Quasi periodic oscilations in active galactic nuclei

Abstract Quasi‐periodic oscillations (QPOs) are coherent peaks of variability power observed in the X‐ray power spectra (PSDs) of stellar mass X‐ray binaries (XRBs). A scale invariance of the accretion process implies they should be present in the active galactic nuclei. The first robust detection was a ∼ 1 h periodicity in the Seyfert galaxy RE J1034+396 from a ∼ 90 ks XMM‐ Newton observation; however, subsequent observations failed to detect the QPO in the 0.3–10.0 keV band. In this talk we present the recent detection of the ∼ 1 h periodicity in the 1.0–4.0 keV band of 4 further low‐flux/spectrally‐harder observations of RE J1034+396 (see Alston et al. 2014). We also present recent work on the discovery of a QPO in the Seyfert galaxy, MS 2254.9–3712, which again is only detected in energy bands associated with the primary power‐law continuum emission (Alston et al. 2015). We conclude these features are most likely analogous to the high‐frequency QPOs observed in XRBs. In both sources, we also see evidence for X‐ray reverberation at the QPO frequency, where soft X‐ray bands and Iron Kα emission lag the primary X‐ray continuum. These time delays may provide another diagnostic for understanding the underlying QPO mechanism observed in accreting black holes. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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