A Systematic Search for Active Galactic Nuclei Obscuration Variability in the Chandra Archive

The nature of the obscuring material in active galactic nuclei (AGN) can be studied by measuring changes in the line-of-sight column density, NH,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 Million Quasars (or Milliquas) catalog to estimate a lower limit on the fraction of sources in the local Universe (z < 0.1) that display spectral variability among observations. We find that 43 sources (54% ± 11%) show indications of NH,LOS variability at 90% confidence level. Interestingly, we also find that the variable fraction is similar for both Seyfert 1 ( fSy1∼61−15+13 %) and Seyfert 2 (fSy2 ∼ 47% ± 15%) galaxies. The slightly higher fSy1 fraction could be due to either a physical difference in the obscurers or the higher data quality in the Seyfert 1 population. We also search for potential dependencies on the timescale between variable and nonvariable 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 NH variability studies.