Ubiquitous column density variability in Seyfert 2 Galaxies
  We present a study of X-ray column density variability in Seyfert 2 galaxies. We show that variations in N_H are observed in almost all the objects with multiple hard X-ray observations. Variation timescales (as short as a few months in several cases) are not in agreement with the standard scenario of a parsec-scale toroidal absorber. We propose that the X-ray absorber in Seyfert galaxies is located much nearer to the center than previously assumed, on the Broad Line Region Scale. An extension of the model by M. Elvis (2000) can explain the observed variability. We also show preliminary results of N_H variability search inside single X-ray observations, which suggest that variations can occur on timescales of a few 10^4 seconds. 
G. Risaliti (1,2), M. Elvis (2), F. Nicastro (2) ((1) Arcetri   Observatory, Italy (2) Harvard-Smithsonian Center for Astrop...