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

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1. Introduction

Unification models adduce the main differences between the observed properties of different classes of AGNs to the anisotropy of the radiation emitted by the active nucleus (see Antonucci 1993and Urry & Padovani 1995for reviews). In particular, for low luminosity radio-loud objects, namely BL Lacs and FR I radio galaxies (Fanaroff & Riley 1974), it is believed that this effect is mainly due to relativistic beaming. In fact, there is growing evidence that obscuration does not play a significant role in these objects, contrary to other classes of AGNs. This is indicated by optical (Chiaberge et al. 1999 hereafter Paper I), radio (Henkel et al. 1998), and X-ray information (e.g. Fabbiano et al. 1984, Worral & Birkinshaw 1994, Trussoni et al. 1999). Within this scenario, the emission from the inner regions of a relativistic jet dominates the observed radiation in BL Lacs, while in FR I, whose jet is observed at larger angles with respect to the line of sight, this component is strongly debeamed. Evidence for this unification scheme includes the power and morphology of the extended radio emission of BL Lacs (e.g. Antonucci & Ulvestad 1985, Kollgaard et al. 1992, Murphy et al. 1993) and the properties of their host galaxies (e.g. Ulrich 1989, Stickel et al. 1991, Urry et al. 1999), which are similar to those of FR I. Furthermore, there is a quantitative agreement among the amount of beaming required by different observational properties (e.g. Ghisellini et al. 1993), the number densities and luminosity functions of the parent and beamed populations in different bands (e.g. Urry & Padovani 1995, Celotti et al. 1993) and the comparison of the radio core emission of beamed and unbeamed objects with similar total radio power (Kollgaard et al. 1996).

Despite this global agreement, it should be stressed that beaming factors inferred from the broad band spectral properties of blazars, more specifically superluminal motions, transparency to the [FORMULA]-ray emission, shape of the SED and time-lags among variations at different frequencies, are significantly and systematically larger than those suggested by radio luminosity data (Dondi & Ghisellini 1995, Ghisellini et al. 1998, Tavecchio et al. 1998).

Thanks to the Hubble Space Telescope (HST), faint optical nuclear components have been recently detected in FR I galaxies (Chiaberge et al. 1999). A strong linear correlation is found between this optical and the radio core emission which strongly argues for a common non-thermal origin. This suggests that the optical cores can be identified with synchrotron radiation produced in a relativistic jet, qualitatively supporting the unifying model for FR I and BL Lacs.

These information offer a new possibility of verifying the unification scheme, by directly comparing the properties of the optical and radio cores of radio galaxies with their putative aligned (beamed) counterparts, analogously to the procedure followed for the radio cores. X-ray observations also provide useful constraints to the nuclear emission of FR I sources (e.g. Hardcastle & Worrall 1999).

The main advantage of using multifrequency data is the possibility of directly comparing the full broad band spectral distributions of these two classes of sources and eventually shed light on the apparent discrepancy in the Lorentz factors inferred from different approaches.

The paper is organized as follows. The (complete) samples of BL Lacs and radio galaxies are presented in Sect. 2. In Sect. 3 we compare separately the core radio and optical emission of beamed and unbeamed objects with similar extended radio power. From this we infer the Lorentz factors requested by the unification scheme within the simplest scenario in which the radiation is emitted by a single uniform region of the relativistic jet. In Sect. 4 the radio and optical data are considered together and, starting from the observed SED of BL Lacs, we derive the expected properties of the nuclear emission of FR I, by taking into account the spectral dependence of the relativistic transformations. As the single-region picture does not account for the observed properties, in Sect. 5 we explore a (simple) alternative scenario and test it also against the X-ray information. Summary and conclusions are presented in Sect. 6.

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© European Southern Observatory (ESO) 2000

Online publication: June 26, 2000