The unified model of active galactic nuclei (AGN) includes a toroidal obscuring structure to explain the differences between Type I and Type II AGN as an effect of inclination angle. This toroidal structure is thought to be 'clumpy' as the line-of-sight column density, NH, has been observed to vary with time in many sources. We present a new method which uses a variation in hardness ratio to predict whether an AGN will have experienced NH variability across different observations. We define two sets of hard and soft bands that are chosen to be sensitive to the energies most affected by changes in NH. We calculate these ratios for Chandra and XMM-Newton observations on a sample of 12 sources with multiple observations, and compare the predictions of this method with the NH values obtained from spectral fitting. We find that the method proposed in this work is effective in preselecting sources for variability studies.
@article{arxiv.2301.07142,
title = {A Simple Method for Predicting $N_H$ Variability in Active Galactic Nuclei},
author = {Isaiah Cox and Nuria Torres-Alba and Stefano Marchesi and Xiurui Zhao and Marco Ajello and Andrealuna Pizzetti and Ross Silver},
journal= {arXiv preprint arXiv:2301.07142},
year = {2023}
}