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

Abstract

We determine the nuclear spectral energy distribution (SED) from the radio to the gamma-ray band for the FR I radio galaxy NGC 6251, by using data from the literature and analyzing Hubble Space Telescope and X-ray archival data. In the log ν- log(νFν) 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 Lac unification model. We model the overall emission in the frame of a synchrotron self-Compton scenario, which well describes the SED of BL Lac objects. The model parameters that we obtain also quantitatively confirm the FR I-BL Lac unification model and imply a rather small viewing angle to this source (θ 20°). NGC 6251 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 NGC 6251, which strengthens the overall result.