Spectral energy distributions of FR I nuclei and the FR I / BL Lac unifying model

We consider archival ROSAT and Hubble Space Telescope (HST) observations of five FR I radio galaxies and isolate their nuclear emission from that of the host galaxy. This enable us to determine the Spectral Energy Distributions (SED) of their nuclei spanning from the radio to the X–ray band. They cannot be described as single power–laws but require the presence of an emission peak located between the IR and soft X-ray band. We found consistency between the SED peak position and the values of the broad band spectral indices of radio galaxies with those of BL Lac, once the effects of beaming are properly taken into account. FR I SED are thus qualitatively similar to those of BL Lacs supporting the identification of FR I sources as their misoriented counterparts. No dependence of the shape of the SED on the FR I orientation is found.