  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 SMBHB at its core. A SMBHB model with a mass of the primary of M_BH = 10^7
M_sun, a mass ratio q=0.08, and a semimajor axis a_b = 0.6 mpc is in good
agreement with the observations. Given that primary mass, introducing an
orbital eccentricity is needed, with e_b = 0.3. Alternatively, a lower mass
primary of M_BH = 10^6 M_sun 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.
