EDP Sciences
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Volume 441, Number 3, October III 2005
Page(s) L13 - L17
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200500174

A&A 441, L13-L17 (2005)
DOI: 10.1051/0004-6361:200500174


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

L. 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
    e-mail: piro@rm.iasf.cnr.it
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 $\tau=0.06\pm0.03$ or with an absorption line with 9.5+1.3-0.6 keV, width $\sigma=0.95^{+1.2}_{-0.7}$ keV and EW= 200 eV. In the first case, we associate the feature to highly ionized iron at rest, like FeXXII-FeXXIII ( ${E_{\rm rest}}=8.4$-8.5 keV). In the second case the feature could be identified with a blend of FeXXV and FeXXVI lines, with an outflow velocity $v \approx (0.09$-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 $\approx$3-5 times lower than observed in the other states, and the covering fraction is reduced by $\approx$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

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