The x-ray variability and reverberation of Active Galactic Nuclei

Abstract

All galaxies are thought to contain a super massive black hole at their core. Some contain an Active Galactic Nuclei (AGN) thought to be powered by an accretion flow. A fraction of AGN are known to exhibit signatures of X-ray reverberation caused by the reflection of photons from the corona off the accretion disc. Inverse Compton scattering of disc photons by energetic electrons in the corona, coupled with disc fluctuations generate the highly variable observed X-ray spectrum, the behaviour of which is still not fully understood. Using 121 observations from the XMM-Newton archives in 3–4 distinct spectral groups, the reverberation lags are calculated as a function of frequency and the relativistic reflection model RELXILL is fitted prior to investigating parameter correlations. The known relationship between the black hole mass and time lag is well recovered and the continuum flux is coupled strongly to the disc reflection. All AGN are well described by reflection and absorption modelling. Investigations of 1H 0707- 495 and IRAS 13224-3809 conclude the reflection fraction and power law photon index may be linked to dynamics of the emitting region. Hints of the power law evolutionary turnover when the Eddington fraction is ∼ 0.02 are also evident. The covering fraction in IRAS 13224- 3809 may contribute to the observed variability via flux modulations from non-uniform orbiting clouds. These 2 AGN have also been explored using an extended corona model which employs two X-ray sources in vertically extended geometry. The model is capable of predicting the observed lag-frequency profile in full and estimating the size of the corona to include well constrained mass estimates. In this scenario, fluctuations propagate vertically from the lower to upper source, inferring the presence of a relativistic jet.

Date of Award	20 Jun 2023
Original language	English
Awarding Institution	University of Bristol
Supervisor	Mark Birkinshaw (Supervisor) & Andrew J Young (Supervisor)