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Astron. Astrophys. 325, 911-914 (1997)

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4. Discussion and conclusions

We can now compare the polarization properties of the extended emission of these six objects with those expected from the Unified Scheme.

In FR I radio sources the magnetic field is parallel to the jet in the vicinity of the core (up to a few kpc), but is dominated by the perpendicular component for the majority of its length (Bridle & Perley 1984, Killeen et al. 1986). On the other hand, in FR II radio sources the magnetic field is parallel to the jet (both on pc and kpc scale), and becomes perpendicular to the source axis in the hot-spots where the magnetic field lines are compressed by a strong shock (Bridle 1986, Bridle et al. 1986, Scheuer 1987).

All the BL Lac objects in Fig. 1 show distinct jet-like features. This is true also for 1147+245 where higher resolution observations (Antonucci et al. 1985) confirm that the polarized southern component is a jet. In all these cases the magnetic field is parallel to the jet axis and follow smoothly any change in direction. In the five sources (0954+658, 1514-241, 1807+698, 2131-021, 2240-260) where we had enough resolution, the jet terminates in a higher brightness and polarized region which could be identified with the terminal hot-spot. This hypothesis is reinforced by the orientation of the magnetic field. In 3 sources (1807+698, 2131-021, 2240-260) the magnetic field in the putative hot-spot is transversal as it is expected when a strong shock compresses the field lines. This seems to be the case also for 1514-241 although we note that the change in the magnetic field direction seems to occur still in the jet region. We also note that when a change in magnetic field orientation is observed, it occurs at a larger distance than observed in FR I radio sources if the observed sizes in our objects are foreshortened by projection.

The polarized emission of BL Lac objects shown in Fig. 1 suggests they are FR II radio galaxies viewed end-on. This conclusion is apparently supported by the results obtained from the extended luminosity. Table 1 lists the extended luminosity derived subtracting the core flux density from the total flux density of the source. Only one object (1514-241) has undoubtedly a luminosity of extended emission typical of a FR I radio galaxy. Two objects (0954+658 and 1807+698) have values about the FR I - FR II break ([FORMULA] W/Hz at 1.4 GHz, Bridle 1984), while three sources have extended luminosities typical of an FR II radio galaxy. On the other hand all the objects shown apart 1147+245 and maybe 1807+698 (which is double lobed in lower resolution images) are one-sided. This suggests that also the extended emission in the majority of the objects shown may be amplified by relativistic boosting, making very difficult any comparison between luminosities.

Several authors have recently debated the BL Lac-FRI Unified Scheme by comparing the host galaxy properties (Wurtz et al. 1996a), the clustering properties (Smith et al. 1995; Wurtz et al. 1996b), the X-ray properties (Brinkmann et al. 1996), and radio properties (Kollgaard et al. 1992; Murphy et al. 1993; Perlman & Stocke 1993) of different samples of BL Lac objects and FRI/FRII radio galaxies. The results presented here raise new problems to the BL Lac-FRI Unification.

The completion of the analysis of our radio data together with the information already available in literature will allow us to investigate in much greater detail than previously possible the problem of the misaligned population of BL Lac objects and to study whether projection effects may influence the observed polarization properties of jets closely aligned to the line of sight.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998