Assess the validity of alternative warm-corona spin models for Cygnus X-1

Determine whether the alternative spectral model incorporating a warm corona layer is applicable to Cygnus X-1, given that it permits a low black-hole spin consistent with zero instead of the previously inferred spin $a\gtrsim0.9985$.

Background

Black-hole spin measurements for Cygnus X-1 depend on the adopted X-ray spectral model. Earlier analyses inferred an extremely high spin, while an alternative model containing a warm corona permits a spin consistent with zero. Because superradiance constraints rely directly on the inferred spin, resolving whether the alternative model is physically applicable is important for determining the robustness of the corresponding boson-mass exclusions.

References

The spin measurement itself can also be model-sensitive: for Cygnus~X-1, alternative spectral models including a ``warm'' corona layer allow low spins consistent with zero, compared with earlier estimates of $a\gtrsim0.9985$, although the applicability of this alternative model remains uncertain.

— Systematic Uncertainties in Ultralight Boson Constraints from Superradiance  (2610.11091 - Aswathi et al., 8 Oct 2026) in Section 2, Constraints from electromagnetic observations