A MILLIPARSEC SUPERMASSIVE BLACK HOLE BINARY CANDIDATE IN THE GALAXY SDSS J120136.02+300305.5

Galaxy mergers play a key role in the evolution of galaxies and the growth of their central supermassive black holes (SMBHs). A search for (active) SMBH binaries (SMBHBs) at the centers of the merger remnants is currently ongoing. Perhaps the greatest challenge is to identify the inactive SMBHBs, which might be the most abundant, but are also the most difficult to identify. Liu et al. predicted characteristic drops in the light curves of tidal disruption events (TDEs), caused by the presence of a secondary SMBH. Here, we apply that model to the light curve of the optically inactive galaxy SDSS J120136.02+300305.5, which was identified as a candidate TDE with XMM-Newton. We show that the deep dips in its evolving X-ray light curve can be well explained by the presence of a SMBIIB at its core. A SMBIIB model with a mass of the primary of M-BH = 10(7) M-circle dot, a mass ratio q similar or equal to 0.08, and a semi-major axis a(b) similar or equal to 0.6 mpc is in good agreement with the observations. Given that primary mass, introducing an orbital eccentricity is needed, with e(b) similar or equal to 0.3. Alternatively, a lower mass primary of M-BH = 10(6) M-circle dot in a circular orbit fits the light curve well. Tight binaries like this one, which have already overcome the "final parsec problem," are prime sources of gravitational wave radiation once the two SMBHs coalesce. Future transient surveys, which will detect TDEs in large numbers, will place tight constraints on the SMBHB fraction in otherwise non-active galaxies.