Searching for energy-resolved quasi-periodic oscillations in AGN

TL;DR

This study searches for energy-resolved X-ray quasi-periodic oscillations in 38 bright AGN using archival data, identifying 6-7 candidates with potential repeat detections, advancing understanding of accretion processes in AGN.

Contribution

It introduces a conservative detection method for AGN QPOs and reports multiple candidates with consistent frequencies across observations, a novel step in the field.

Findings

QPO candidates found in 6-7 AGN.

Candidates tend to occur at specific energies.

Some candidates show consistent frequencies across observations.

Abstract

X-ray quasi-periodic oscillations (QPOs) in 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 lightcurves of 38 bright AGN. 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 6 AGN (7 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.