Supermassive black hole binaries ( SMBHBs ) should be an inevitable consequence of the hierarchical growth of massive galaxies through mergers and the strongest sirens of gravitational waves ( GWs ) in the cosmos .
Yet , their direct detection has remained elusive due to the compact ( sub-parsec ) orbital separations of gravitationally bound SMBHBs .
Here , we exploit a theoretically predicted signature of an SMBHB in the time domain : periodic variability caused by a mass accretion rate that is modulated by the binary ’ s orbital motion .
We report our first significant periodically varying quasar detection from the systematic search in the Pan-STARRS1 ( PS1 ) Medium Deep Survey .
Our SMBHB candidate , PSO J334.2028+01.4075 , is a luminous radio-loud quasar at z = 2.060 , with extended baseline photometry from the Catalina Real-Time Transient Survey , as well as archival spectroscopy from the FIRST Bright Quasar Survey .
The observed period ( 542 \pm 15 days ) and estimated black hole mass ( \log ( M _ { BH } / M _ { \odot } ) = 9.97 \pm 0.50 ) , correspond to an orbital separation of 7 ^ { +8 } _ { -4 } Schwarzschild radii ( \sim 0.006 ^ { +0.007 } _ { -0.003 } pc ) , assuming the rest-frame period of the quasar variability traces the orbital period of the binary .
This SMBHB candidate , discovered at the peak redshift for SMBH mergers , is in a physically stable configuration for a circumbinary accretion disk and within the regime of GW-driven orbital decay .
Our search with PS1 is a benchmark study for the exciting capabilities of LSST , which will have orders of magnitude larger survey power and will potentially pinpoint the locations of thousands of SMBHBs in the variable night sky .