Quasi-periodic oscillations (QPOs) are important observables in accretion disk systems and have been studied extensively in galactic x-ray binaries (XRB). Since the physical processes of accretion should be scale invariant, QPOs should be present in Active Galactic Nuclei (AGN). Due to the higher mass of AGN, the longer timescales complicate the search for QPOs. Physical models to explain QPOs include various combinations of the keplerian motion of matter in the disk, black hole (BH) spin, general relativistic (GR) effects, or beat frequencies between two of the previous mechanisms. Lense-Thirring precession is a GR effect often suggested to explain QPO frequencies in galactic BH and supermassive black holes (SMBH).
AGN monitoring performed with the Rossi X-ray Timing Explorer (RXTE) satellite from 1996 to 2011 provides a unique resource to search for low-frequency QPOs. Our analysis is based on data from reduced 3-color light curves prepared and archived at the University of California, San Diego (UCSD). We have investigated more than 50 sources for possible QPOs using the Lomb-Scargle periodogram (LSP), which is appropriate for unevenly sampled data. A QPO with a period of 42 days has been reported in NGC 4945 (Smith et al. 2020). Candidate QPOs have also been reported in 3C 111 (499 d) and MKN 110 (1084 d) (Hursh et al. 2020). We report here additional candidates: PKS 1510-089 (335 & 218 d), BL Lac (774 d), and Cen A (207 d).