The XMM-Newton view of NGC 6251

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

Corresponding author: M. Gliozzi, mario@physics.gmu.edu

Received: 4 July 2003
Accepted: 20 September 2003

Abstract

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 ~. The 0.4–10 keV EPIC pn continuum is best fitted by two thermal components ( and 1.4 keV, respectively), plus a power law with photon index absorbed by a column density 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 ( 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.