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

TL;DR

This study investigates the variability of the SMBHB candidate quasar PG1302-102, testing for multiple periodic signals as predicted by hydrodynamical simulations, and finds no evidence for additional periods with current data.

Contribution

The paper provides the first detailed analysis of secondary periodic components in PG1302-102, setting upper limits and proposing follow-up observations to test simulation predictions.

Findings

No compelling evidence for additional periodic peaks.

Current data can detect secondary signals with amplitudes ≥0.07 mag.

Follow-up monitoring can improve detection sensitivity by ~50%.

Abstract

Graham et al. (2015) 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 derive upper limits on any additional periodic modulations in the available data, by modeling the light-curve as the sum of red 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%) secondary periodic terms with periods in the range predicted by the simulations, if the amplitude of the variability was at least ~0.07 mag (compared to 0.14 mag for the main peak). A three-year follow-up monitoring campaign with weekly observations can increase the sensitivity for detecting secondary peaks ~50%, and would allow a more robust test of predictions from hydrodynamical simulations.