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 logν-log(νF_ν) 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^deg). 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.

Based on observations obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS 5-26555.