  Studying a centi-parsec supermassive black hole binary (SMBHB) would allow us
to explore a new parameter space in active galactic nuclei, and these objects
are also potential sources of gravitational waves. We report evidence that an
SMBHB with an orbital period of about 30 yr may be resident in the nearby
galactic nucleus M81. This orbital period and the known mass of M81 imply an
orbital separation of about 0.02 pc. The jet emanating from the primary black
hole showed a short period of jet wobbling at about 16.7 yr, superposing a
long-term precession at a timescale of several hundred years. Periodic radio
and X-ray outbursts were also found two times per orbital period, which could
be explained by a double-peaked mass accretion rate variation per binary orbit.
If confirmed, M81 would be one of the closest SMBHB candidates, providing a
rare opportunity to study the final parsec problem.
