Rapid NH changes in NGC 4151

We have analysed the two longest (elapsed time >~3 d) BeppoSAX observations of the X-ray brightest Seyfert galaxy, NGC 4151, to search for spectral variability on time-scales from a few tens of kiloseconds to years. We found in both cases highly significant spectral variability below ~6 keV down to the shortest time-scales investigated. These variations can be naturally explained in terms of variations in the low energy cut-off due to obscuring matter along the line of sight. If the cut-off is modelled by two neutral absorption components, one fully covering the source and the second covering only a fraction of the source, the shortest time-scale of variability of a few days constrains the location of the obscuring matter to within 3.4 × 104 Schwarzschild radii from the central X-ray source. This is consistent with the distance of the broad emission-line region, as inferred from reverberation mapping, and difficult to reconcile with the parsec scale dusty molecular torus of Krolik & Begelman. We have also explored a more complex absorption structure, namely the presence of an ionized absorber. Although the behaviour of the ionization parameter is nicely consistent with the expectations, the results are not completely satisfactory from the statistical point of view. The overall absorption during the 2001 December observation is lower than in all other historical observations with similar 2-10 keV flux. This suggests that absorption variability plays a crucial role in the observed flux variability of this source.