Search for X-ray occultations in Active Galactic Nuclei

TL;DR

This study investigates the prevalence of X-ray occultations by Broad Line Region clouds in Active Galactic Nuclei, revealing that such events are common and significantly contribute to spectral variability in bright AGN.

Contribution

First homogeneous analysis of X-ray occultations in a representative AGN sample, establishing the commonality and impact of BLR cloud occultations on spectral variability.

Findings

X-ray eclipses are common in sources with compatible occultation times.

Occultations are not observed in sources with longer occultation times.

BLR clouds likely cause most high-energy spectral variability in AGN.

Abstract

Recent time-resolved spectral studies of a few Active Galactic Nuclei in hard X-rays revealed occultations of the X-ray primary source probably by Broad Line Region (BLR) clouds. An important open question on the structure of the circumnuclear medium of AGN is whether this phenomenon is common, i.e. whether a significant fraction of the X-ray absorption in AGN is due to BLR clouds. Here we present the first attempt to perform this kind of analysis in a homogeneous way, on a statistically representative sample of AGN, consisting of the ~40 brightest sources with long XMM-Newton and/or Suzaku observations. We describe our method, based on a simple analysis of hardness-ratio light curves, and its validation through a complete spectroscopic analysis of a few cases. We find that X-ray eclipses, most probably due to clouds at the distance of the BLR, are common in sources where the expected occultation time is compatible with the observation time, while they are not found in sources with longer estimated occultation times. Overall, our results show that occultations by BLR clouds may be responsible for most of the observed X-ray spectral variability at energies higher than 2 keV, on time scales longer than a few ks.