A systematic search for AGN obscuration variability in the Chandra archive

TL;DR

This study analyzes archival Chandra data of 79 Seyfert galaxies to estimate the fraction exhibiting obscuration variability, revealing over half show significant changes in line-of-sight column density, with variability increasing over longer timescales.

Contribution

It provides the first population-level estimate of obscuration variability in Seyfert galaxies using archival data, highlighting the prevalence and timescale dependence of such variability.

Findings

54% of sources show $N_{H,los}$ variability at 90% confidence

Variable fraction is similar for Seyfert 1 and Seyfert 2 galaxies

Longer timescales are associated with increased variability

Abstract

The nature of the obscuring material in active galactic nuclei (AGN) can be studied by measuring changes in the line-of-sight column density, $N_{\rm H,los}$, over time. This can be accomplished by monitoring AGN over long periods of time and at all timescales. However, this can only be done for a few selected objects as it is resource intensive. Therefore, the best option currently is to focus on population statistics based on the available archival data. In this work, we study 79 Seyfert 1 and Seyfert 2 galaxies from the Milliquas catalog to estimate a lower limit on the fraction of sources in the local $(z<0.1)$ universe that display spectral variability among observations. We find that 43 sources $(54\pm11\%)$, show indications of $N_{\rm H,los}$ variability at 90% confidence level. Interestingly, we also find that the variable fraction is similar for both Seyfert 1 $(f_{\rm Sy1}\sim61^{+13}_{-15}\%)$ and Seyfert 2 $(f_{\rm Sy2}\sim47\pm15\%)$ galaxies. The slightly higher $f_{\rm Sy1}$ fraction could be due to either a physical difference in the obscurers or the higher data quality in the Sy1 population. We also search for potential dependencies on the timescale between variable and non-variable observation pairs within a given source. In agreement with previous studies, we find evidence that more variability occurs on longer timescales than on shorter timescales. We present the 43 variable sources as a promising sample for future $N_{\rm H}$ variability studies.