Ubiquitous Variability of X-Rayabsorbing Column Densities in Seyfert 2 Galaxies

摘要

We present a study of the variations in the absorbing column density of 25 X-raydefined Seyfert 2 gal- axies, as inferred from hard X-ray observations, on timescales from months to several years. We show that a significant variation of N_H (from 20/100 to 80/100) is observed in almost all (22 of 25) of the sources with multiple X-ray observations, although X-ray absorption never vanishes. For a subsample of 11 sources observed at least five times, the typical variation time, as defined by a structure function, is less than 1 yr for both heavily absorbed (N_H 10～~23 cm~-2) and moderately absorbed (N_H ～10~22 cm~-2) sources.