concentrated near $\sim 0.05$ with small component spins. Determine whether this reflects different formation histories (spin-up by accretion or tidal locking versus slow birth spins from efficient core-envelope coupling), systematic errors in X-ray methods, or gravitational-wave prior choices. Answer: a model consistent with both populations or a demonstrated systematic error.

Independent spin measurements of X-ray binary black holes (for example X-ray polarimetry or quasi-periodic oscillations) agreeing or disagreeing with spectral methods, and prior-robust gravitational-wave spin distributions.