EDP Sciences
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Volume 413, Number 1, January I 2004
Page(s) 139 - 144
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20034053

A&A 413, 139-144 (2004)
DOI: 10.1051/0004-6361:20034053

The XMM-Newton view of NGC 6251

M. Gliozzi1, R. M. Sambruna1, W. N. Brandt2, R. Mushotzky3 and M. Eracleous2

1  George Mason University, Dept. of Physics & Astronomy & School of Computational Sciences, 4400 University Drive, MS 3F3, Fairfax, VA 22030, USA
2  The Pennsylvania State University, Department of Astronomy & Astrophysics, 525 Davey Lab, University Park, PA 16802, USA
3  NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA

(Received 4 July 2003 / Accepted 20 September 2003 )

We report on the nuclear X-ray properties of the radio galaxy NGC 6251 observed with XMM-Newton. NGC 6251 is a well-known radio galaxy with intermediate FRI/II radio properties. It is optically classified as a Seyfert 2 and hosts a supermassive black hole with mass ~ $ 6\times10^8~ M_{\odot}$. The 0.4-10 keV EPIC pn continuum is best fitted by two thermal components ( $kT \sim 0.5$ and 1.4 keV, respectively), plus a power law with photon index $\Gamma \sim 1.9$ absorbed by a column density $N_{\rm H} \sim 5 \times 10^{20}$ cm -2. We confirm the previous ASCA detection of a strong iron line. The line, resolved in the EPIC pn spectrum, is adequately fitted with a broad ( $\sigma\sim 0.6$ keV) Gaussian at rest-frame energy 6.4 keV with EW ~ 220 eV. We also detect, for the first time, short-term, low-amplitude variability of the nuclear flux on a timescale of a few ks. The spectral properties argue in favor of the presence of a standard accretion disk, ruling out the base of the jet as the sole origin of the X-rays. The moderate X-ray luminosity and lack of strong intrinsic absorption suggest that NGC 6251 is a "pure" type 2 AGN which lacks a broad-line region.

Key words: galaxies: active -- galaxies: nuclei -- X-rays: galaxies

Offprint request: M. Gliozzi, mario@physics.gmu.edu

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