Supermassive black hole binaries ( SMBHBs ) at sub-parsec separations should be common in galactic nuclei , as a result of frequent galaxy mergers .
Hydrodynamical simulations of circumbinary discs predict strong periodic modulation of the mass accretion rate on time-scales comparable to the orbital period of the binary .
As a result , SMBHBs may be recognized by the periodic modulation of their brightness .
We conducted a statistical search for periodic variability in a sample of 35,383 spectroscopically confirmed quasars in the photometric database of the Palomar Transient Factory ( PTF ) .
We analysed Lomb-Scargle periodograms and assessed the significance of our findings by modeling each individual quasar ’ s variability as a damped random walk ( DRW ) .
We identified 50 quasars with significant periodicity beyond the DRW model , typically with short periods of a few hundred days .
We find 33 of these to remain significant after a re-analysis of their periodograms including additional optical data from the intermediate-PTF and the Catalina Real-Time Transient Survey ( CRTS ) .
Assuming that the observed periods correspond to the redshifted orbital periods of SMBHBs , we conclude that our findings are consistent with a population of unequal-mass SMBHBs , with a typical mass ratio as low as q \equiv M _ { 2 } / M _ { 1 } \approx 0.01 .