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 similar to 1.1 yr beyond a damped random walk model in the CRTS+ASASSN optical data set. Mrk 231 has been previously proposed as an SMBBH candidate with a highly unequal mass ratio (q similar to 0.03), very tight mutual separation of similar to 590 au, and an orbital period of similar to 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 the CRTS data with limited sampling cadence and photometric accuracy, and further monitoring of Mrk 231 is crucial to confirm the periodicity.