2014MNRAS.439.1403M

We present an analysis of multitime-scale variability in line-of-sight X-ray-absorbing gas as a function of optical classification in a large sample of Seyfert active galactic nuclei (AGN) to derive the first X-ray statistical constraints for clumpy-torus models. We systematically search for discrete absorption events in the vast archive of Rossi X-ray Timing Explorer monitoring of dozens of nearby type I and Compton-thin type II AGN. We are sensitive to discrete absorption events due to clouds of full-covering, neutral or mildly ionized gas with columns ≳ 10^22–25/cm2 transiting the line of sight. We detect 12 eclipse events in 8 objects, roughly tripling the number previously published from this archive. Peak column densities span ∼ 4-26x10²²/cm2, i.e. there are no full-covering Compton-thick events in our sample. Event durations span hours to months. The column density profile for an eclipsing cloud in NGC 3783 is doubly spiked, possibly indicating a cloud that is being tidally sheared. We infer the clouds' distances from the black hole to span ∼ 0.3-140x10⁴Rg. In seven objects, the clouds' distances are commensurate with the outer portions of broad line regions (BLR), or outside the BLR by factors up to ∼ 10 (the inner regions of infrared-emitting dusty tori). We discuss implications for cloud distributions in the context of clumpy-torus models. Eight monitored type II AGN show X-ray absorption that is consistent with being constant over time-scales from 0.6 to 8.4 yr. This can either be explained by a homogeneous medium, or by X-ray-absorbing clouds that each have NH ≪ 10²²/cm2. The probability of observing a source undergoing an absorption event, independent of constant absorption due to non-clumpy material, is 0.006^+0.160_-0.003 for type Is and 0.110^+0.461_-0.071 for type IIs.