A Simple Method for Predicting NH Variability in Active Galactic Nuclei

The unified model of active galactic nuclei (AGNs) includes a geometrically thick obscuring medium to explain the differences between type I and type II AGNs as an effect of inclination angle. This medium is often referred to as the torus and is thought to be "clumpy" as the line-of-sight column density, N-H, has been observed to vary in time for many sources. We present a method which uses a variation in the hardness ratio to predict whether an AGN has experienced N-H 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 N-H. We calculate hardness ratios for Chandra and XMM-Newton observations of a sample of 12 sources with multiple observations, and compare the predictions of this method to the N-H values obtained from spectral fitting with physically motivated torus models (borus02, MYTorus, and UXCLUMPY). We also provide a calibrated correction factor that allows comparison between Chandra and XMM-Newton observations which is otherwise not possible due to differences in the instrument response functions. The sensitivity of this method can be easily adjusted. As we decrease the sensitivity, we find that the false positive rate becomes small while the true positive rate remains above 0.5. We also test the method on simulated data and show that it remains reliable for observations with as few as 100 counts. Therefore, we conclude that the method proposed in this work is effective in preselecting sources for variability studies.