A Simple Method for Predicting $N_H$ Variability in Active Galactic Nuclei

TL;DR

This paper introduces a new method using hardness ratio variations to predict $N_H$ variability in AGN, aiding in the selection of sources for detailed variability analysis.

Contribution

The paper presents a novel, simple approach to predict $N_H$ variability in AGN using hardness ratios, validated on X-ray observations from Chandra and XMM-Newton.

Findings

Method effectively predicts $N_H$ variability.

Predictions align well with spectral fitting results.

Useful for preselecting AGN for variability studies.

Abstract

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, $N_{H}$, 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 $N_H$ 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 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 $N_H$ values obtained from spectral fitting. We find that the method proposed in this work is effective in preselecting sources for variability studies.