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 \sim 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 \sim 0.03 ) , very tight mutual separation of \sim 590 AU , and an orbital period of \sim 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 .