A QPO in NGC 4945 from Archival RXTE Data

TL;DR

This paper reports the discovery of a ~6-week quasi-periodic oscillation in the X-ray data of NGC 4945, suggesting potential common physical processes in accretion disks across different black hole systems.

Contribution

First detection of a low-frequency QPO in an active galactic nucleus using archival RXTE data, linking AGN variability to black hole accretion physics.

Findings

Detected a QPO at approximately 42 days in NGC 4945

QPO possibly related to Lense-Thirring precession

Demonstrates AGN can exhibit low-frequency QPOs similar to X-ray binaries

Abstract

We report the discovery of a ~6-week quasi-periodic oscillation (QPO) in archival NGC 4945 data observed by the Rossi X-ray Timing Explorer (RXTE) satellite. QPOs are an important observable 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 black holes and supermassive black holes (SMBH) are governed by a common set of physical processes. The search for QPOs in AGN has proven difficult because the timescales 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 $\mu$Hz (period $\approx$ 42.2 days). We estimate the uncertainties using the False Alarm Probability (FAP). We discuss the possible identification of this feature with the Lense-Thirring precession period.