Searching for energy-resolved quasi-periodic oscillations in AGN

Abstract

X-ray quasi-periodic oscillations (QPOs) in active galactic nucleus (AGN) allow us to probe and understand the nature of accretion in highly curved space–time, yet the most robust form of detection (i.e. repeat detections over multiple observations) has been limited to a single source to-date, with only tentative claims of single observation detections in several others. The association of those established AGN QPOs with a specific spectral component has motivated us to search the XMM–Newton archive and analyse the energy-resolved light curves of 38 bright AGNs. We apply a conservative false alarm testing routine folding in the uncertainty and covariance of the underlying broad-band noise. We also explore the impact of red-noise leak and the assumption of various different forms (power-law, broken power-law, and Lorentzians) for the underlying broad-band noise. In this initial study, we report QPO candidates in six AGNs (seven including one tentative detection in MRK 766) from our sample of 38, which tend to be found at characteristic energies and, in four cases, at the same frequency across at least two observations, indicating they are highly unlikely to be spurious in nature.

A study of natural frequencies in a dynamic corona – disk system

“…The natural frequency of the corona-disk system ranges between ∼ 1 Hz and tens of Hz for typical parameter values for BHXRBs, with a tendency to increase with ṁ increasing. Because of the derived scaling of f 0 with mass, our model predicts quasiperiodicity on timescales of a few hours (10 −5 − 10 −4 Hz) to several weeks (10 −8 − 10 −7 Hz) for supermassive black holes in AGN with masses between 10 6 and 10 9 M (for candidate QPOs in AGN, see e.g., Middleton & Done 2010;Carpano & Jin 2018;Ashton & Middleton 2021). On the other end, it predicts kHz QPOs for neutron stars with masses of 1 M , which indeed seems to be the case from the Rossi X-ray Timing Explorer observations of 4U 1728-34 and Sco X-1 made more than 25 years ago (Strohmayer et al 1996;van der Klis et al 1996).…”

Constraining energy-dependent emissivity profiles of AGN inflows

“…Where data quality permits, the energydependence of the variability can provide even greater insight via, e.g. energy-lag spectra (Kara et al 2019), deeper QPO searches (Ashton & Middleton 2021), rms and covariance spectra (Middleton et al 2009(Middleton et al , 2011Uttley et al 2014).…”

Evidence for a compact object in the aftermath of the extragalactic transient AT2018cow

“…There is no clear consensus on the exact mechanisms that produce these so-called High-Frequency Quasi-Periodic Oscillations (HFQPOs) but it is generally agreed upon that they originate from a region close to the compact object where the dynamics of motion are dictated by the compact object's strong gravitational field (see (28) and references therein), and they represent a direct evidence for the presence of a compact object. Some active galactic nuclei have also shown evidence for QPOs which have been argued to be analogous to HFQPOs of stellar-mass black holes (29)(30)(31). More recently, HFQPO analogs (frequencies of a few mHz) have also been found in stellar tidal disruption events involving ∼10 6 M black holes (32)(33)(34) suggesting that, perhaps, such QPOs are universal among all compact object systems that undergo extreme changes in accretion.…”