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

Abstract

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^+0.34_-0.52 keV 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^+1.2_-0.7 keV and EW= 200 eV. In the first case, we associate the feature to highly ionized iron at rest, like FeXXII-FeXXIII ({E_rest}=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.