NANOGrav-fitted SMBH binary models flag 3C 66B and Mrk 501 as individually resolvable GW sources for SKAO

The analysis employs an SMBH binary population model calibrated directly to the NANOGrav 15-year stochastic background amplitude and spectrum. It then injects the observed electromagnetic candidates into the simulated ensemble to test whether their expected GW strains exceed the confusion noise from the unresolved population. Strong environmental hardening increases the number of resolvable binaries at nanohertz frequencies relative to purely gravitational-wave-driven evolution. The same framework forecasts that SKAO will resolve tens of individual binaries over its lifetime, with 3C 66B remaining detectable even under conservative confusion-noise assumptions.

This result links pulsar-timing arrays to future radio arrays and shows that multimessenger selection can test binary hardening physics. Environmental coupling (stellar scattering or circumbinary disks) alters the residence time at nanohertz frequencies, directly changing the expected number of individually detectable sources. The finding therefore supplies an observational discriminant between hardening models that is independent of the overall background amplitude.

Because the paper is a population study rather than a new detection, confirmation will require continued PTA monitoring and SKAO early-science observations. If 3C 66B is recovered as a deterministic source, the measured strain and frequency evolution can be compared against the environmental-hardening predictions to constrain the dominant interaction channel.

Future work should incorporate updated PTA data releases and refined electromagnetic candidate lists to tighten the forecast uncertainties.