Benjamin Pottie

The focus of my research is on studying obscuration in active galactic nuclei (AGNs). Obscuration of AGN offers a window into the AGN environment, allowing us to better understand what occupies these inner regions. Transient obscuration events generally result in X-ray eclipsing of the AGN. One such AGN, known as NGC 6814, had nearly the entirety of an eclipsing event observed by XMM-Newton in 2016, which is a rarity. This event, which had a similar light curve shape to that seen with exoplanet transits of a star, seemingly pointed towards the eclipsing object being a simple homogeneous cloud (Gallo et al., 2021). My work analyzed this further by applying colour-colour grids to the source. By using colours, we get to understand the behaviour of data in different energy bands, as colours are defined as count rates in one energy band divided by count rates in another energy band. These grids are constructed by varying two different parameters within a given model. Comparing data from the 2016 eclipse to a colour-colour grid, we were then able to study properties of the eclipsing absorber. This revealed a more complex scenario than was initially thought, with significant variations in both the fraction of the source being covered by the absorber (covering fraction) and the amount of absorber material along the line-of-sight (column density). My research showed that these changes could not be described by a simple homogenous cloud and that a more complex geometry is required to explain the variations seen. As the uniqueness of NGC 6814 continues to be studied, further plans for my research are applying these grids to other eclipsed or obscured sources.

This plot shows the colour-colour grid created for NGC 6814. The soft colour (x-axis) is taken as the counts in the 0.3-1 keV band over those in the 1-4 keV band, while the hard colour (y-axis) is taken as the counts in the 1-4 keV band over those in the 4-10 keV band. Tracks on the grid indicate changes in two parameters of the eclipsing absorber: column density (amount of absorber material along the line-of-sight) and covering fraction (fraction of source being covered by the absorber). Smoothed data from the 2016 observation of the NGC 6814 eclipse by the XMM-Newton observatory is compared to the tracks on the grid. This shows data from the high-state (pre-eclipse), ingress (starting to be eclipsed), low-state (during the eclipse), and egress (going towards the non-eclipsed phase) phases of NGC 6814 during this eclipsing event.