A&A 441, L13-L17 (2005)
Clearing up the clouds around NGC 4151: evidence of a highly ionized absorberL. Piro1, A. De Rosa1, G. Matt2 and G. C. Perola2
1 Istituto di Astrofisica Spaziale e Fisica Cosmica, INAF, via Fosso del Cavaliere, 00133 Roma, Italy
2 Dipartimento di Fisica, Università degli Studi "Roma Tre", via della Vasca Navale 84, 00146 Roma, Italy
(Received 4 July 2005 / Accepted 18 August 2005 )
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 or with an absorption line with 9.5+1.3-0.6 keV, width keV and EW= 200 eV. In the first case, we associate the feature to highly ionized iron at rest, like FeXXII-FeXXIII ( -8.5 keV). In the second case the feature could be identified with a blend of FeXXV and FeXXVI lines, with an outflow velocity -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.
Key words: galaxies: Seyfert -- X-rays: galaxies -- galaxies: individual: NGC 4151
© ESO 2005