Clearing up the clouds around NGC 4151 : evidence of a highly ionized absorber

The Seyfert 1 galaxy NGC 4151 is characterized by complex X-ray absorption, well described by a dual absorber, composed of a uniform mildly ionized gas and a cold system that partially covers the central source. However, in one of the 5 BeppoSAX observations, the spectrum shows two peculiar features. An absorption feature is detected around 8.5-9 keV with a statistical significance of 99.96%. This feature can be fitted either with an absorption edge at E= 8.62 −0.52keV with optical depth τ = 0.06 ± 0.03 or with an absorption line with 9.5 +1.3 −0.6 keV, width σ = 0.95 −0.7 keV and EW= 200 eV. In the first case, we associate the feature to highly ionized iron at rest, like FeXXII-FeXXIII (Erest=8.4-8.5 keV). In the second case the feature could be identified with a blend of FeXXV and FeXXVI lines, with an outflow velocity v≈ (0.09 − 0.26)c. This spectrum is also characterized by a substantial reduction of the absorption column density and the covering fraction of the dual absorber. In particular the column density of the mildly ionized and cold absorbers is ≈ 3 − 5 times lower than observed in the other states, and the covering fraction is reduced by ≈ 40 per cent. We propose a possible explanation linking the two properties in terms of a multi-phase ionized absorber.