Characterizing the X-ray variability of QSOs to the highest Eddington ratios and black hole masses with eROSITA light curves

ABSTRACT An important diagnostic of the inner structure of accretion flows onto supermassive black holes are the stochastic flux variations at X-ray wavelengths. Despite its significance, a systematic characterization of the statistical properties of the X-ray variability to the highest Eddington ratios and most massive black holes is still lacking. In this paper, we address this issue using SRG/eROSITA (Spectrum-Roentgen-Gamma/extended ROentgen Survey with an Imaging Telescope Array) five-epoch light curves to characterize the mean X-ray variability of optically selected Sloan Digital Sky Survey quasars extending to black holes masses of $10^{10}$ solar and accretion rates close to the Eddington limit. The adopted variability statistic is the ensemble normalized excess variance, which is measured using a novel hierarchical Bayesian model (eBExVar) tailored to the Poisson nature of the X-ray light curves. We find a clear anticorrelation of the ensemble variability with black hole mass, extending previous results to time-scales of months. This can be interpreted as evidence for an X-ray corona size and/or physical conditions that scale with black holes mass. We also find an unexpected increase of the ensemble normalized excess variance close to the Eddington limit, which is contrary to the predictions of empirical variability models. This result suggests an additional variability component for fast growing black holes that may be related to systematic variations of the hot corona size with Eddington ratio or shielding of the hot corona by an inner puffed-up disc and/or outflows.