A Population of Short-Period Variable Quasars from PTF as Supermassive Black Hole Binary Candidates

TL;DR

This study searches for periodic brightness variations in over 35,000 quasars from PTF data, identifying 50 candidates that could be supermassive black hole binaries, providing insights into their population and characteristics.

Contribution

It presents the first large-scale statistical search for SMBHB candidates using optical variability, identifying 50 promising candidates and analyzing their properties.

Findings

50 quasars show significant periodicity beyond stochastic variability

33 candidates remain significant after additional data re-analysis

Results suggest a population of unequal-mass SMBHBs with low mass ratios (~0.01)

Abstract

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 = M2/M1 ~ 0.01.