The XMM-Newton view of NGC 6251

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 ∼ 6×10⁸ M_⊙. The 0.4-10 keV EPIC pn continuum is best fitted by two thermal components (kT ∼ 0.5 and 1.4 keV, respectively), plus a power law with photon index Γ ∼ 1.9 absorbed by a column density N_H ∼ 5 × 10²⁰ 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 (σ∼ 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.