2020ApJ...902...65S

We report the discovery of an ∼6 week quasi-periodic oscillation (QPO) in archival NGC 4945 data observed by the Rossi X-ray Timing Explorer (RXTE) satellite. QPOs are important observables in accretion disks and have been studied extensively in both neutron star (NS) and black hole (BH) X-ray binaries (XRB). QPOs should be present in active galactic nuclei (AGN) if galactic BHs and supermassive black holes are governed by a common set of physical processes. The search for QPOs in AGN has proven difficult because the time scales would be much longer, due to their higher mass. RXTE AGN light curves spanning 1996 to 2011 provide an excellent and perhaps unique opportunity to search for low-frequency QPOs. We investigated the 533 RXTE observations made of the Seyfert-2 AGN, NGC 4945. During a large cluster of observations in 2006-2007 (194 observations, spanning 396 days), the Lomb-Scargle periodogram shows a candidate QPO at 0.274 µHz (period ≃42.2 days). We estimate the uncertainties using the false alarm probability. We discuss the possible identification of this feature with the Lense-Thirring precession period.