Identification of periodicities with arbitrary shapes in light curves of active galactic nuclei

Massive black hole binaries are expected to be observable as periodic active galactic nuclei in time-domain photometric surveys. Periodicities may originate from different physical processes, including the intermittent gas feeding of the black holes caused by the time-varying non-axisymmetric binary potential, the Doppler boosting of the flux emitted by individual accretion discs bound to the orbiting black holes, and the gravitational lensing of the accretion disc of one black hole by another such disc. Only the Doppler boost scenario applied to circular binaries with non-modulated accretion predicts a sinusoidal light curve, while in the general case, binary signals are expected to show more complex periodic patterns. Current searches for massive black hole binaries rely on techniques tailored to quasi-sinusoidal light curves, but fail to identify the more complex predicted periodicities. We present an alternative method that leverages Gaussian processes, making use of a generic periodic kernel that is flexible enough to identify arbitrary periodicities in unevenly sampled light curves with realistic quasar noise. We demonstrate that it outperforms previously proposed strategies in identifying general periodicities by analysing mock light curves with different baselines. Specifically, our analysis can detect non-sinusoidal periodicities (e.g. sawtooth-shaped or symmetric flares) and retrieves a higher fraction of true periodicities when compared to a periodogram analysis or a Gaussian process analysis with less flexible periodic kernels. Furthermore, by comparing the retrieved fraction of periodicities between mock Palomar transient factory light curves and mock Legacy Survey of Space and Time light curves, we found that our analysis is most sensitive to the number of observed cycles. The application of this analysis has the potential to greatly increase the scientific return of current and upcoming large time-domain photometric surveys.