EDP Sciences
Free access
Volume 410, Number 1, October IV 2003
Page(s) 131 - 138
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20031195

A&A 410, 131-138 (2003)
DOI: 10.1051/0004-6361:20031195

The hard X-ray view of the low-luminosity blazar in the radio galaxy NGC 6251

M. Guainazzi1, P. Grandi2, A. Comastri3 and G. Matt4

1  XMM-Newton Science Operations Center, VILSPA, ESA, Apartado 50727, 28080 Madrid, Spain
    e-mail: mguainaz@xmm.vilspa.esa.es
2  Istituto di Astrofisica Spaziale e Fisica Cosmica (IASF-C.N.R.), Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy
3  I.N.A.F., Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
4  Dipartimento di Fisica, Università degli Studi "Roma Tre", via della Vasca Navale 84, 00146 Roma, Italy

(Received 21 February 2003 / Accepted 31 July 2003 )

We present results from a BeppoSAX (July 2001) observation of the FR I radio galaxy NGC 6251, together with a re-analysis of archival ASCA (October 1994) and Chandra (September 2000) data. The weak detection above 10 keV and the lack of iron fluorescent K $_{\alpha}$ emission lines in the BeppoSAX spectrum rule out that the bulk of the X-ray emission is due to an obscured Seyfert nucleus. The study of the multiwavelength spectral energy distribution suggests instead that X-rays probably originate as inverse-Compton of synchrotron seed photons in a relativistic jet, indicating that NGC 6251 hosts a low radio luminosity ( $L_{{\rm 5 \ GHz}} \sim 10^{40}$ erg s -1) blazar. The BeppoSAX spectrum is flatter than in the earlier ASCA observation. This might be due to the emergence of a different spectral component during phases of lower X-ray flux. In this context, we discuss some possible explanations for the intense and mildly-ionized fluorescent iron line measured by ASCA.

Key words: galaxies: active -- galaxies: individual: NGC 6251 -- galaxies: jets -- galaxies: nuclei -- X-rays: galaxies

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

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