Obscuration Variability in Nearby AGN
Núria Torres-Alba (Univeristy of Virginia)
Tuesday 28/10/2025 @ 14:00, Sala Antonio Sollima (IV piano Battiferro)
Active galactic nuclei (AGN) are powered by accreting supermassive black holes, surrounded by a torus of obscuring material. The exact geometry of this material has been a subject of debate, as models have advanced from the initial homogeneous torus to a variety of possibilities, ranging from cloud distributions to warped disks, to outflows. It is clear, however, that this distribution is clumpy: X-ray determinations of line-of-sight (l.o.s.) obscuration show variability in timescales from <1 day to years. However, studies of large samples of sources tend to find variability in <50% of Sy2s analyzed; and it is unclear whether this result is compatible with our understanding of the clumpy torus. X-ray observations are the only way to probe the obscuring column density in the line of sight at any given time, and thus the optimal tool to place constraints on the exact distribution of this material. Here, I present our group's efforts in using archival X-ray data to characterize obscuration variability of a large sample of nearby AGN for the first time; from the systematic analysis of more than 30 sources, to a search for variability in the Chandra archive, to how the data can be use to calibrate clumpy torus models.