Letter to the Editor

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

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

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  keV with optical depth or with an absorption line with  keV, width keV and 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.