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Astron. Astrophys. 358, 104-112 (2000)

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6. Summary and conclusions

With the aim of exploring the viability of the unification scenario between (HBL, LBL) BL Lacs and FR I radio galaxies we have compared their nuclear emission in the radio, optical and X-ray bands.

We have firstly considered these spectral regions separately, comparing the nuclear emission of the two classes of objects for similar extended radio power. As the core radiation of BL Lacs is enhanced by relativistic beaming, we derived the bulk Lorentz factors requested to account for the observed distribution. The values of [FORMULA] thus inferred are not compatible with the higher bulk velocities requested by theoretical arguments, such as the pair production opacity and the spectral modeling of the SED of BL Lacs.

We then examined the core emission of three samples in the [FORMULA] plane. In the frame of the simplest one-zone emission model, we calculated debeaming trails of the BL Lac broad band emission as predicted by the relativistic transformation for an increasing angle of sight. We found that the model does not account for the observed spectral properties of FR I, as expected from the above inconsistency of the Lorentz factors.

The simplest and rather plausible hypothesis to account for this discrepancy within the unification scenario is to assume a structure in the jet velocity field, in which a fast spine is surrounded by a slow layer. Note however that the slower jet component must be relativistic in order to explain the anisotropic radiation of radio galaxy cores (e.g. Capetti & Celotti 1999). The observed flux is dominated by the emission from either the spine or the slower layer, in the case of aligned and misaligned objects, respectively.

Interestingly, the existence of velocity structures in the jet has been suggested by various authors (Komissarov 1990, Laing 1993) in order to explain some observed properties of FR I (and FR II) jets, such as the structure of the magnetic field in FR I which appears to be longitudinal close to the jet axis and transverse at the edges. Swain et al. 1998obtained VLA images of 3C 353 (an FR II with straight jets), finding that a model consisting in a fast relativistic spine ([FORMULA]) plus a slower outer layer ([FORMULA], but still relativistic in order to produce the observed jet-counterjet intensity asymmetry) could account for the apparently lower emissivity near the jet axis. Similar behaviours have been inferred for the two low luminosity radio galaxies M 87 (Owen et al. 1989) and B2 1144+35 (Giovannini et al. 1999). Furthermore, Laing et al. (1999) showed that the jet asymmetries in FR I can be explained by means of a two-speed model. As a consequence, they argued that the lower velocity component dominates in the cores of the edge-on sources, while the fast spine emission dominates the end-on ones. This possibility might be also supported by recent numerical simulations of relativistic jets (Aloy et al. 2000).

The same indication has been found through different approaches. Capetti & Celotti 1999reveal a trend in the radio galaxy/BL Lac relative powers with the line of sight, which is consistent with a slower (less beamed) component dominating at the largest angles. Capetti et al. 2000consider the same issue by examining the more detailed SED of five radio galaxies and consider their beamed counterparts. They found that while the spectral shapes of 3C 264 and 3C 270 can be reconducted to those of BL Lacs, the required ratio of beaming factors, i.e. [FORMULA], implies that the corresponding BL Lacs would be overluminous. The inclusion of a slower (less beamed) jet component seems to be a plausible explanation.

We found that Lorentz factors of the layer [FORMULA] can account for the unification of FR I (of the 3CR) with LBL and intermediate luminosity BL Lacs. Instead the debeaming trails for the lowest luminosity HBL do not cross the FR I region in the [FORMULA] plane. While the HBL behavior should be compared with that of radio galaxies with which they share the extended radio power (e.g. those of the B2 catalogue), our simple two-component jet model could not account for the observed properties if the cores of such low-power FR I radio galaxies lied on the extrapolation of the 3CR radio-optical correlation. The properties of such weak sources can be instead reproduced if their radio emitting region is less beamed than the optical one, as could be expected if the jet decelerates after the higher energy emitting zone.

Finally, the presence of velocity structures in jets of course affects the number counts of beamed and unbeamed sources: for example, the lack of BL Lacs in clusters (Owen et al. 1996) could be attributed to values of typical bulk Lorentz factors higher than those derived from statistical arguments (Urry et al. 1991). Intriguingly, the very latter authors had to require a wide distribution of Lorentz factors to account for the number densities of FR I and BL Lacs in the radio band.

Much has still to be understood on the dynamics and emitting properties of relativistic jets. Multifrequency studies of the nuclear properties of beamed sources and their parent populations and their comparison - according to unification scenarios which are well supported by other independent indications - constitute a new and powerful tool to achieve that, both for well studied individual sources as well as complete samples. Near IR observations by HST, mm data and higher resolution and sensitivity by Chandra in X-rays will further open this possibility.

Concluding, the radio, optical and X-ray nuclear emission of FR I and BL Lacs strongly indicate the presence of a velocity structure in the jet if indeed these sources are intrinsically identical. In other words, by considering the indications of trends in the SED of blazars emerged in the last few years (Giommi & Padovani 1994, Fossati et al. 1998) together with the constraints derived from their unification with radio galaxies, it appears that the phenomenology of these sources is characterized and determined by differences both in the intrinsic SED and in beaming properties.

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000