Astronomy and Astrophysics, volume 444, 119-132 (2005/12-2)
X-ray obscuration and obscured AGN in the local universe.
GUAINAZZI M., MATT G. and PEROLA G.C.
Abstract (from CDS):
We discuss the X-ray properties of 49 local (z<0.035) Seyfert 2 galaxies with HST/WFC2 high-resolution optical coverage. It includes the results of 26 still unpublished Chandra and XMM-Newton observations, which yield 25 (22) new X-ray detections in the 0.5-2keV (2-10keV) energy band. Our sample covers a range in the 2-10 keV observed flux, F2–10, from 3x10–11 to 6x10–15erg/cm2/s. The percentage of objects that are likely obscured by Compton-thick matter (column density, NH≥(σt)–1 ≃1.6x1024cm–2) is ≃50%, and reaches ≃80% for log (F2–10)<12.3. Hence, Kα fluorescent iron lines with large Equivalent Width (EW>0.6keV) are common in our sample (6 new detections at a confidence level ≥2σ). They are explained as due to reflection off the illuminated side of optically thick material. We confirm a correlation between the presence of a ∼100-pc scale nuclear dust in the WFC2 images and Compton-thin obscuration. We interpret this correlation as due to the large covering fraction of gas associated with the dust lanes. The X-ray spectra of highly obscured AGN invariably present a prominent soft excess emission above the extrapolation of the hard X-ray component. This soft component can account for a very large fraction of the overall X-ray energy budget. As this component is generally unobscured - and therefore likely produced in extended gas structures - it may lead to a severe underestimation of the nuclear obscuration in z∼1 absorbed AGN, if standard X-ray colors are used to classify them. As a by-product of our study, we report the discovery of a soft X-ray, luminous (≃7x1040erg/s) halo embedding the interacting galaxy pair Mkn 266.