Probing scaling Relations of AGN accretion parameters through the ensemble QSO X-ray variability in XMM- Newton light Curves

Active Galactic Nuclei exhibit characteristic stochastic flux fluctuations in their X-ray emission, providing important insights into the accretion physics around supermassive black holes. Despite this, few systematic studies have targeted variability in AGNs with high Eddington ratios and massive black holes, particularly on shorter timescales that closely reflect conditions near the central engine. In this thesis, we study intrinsic X-ray variability through normalized excess variance (NXSV) using a sample of optically-selected SDSS QSOs also found in observations from XMM-Newton’s Serendipitous Source Catalog. Our sample spans Eddingtonratios λEdd ≈ 10^−2 to the Eddington limit and black hole masses MBH ≈ 10^8–10^10 M⊙. We employ a custom processing pipeline for optimal light curve extraction and utilize a new Bayesian hierarchical methodology fit for the Poisson nature of the light curves to estimate mean NXSV across populations of physically similar QSOs. Our analysis reveals a positive correlation between NXSV and λEdd, challenging earlier studies reporting an anti-correlation. This increased short-term X-ray variability at λEdd may result from changes in the corona’s geometry or from enhanced turbulence associated with transitions to slim-disk accretion. Conversely, variability shows no clear scaling with MBH, although comparisons with theoretical models suggest a subtle correlation could be obscured by the uncertainties inherent to our values.