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AGN QPOs from the RXTE Archive

Presentation #209.02D in the session “AGN and Quasars 2”.

Published onJan 11, 2021
AGN QPOs from the RXTE Archive

AGN QPOs from the RXTE ArchiveQuasi-periodic oscillations (QPOs) offer an important probe of the motion of matter in strong gravity, within a few Schwarzschild radii (RS) of the central black hole (BH). Studied extensively in galactic x-ray binaries (XRB), QPOs should be present in Active Galactic Nuclei (AGN) if accretion physics is scale invariant. QPOs have been identified as orbits limited by the innermost stable circular orbit (ISCO), as predicted by general relativity (GR), and with frame dragging associated with the BH spin. The search for AGN QPOs has been complicated by their longer timescale, caused by their higher mass. A few “high-frequency” QPOs have been reported, mainly in the range of a few hours based on observations spans up to a few days. AGN monitoring with the Rossi X-ray Timing Explorer (RXTE) satellite (1996 – 2011) provide a unique resource to search for low-frequency QPOs. Reduced 3-color light curves prepared and archived at University of California, San Diego (UCSD) make these data readily available for analysis. Of the 124 AGN included in the UCSD archive, we found 76 AGN which had both a large quantity of observations and appropriate spacing to support the search for QPOs. The campaign lengths, 15 years in some cases, provide time series sensitive to QPOs of longer duration than could be detected in other datasets. We used the Lomb-Scargle periodogram (LSP) to look for QPOs in unevenly sampled AGN light curves. To model the power spectrum of the noise and better understand the probability these results could be attained solely by chance, we compared to the LSP of the window function and the false alarm probability (FAP). We have reported a QPO with period 42 days in NGC 4945 (Smith et al 2020). Other candidate QPOs include 3C 111 (499 d) and MKN 110 (1084 d) (Hursh et al 2020). In an accompanying paper (Oramas et al 2021) we report additional candidates: PKS 1510-089 (335 & 218 d), BL Lac (774 d) and Cen A (207 d). Based on previously published estimates for the mass and spin of any AGN with a detected QPO, we can make educated guesses about the physical dimensions of the accretion disk and location of the emitting matter. Physical models of QPOs often include the Keplerian motion of matter in the disk and Lense-Thirring (LT) precession, a GR effect often suggested to explain QPO frequencies in galactic BH and supermassive black holes (SMBH). We favor the LT model because observed QPOs correspond to features at order 10 RS rather than order 1000 RS for the Keplerian model, and the X-ray emission is expected to occur deep in the gravity well at higher temperatures.

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