Multiple periods in the variability of the supermassive black hole binary candidate quasar PG1302-102?

Graham et al. discovered a supermassive black hole binary (SMBHB) candidate and identified the detected 5.2 yr period of the optical variability as the orbital period of the binary. Hydrodynamical simulations predict multiple periodic components for the variability of SMBHBs, thus raising the possibility that the true period of the binary is different from 5.2 yr. We analyse the periodogram of PG1302 and find no compelling evidence for additional peaks. We also point out that, despite the 5.2 yr peak being significant if a single source is considered, further analysis is required to account for the fact that PG1302 was selected among a large sample of 247 000 quasars. We derive upper limits on any additional periodic modulations in the available data, by modelling the light curve as the sum of stochastic noise and the known 5.2 yr periodic component, and injecting additional sinusoidal signals. We find that, with the current data, we would be able to detect with high significance (false alarm probability < 1 per cent) secondary periodic terms, with periods in the range predicted by the simulations, if the amplitude of the variability was at least similar to 0.06 mag (compared to 0.14 mag for the main sinusoid). A three-year follow-up monitoring campaign with weekly observations can increase the sensitivity for detecting secondary peaks by approximate to 50 per cent, and would allow a more robust test of predictions from hydrodynamical simulations.