The structure of AGNs from X-ray absorption variability

TL;DR

This paper provides evidence that X-ray absorption variability in AGNs is caused by clouds with properties similar to BLR clouds, revealing their shape and dynamics through time-resolved spectroscopy.

Contribution

It demonstrates that X-ray absorption variability is due to BLR-like clouds with cometary shapes, and shows how time-resolved spectroscopy can measure their physical and geometrical properties.

Findings

X-ray absorption varies on hours to days timescales.

Absorbing clouds have velocities >1000 km/s and resemble BLR clouds.

Clouds have a cometary shape with a dense head and trailing tail.

Abstract

We present new evidence of X-ray absorption variability on time scales from a few hours to a few days for several nearby bright AGNs. The observed N_H variations imply that the X-ray absorber is made of clouds eclipsing the X-ray source with velocities in excess of 10^3 km/s, and densities, sizes and distances from the central black hole typical of BLR clouds. We conclude that the variable X-ray absorption is due to the same clouds emitting the broad emission lines in the optical/UV. We then concentrate on the two highest signal-to-noise spectra of eclipses, discovered in two long observations of NGC 1365 and Mrk 766, and we show that the obscuring clouds have a cometary shape, with a high density head followed by a tail with decreasing N_H. Our results show that X-ray time resolved spectroscopy can be a powerful way to directly measure the physical and geometrical properties of BLR clouds.