EDP Sciences
Free access
Volume 414, Number 3, February II 2004
Page(s) 885 - 894
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20031657

A&A 414, 885-894 (2004)
DOI: 10.1051/0004-6361:20031657

The XMM-Newton view of the X-ray halo and jet of NGC 6251

R. M. Sambruna1, M. Gliozzi1, D. Donato1, F. Tavecchio2, C. C. Cheung3 and R. F. Mushotzky4

1  George Mason University, Dept. of Physics and Astronomy and School of Computational Sciences, MS 3F3, 4400 University Drive, Fairfax, VA 22030, USA
2  Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy
3  Brandeis University, Department of Physics, MS 057, Waltham, MA 02454, USA
4  NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA

(Received 21 July 2003 / Accepted 24 October 2003)

We present an XMM-Newton observation of the radio jet and diffuse halo of the nearby radio galaxy NGC 6251. The EPIC spectrum of the galaxy's halo is best-fitted by a thermal model with temperature $kT \sim 1.6$ keV and sub-solar abundances. Interestingly, an additional hard X-ray component is required to fit the EPIC spectra of the halo above 3 keV, and is independently confirmed by an archival Chandra observation. However, its physical origin is not clear. Contribution from a population of undetected Low Mass X-ray Binaries seems unlikely. Instead, the hard X-ray component could be due to inverse Compton scattering of the CMB photons (IC/CMB) off relativistic electrons scattered throughout the halo of the galaxy, or non-thermal bremsstrahlung emission. The IC/CMB interpretation, together with limits on the diffuse radio emission, implies a very weak magnetic field, $B \ll 1~\mu$Gauss, while a non-thermal bremsstrahlung origin implies the presence of a large number of very energetic electrons. We also detect X-ray emission from the outer (~3.5 $^{\prime}$) jet, confirming previous ROSAT findings. Both the EPIC and ACIS spectra of the jet are best-fitted by a power law with photon index $\Gamma
\sim 1.2$ , fixed Galactic column density, and 1 keV flux $F_{\rm 1~keV}=2.1$ nJy. A thermal model is formally ruled out by the data. Assuming an origin of the X-rays from the jet via IC/CMB, as suggested by energetic arguments, and assuming equipartition implies a large Doppler factor ( $\delta \sim 10$). Alternatively, weaker beaming is possible for magnetic fields several orders of magnitude lower than the equipartition field.

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

Offprint request: R. M. Sambruna, rms@physics.gmu.edu

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