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

Abstract

The Seyfert 1 galaxy NGC4151 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\pm0.03 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 \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.

5 pages, 4 figures. Accepted for publication in Astronomy & Astrophysics Letters

References in corpus (7)

• A high velocity ionised outflow and XUV photosphere in the narrow emission line quasar PG1211+143
• A Massive X-Ray Outflow from the Quasar PDS 456
• Chandra Detects Relativistic Broad Absorption Lines from APM 08279+5255
• Evidence of a high velocity ionised outflow in a second narrow line quasar PG0844+349
• Fe XXV and Fe XXVI lines from low velocity, photoionised gas in the X-ray spectra of AGN
• O VII and O VIII absorption by hot gas in the vicinity of the Galaxy
• High resolution X-ray spectra of quasars

Cited by in corpus (7)

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• Extreme flux states of NGC 4151 observed with INTEGRAL
• X-ray spectral properties of Seyfert galaxies and the unification scheme
• The broad band spectrum and variability of NGC 4151 observed by BeppoSAX
• The hard X-ray spectrum of NGC 1365: scattered light, not black hole spin