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(-0.52)(+0.34) keV with optical depth tau = 0.06 +/- 0.03 or with an absorption line with 9.5(-0.6)(+1.3) keV, width sigma = 0.95(-0.7)(+1.2) 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 approximate to (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 approximate to 3-5 times lower than observed in the other states, and the covering fraction is reduced by approximate to 40 per cent. We propose a possible explanation linking the two properties in terms of a multi-phase ionized absorber.
Piro, L., De Rosa, A., Matt, G., Perola, G.c. (2005). Clearing up the clouds around NGC 4151: evidence of a highly ionized absorber. ASTRONOMY & ASTROPHYSICS, 441(3), L13-L17 [10.1051/0004-6361:200500174].
Clearing up the clouds around NGC 4151: evidence of a highly ionized absorber
MATT, Giorgio;
2005-01-01
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(-0.52)(+0.34) keV with optical depth tau = 0.06 +/- 0.03 or with an absorption line with 9.5(-0.6)(+1.3) keV, width sigma = 0.95(-0.7)(+1.2) 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 approximate to (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 approximate to 3-5 times lower than observed in the other states, and the covering fraction is reduced by approximate to 40 per cent. We propose a possible explanation linking the two properties in terms of a multi-phase ionized absorber.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
