Hydrogen Column Density Variability in a Sample of Local Compton-thin AGN II

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Abstract We present the multiepoch analysis of 13 variable, nearby (z ≲ 0.1), and Compton-thin (1022 < N H < 1.5 × 1024 cm−2) active galactic nuclei (AGN) selected from the 105-month BAT catalog. Analyzing all available archival soft and hard X-ray observations, we investigate the line-of-sight hydrogen column density (N H,los) variability on timescales ranging from a few days to approximately 20 yr. Each source is analyzed by simultaneously modeling the data with three physical torus models, providing tight constraints on torus properties, including the covering factor, the cloud dispersion, and the torus average hydrogen column density (N H,av). Our final sample includes 27 variable, Compton-thin AGN after implementing another 14 AGN analyzed in our previous work. We find that all sources require either flux or N H,los variability. Based on the degree of variability, we classify 37% of them as “N H,los Variable,” 44% as “Nonvariable in N H,los,” and 19% as “Undetermined.” Noticeably, there is no discernible difference between geometrical and intrinsic properties among the three variability classes, suggesting no intrinsic differences between the N H,los-variable and nonvariable sources within the considered sample. We measure the median variation in N H,los between any observation pair of the same source to be 25% with respect to the lowest N H,los measure in the pair. Furthermore, 48% of the analyzed sources require the inclusion of a Compton-thick reflector in the spectral fitting. Among these, 30% exhibit recorded 22 GHz water megamaser emission, suggesting a potential shared nature between the two structures.