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The nuclear SED of NGC6251 a BL Lac in the center of an FR I radio galaxy

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The nuclear SED of NGC 6251:a BL Lac in the center of an FR I radio galaxy 1Marco Chiaberge 2Space Telescope Science Institute,3700San Martin Drive,Baltimore,MD 21218Istituto di Radioastronomia del CNR -Via P.Gobetti 101,I-40129Bologna,Italy chiab@stsci.edu Roberto Gilli Istituto Nazionale di Astrofisica (INAF)–Osservatorio Astrofisico di Arcetri,Largo E.Fermi 5,I-50125Firenze,Italy Alessandro Capetti Istituto Nazionale di Astrofisica (INAF)–Osservatorio Astronomico di Torino,Strada Osservatorio 20,I-10025Pino Torinese,Italy F.Duccio Macchetto 3Space Telescope Science Institute,3700San Martin Drive,Baltimore,MD 21218ABSTRACT 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 both data from the literature and analyzing HST and X-ray archival data.In the log ν−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

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 (θ<∼20◦).NGC 6251is 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.Subject headings:galaxies:active —galaxies:nuclei —galaxies:jets —galaxies:individual (NGC 6251)

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 (θ<∼20◦).NGC 6251is 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.Subject headings:galaxies:active —galaxies:nuclei —galaxies:jets —galaxies:individual (NGC 6251)