A Colourful Analysis: Probing the Eclipse of the Black Hole and Central Engine in NGC 6814 Using X-ray Colour-Colour Grids

TL;DR

This study uses X-ray colour-colour analysis of NGC 6814 to investigate eclipse behavior, revealing complex absorber geometries and estimating cloud coverage, advancing understanding of AGN environments.

Contribution

It introduces a colour-colour grid method for analyzing AGN eclipses, providing new insights into absorber structures and dynamics during eclipses.

Findings

Absorber variations are dominated by column density and covering fraction changes.

Maximum eclipse behavior indicates multiple clouds or a halo, not a single cloud.

Approximately 2-4% of the orbit is obscured by clouds, with a non-isotropic distribution.

Abstract

Eclipsing of the X-ray emitting region in active galactic nuclei (AGN) is a potentially powerful probe to examine the AGN environment and absorber properties. Here we study the eclipse data from the 2016 XMM-Newton observation of NGC 6814 using a colour-colour analysis. Colours (i.e. hardness ratios) can provide the advantage of better time-resolution over spectral analysis alone. Colour-colour grids are constructed to examine the effects of different parameters on the observed spectral variability during the eclipse. Consistent with previous spectral analysis, the variations are dominated by changes in the column density and covering fraction of the absorber. However, during maximum eclipse the behaviour of the absorber changes. Just after ingress, the eclipse is described by changes in column density and covering fraction, but prior to egress, the variations are dominated by changes in column density alone. Simulations are carried out to consider possible absorber geometries that might produce this behaviour. The behaviour is inconsistent with a single, homogeneous cloud, but simulations suggest that multiple clouds, perhaps embedded in a highly ionised halo, could reproduce the results. In addition, we determine the orbital covering factor (fraction of orbital path-length) based on evidence of several eclipses in the 2016, 64-day Swift light curve. We estimate that ~ 2-4 per cent of the orbit is covered by obscuring clouds and that the distribution of clouds is not isotropic.