The energy dependence of AGN variability as a means to probe the nature of accretion

Abstract

The study of X-ray variability in AGN allows us to probe the nature of accretion in highly curved space-time. This work sets out to study energy-resolved XMM-Newton observations of a large sample of AGN using Fourier analysis techniques, in order to search for X-ray quasi-periodic oscillations (QPOs) across as yet unexplored energy bands, and more broadly investigate the energy dependence of X-ray variability in AGN. Robust detection of QPOs in AGN (i.e. repeat detections over multiple observations) have been limited to a single source to-date, with only tentative claims of single observation detections in several others. I develop a deep QPO search using the XMM Newton archive to analyse the energy-resolved light curves of 38 bright AGN, and apply a conservative false alarm testing routine folding in the uncertainty of the modelled broad-band noise. I report statistically significant QPO candidates in 6 AGN (+1 tentative detection) from this initial study, including four cases of candidates at the same frequency across at least two observations, indicating they are highly unlikely to be spurious in nature. I additionally explore the impact of a number of effects which may act to bias the detection significance and scrutinise the validity of these detections. I also explore the energy-dependence of the power spectrum for five AGN across the XMM-Newton bandpass and find a ubiquitous flattening of the power spectrum towards harder energies. I develop a theoretical framework to explore this behaviour, considering both propagation and reflection components, and investigate the nature of the radial emissivity of the accretion flow. Assuming a simple disc-like geometry, I extract the energy dependence of the emissivity, finding it to range from ∝∼ R −2 at energies around the soft excess to ∝∼ R −4 or steeper at harder energies. I attempt to correlate the gradient of the linear function used to parameterise the emissivity index against key AGN parameters but, as yet, the sample size is too small to confirm hints of a correlation with Eddington ratio.

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