Unification of radio-loud AGN

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.