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Issue A&A
Volume 421, Number 2, July II 2004
Page(s) 461 - 471
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20047051



A&A 421, 461-471 (2004)
DOI: 10.1051/0004-6361:20047051

XMM-Newton discovery of a Compton-thick AGN in the GPS galaxy Mkn 668

M. Guainazzi1, A. Siemiginowska2, P. Rodriguez-Pascual1 and C. Stanghellini3

1  XMM-Newton Science Operation Center, VILSPA, ESA, Apartado 50727, 28080 Madrid, Spain
2  Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
3  Istituto di Radioastronomia CNR, Noto, Italy

(Received 12 January 2004 / Accepted 3 March 2004 )

Abstract
We report the XMM-Newton discovery of the first Compton-thick obscured AGN in a Broad Line Radio Galaxy, the Gigahertz Peaked-Spectrum source Mkn 668 (OQ+208). The remarkably flat 2-10 keV X-ray spectrum (observed photon index, $\Gamma \simeq$ 0.7), with a prominent iron K $_{\alpha}$ fluorescent emission line, is a clear signature of a Compton-reflection dominated spectrum. Mkn 688 represents a remarkable example of discrepancy between X-ray spectral properties and optical classification, as its optical spectrum is characterized by broad and asymmetric Balmer lines. The obscuring matter is constrained to be located within the radio hotspots, in turn separated by about 10 pc. If the jets are piercing their way through a Compton-thick medium pervading the nuclear environment, one could be largely underestimating the radio activity dynamical age determined from the observed hotspot recession velocity. The soft X-ray spectrum is dominated by a much steeper component, which may be due to nuclear continuum electron scattering, or inverse Compton of the - remarkably large - far infrared emission. Soft X-rays are suppressed by a further Compton-thin ( $N_{\rm H} \sim 10^{21}$ cm -2) absorbing system, that we identify with matter responsible for free-free absorption of the radio lobes.


Key words: galaxies: individual: Mkn 668 -- galaxies: jets -- galaxies: nuclei -- galaxies: Seyfert -- X-ray: galaxies

Offprint request: M. Guainazzi, mguainaz@xmm.vilspa.esa.es

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© ESO 2004


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