Is there any confirmed sub-parsec supermassive black hole binary?
In plain words
Several quasars brighten and dim periodically as if two black holes orbit each other, but none has been confirmed. A single proven close pair would anchor the whole population.
Precise statement
Determine whether any electromagnetic candidate (periodic quasars, double-peaked or shifting broad lines, the blazar OJ 287) is a gravitationally bound binary with separation $< 1\ \mathrm{pc}$, or detect a continuous-wave pulsar timing source with an identified host galaxy. The answer is a confirmed system with measured orbital period and mass ratio.
What would settle it
Coincident detection of a continuous-wave gravitational signal in pulsar timing data and a periodic electromagnetic counterpart at the same sky position and period.
Status in the literature
Unverified note
Candidates include OJ 287 and the radio blazar PKS 2131-021 (sinusoidal radio light curve, 2022); none is confirmed as of 2026.