Abstract

In this thesis we address the subject of the unification of radio-loud Active Galactic Nuclei (AGN) (FRI/FRII galaxies, BL Lac objects and quasars) with a statistical and multiwavelength approach, paying particular attention to the X-ray band which carries precious information on the innermost regions of AGN. A large sample of 2260 AGN of different kinds was created by cross-correlating the ROSAT catalogs with various radio surveys and, for each source, data were collected in the radio, optical and soft X-ray bands. 1682 objects are formally classified as radio-loud and are analyzed in this thesis. The main purpose of this thesis is to test the unified scheme for radio-loud AGN by investigating if the correlations between luminosities at different frequencies are consistent with the basic assumption that BL Lac objects and radio-loud quasars are relativistically beamed counterparts of FRI and FRII radio galaxies, respectively. Although this is the key hypothesis of the unification scheme for radio-loud AGN, these questions have not been carefully analyzed so far. The results of this thesis are in general agreement with the relativistic beaming scenario, however, some complications have been highlighted. Tight relationships between the nuclear emission in the three wavebands considered are confirmed for all classes and we were able to better constrain the parameters of the correlations due to the larger number of objects compared to previous investigations. However, for FRI galaxies and BL Lac objects, more than one emission component is required, at least in the X-ray band, to explain in a relativistic beaming scenario the different parameters observed for the correlations. One component can easily be associated with the jet, whereas the other remains so far unknown. A possible problematic aspect of the unified scheme is that, among the FRI galaxies, the counterparts of Low-energy-peaked BL Lacs have not been found. Absorption with N_H > 10^{22} cm^{-2} is present in FRII galaxies and plays, together with relativistic beaming, a major role in the unification with radio-loud quasars. Allowing for absorption, only one emission component is required at all frequencies to account for the observed correlations in both classes. Flat-spectrum quasars (FSRQ) appear to be more beamed than steep-spectrum quasars (SSRQ) only at radio frequencies, but not in the optical and X-ray bands. This might imply that quasars are disk-dominated in the X-ray and optical band, and jet-dominated at radio frequencies. On the other hand, FRI galaxies and BL Lacs appear to be globally jet-dominated. A parameter which might account for the FRI/BL Lac - FRII/quasar dichotomy is the accretion rate. At low, sub-Eddington values jets are weak but the emission from them dominates that from the disk and these sources are classified as FRI galaxies or BL Lacs, depending on the viewing angle. At values close to the Eddington limit the disk emission is comparable to that from the jet, which is also more powerful due to the higher accretion rate, and either a FRII galaxy or a radio-loud quasar is produced in this case.