Explore detectable signatures of binaries below the adopted mass-ratio limit

Investigate the detectable observational signatures of supermassive black hole binaries with mass ratios below q=0.05 using accretion-flow models appropriate to the regime in which the secondary black hole and its minidisc act as a perturbation within the primary’s disc.

Background

The analysis adopts q=0.05 as its minimum mass ratio because the assumed linear superposition of two minidiscs and one circumbinary disc becomes invalid below this value. In the lower-mass-ratio regime, the secondary and its minidisc are expected to behave more like a perturbation embedded in the primary’s disc.

Such systems could produce observational signatures different from those predicted by the binary advection-dominated accretion flow model used in the paper. Developing suitable models and determining whether these signatures are detectable with ground or spaceborne VLBI would extend the parameter space of the analysis.

References

This low q case may present other detectable signatures but we defer their exploration to future work when models of such systems exist.

— Supermassive black hole binaries in the multi-messenger context of ground and spaceborne VLBI  (2609.08704 - Hudson et al., 8 Sep 2026) in Section 7, “Discussion”