Confirmed short periodic variability of subparsec supermassive binary
  black hole candidate Mrk 231
  Here we confirm the short periodic variability of a subparsec supermassive
binary black hole (SMBBH) candidate Mrk 231 in the extended optical photometric
data set collected by the Catalina Real-Time Transient Survey (CRTS) and
All-Sky Automated Survey for Supernovae (ASAS-SN). Using the Lomb-Scargle
periodogram and 2DHybrid method, we detected the significant periodicity of ~
1.1 yr beyond a damped random walk model in the CRTS+ASAS-SN optical data set.
Mrk 231 has been previously proposed as a SMBBH candidate with a highly unequal
mass ratio (q~ 0.03), very tight mutual separation of ~590 AU, and an orbital
period of ~1.2 yr. Hence, our result further supports, even though not prove,
the intriguing hypothesis that SMBBHs with low mass ratios may be more common
than close-equal mass SMBBHs. This result, however, was obtained from the
contribution of CRTS data with limited sampling cadence and photometric
accuracy, and further monitoring of Mrk 231 is crucial to confirm the
periodicity.
