Determine whether black holes in high-mass X-ray binaries are truly rapidly rotating

Determine whether the black holes in high-mass X-ray binaries proposed to result from weak core–envelope coupling are truly rapidly rotating.

Background

The paper notes that weak core–envelope coupling has been proposed as an explanation for the high spins inferred in black-hole high-mass X-ray binaries. However, the actual spin measurements or interpretations of these systems remain uncertain, making their relevance to the formation of high-spin black holes in isolated binaries unresolved.

References

However, it is unclear whether these BHs are truly fast rotating.

— Can isolated binaries form unequal-mass binary black-hole mergers with a high-spin primary black hole?  (2609.05128 - Xu et al., 4 Sep 2026) in Introduction, footnote 1

In general, the origin of black hole spins remains an open question.

— The School on Gravity: from motion to commotion  (2610.13514 - Cardoso et al., 9 Oct 2026) in Chapter 1, Section 1.1.4, subsection “Comparing predicted to observed rotation rates”