The Nuclear Spectral Energy Distribution of NGC 6251: A BL Lacertae Object in the Center of an FR I Radio Galaxy

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Abstract
We determine the nuclear spectral energy distribution (SED) from the radio to the gamma-ray band for the FR I radio galaxy NGC6251, by using both data from the literature and analyzing HST and X-ray archival data. In the Log nu - Log nu F(nu) representation, the SED has two broad peaks, and it is remarkably similar to those of blazars. We show that the low-energy peak can be explained in terms of synchrotron radiation, while the high energy peak is most plausibly produced by inverse Compton scattering. This brings direct support to the FR I-BL Lacs unification model. We model the overall emission in the frame of a synchrotron self-Compton scenario, which well describes the SED of BL Lacs. The model parameters we obtain confirm also quantitatively the FR I-BL Lac unification model and imply a rather small viewing angle to this source (theta <~ 20deg). NGC6251 is the second radio galaxy, in addition to Centaurus A, for which a similar analysis has been performed. A significant improvement with respect to the case of Cen A is the absence of obscuration in NGC6251, which strengthens the overall result.Comment: 14 pages, 6 figures, accepted for publication in the Ap
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