Lomb-Scargle periodograms struggle with non-sinusoidal supermassive BH binary signatures in quasar lightcurves

Abstract

Supermassive black hole binary (SMBHB) systems are expected to form as a consequence of galaxy mergers. At sub-parsec separations, SMBHBs can be identified as quasars with periodic variability with previous periodicity searches uncovering significant candidates. However, these searches focused primarily on sinusoidal signals, while theoretical models and hydrodynamical simulations predict that binaries produce more complex non-sinusoidal pulse shapes. Here we examine the efficacy of the Lomb-Scargle periodogram (LSP; one of the most popular tools for periodicity searches in unevenly sampled lightcurves) to detect periodicities with a sawtooth shape mimicking results of hydrodynamical simulations. We simulate idealised well-sampled lightcurves, lightcurves that mimic the data in the Palomar Transient Factory (PTF) analyzed in Charisi et al., 2016, and lightcurves that resemble our expectations for single-band data in the upcoming Legacy Survey of Space and Time (LSST) of the Rubin Observatory. We approximate quasar variability with a damped random walk (DRW) model, inject sinusoidal and sawtooth pulse shapes and assess their statistical significance. We find that in the presence of red noise the LSP detects a relatively low fraction of the sinusoidal signals (~45%, ~24% and ~23%, in the PTF-like, idealised, and LSST-like lightcurves, respectively). The fraction is significantly reduced for sawtooth periodicity (with only ~9% in PTF-like and ~1% in idealised and LSST-like lightcurves). These low recovery rates imply that previous searches have missed the large majority of binaries. They also have significant implications for the detection of SMBHBs in upcoming LSST necessitating the developement of advanced tools that go beyond the simple LSP.