Using X-ray Hardness Ratios to Predict NH,los Variability in Nearby AGN

Active galactic nuclei (AGN) are accreting supermassive black holes that emit across the electromagnetic spectrum. Type II AGN display obscured X-ray emission with column densities N_H>10²²cm^-2. The physical nature of this obscuring material is not well understood, however, it is commonly attributed to an inhomogeneous, dusty torus structure. Studying the variability in the column density along the line of sight provides insight into the properties of the obscuring medium. Here, we present a method to find sources with variable line-of-sight column density (N_H,los) by using hardness ratios in X-ray data from Chandra and XMM-Newton. We test our method on a sample of sources with N_H,los determined by fitting X-ray spectra with physically-motivated torus models. The AUC-ROC score for this classification method is >0.8, indicating a good classifier. Therefore, our results show that our method is able to quickly and reliably predict whether a source has variable N_H,los between two observations without the need for time-consuming spectral fitting.