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Astron. Astrophys. 327, 947-951 (1997)

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1. Introduction

BL Lac objects are generally described as a subclass of active galactic nuclei(AGNs). Hewitt & Burbidge (1993) designate 90 sources in their catalog as BL Lac objects. Veron-Cetty & Veron (1996) and Padovani & Giommi (1995a) list 220 and 233 BL Lac objects in their catalogs respectively. BL Lac objects are always radio-loud and highly polarized objects characterized by weak or absent line feature. Some of them are core-dominated radio sources displaying superluminal motion, variability and gamma-ray loud (Angel & Stockman 1980; Zensus 1989; Vermeulen & Cohen 1994; Ghisellini et al 1993; Fan et al 1996a; Fichtel et al 1994; von Montigny et al 1995; Thompson et al 1993, 1995, 1996; Lin 1996; Quinn et al 1996). According to the surveys, BL Lac objects are divided into radio-selected BL Lac objects (RBLs) and X-ray selected BL Lac objects (XBLs). But some so-called RBLs have been observed in the ROSAT all sky survey and the Einstein Slew Survey (Perlman et al. 1996). For these BL Lac objects, their classification can be made by their relative fluxes at radio and X-ray frequencies, [FORMULA]. They are classified as XBLs if their [FORMULA] (Urry & Padovani 1995) or 0.80 (Sambruna et al. 1996), otherwise they are classified as RBLs. Complete radio flux-limited samples have been compiled for RBLs (Kuhr & Schmidt, 1990; Stickel et al. 1991). A complete X-ray-flux-limited sample of BL Lac objects (XBLs) has also been compiled from the Einstein Extended Medium Sensitivity Survey (EMSS) (Gioia et al 1990; Morris et al 1991; Stocke et al 1990).

The properties of RBLs are systematically different from those of XBLs. The latter have flatter spectral energy distribution from radio through X-ray (Ledden & O'Dell 1985), a higher starlight fraction (Morris et al 1991), a higher observed peak of the emitted power from radio through X-ray spectral energy distribution (Giommi et al 1995) and convex optical-to-X-ray continua (Sambruna et al 1996). XBLs fit the Hubble diagram much better than RBLs (Burbidge & Hewitt 1987; Fan et al 1994) and show good correlations between X-ray, optical magnitude, and radio flux while RBLs do not (Maccagni et al 1989; Fan et al 1993; 1994). RBLs and XBLs occupy different places not only in the [FORMULA] - [FORMULA] diagram (Schwartz et al 1989; Stocke et al 1989; Tagliaferri et al 1989) but also in the [FORMULA] - [FORMULA] and [FORMULA] - [FORMULA] diagrams (Fan & Xie 1996). On the other hand, the radio and optical luminosities for RBLs are higher than those for XBLs, but the X-ray luminosities are almost the same for the both (Maraschi et al 1989; Urry et al 1991; Laurent-Muehleison et al 1993). XBLs generally have lower optical polarization (Jannuzi et al 1993a, b; 1994) with an average polarization [FORMULA] (except for 1722+119, Brissenden et al 1990), while RBLs have an average optical polarization [FORMULA].

Some arguments have been proposed to explain the differences between RBLs and XBLs. First, the location of the high energy cutoffs of the synchrotron emission for XBLs is suggested, which can explain why XBLs have relatively lower ratios of radio-to-X-ray flux (Giommi & Padovani 1994, Kollgaard 1994). Second, XBLs are intrinsically less luminous which can explain the extended power difference (Padovani & Giommi 1995b). However, the most natural way to explain the differences between RBLs and XBLs is the relativistic beaming model proposed by Blandford & Rees (1978) and developed by others (Blandford & Konigl 1979; Marscher & Gear 1985), in which RBLs and XBLs are the same objects seen from different directions (Celotti et al 1993; Urry 1989; Urry & Padovani 1991; Urry et al 1991; Fan & Xie 1996). The milder radio-optical properties of XBLs are generally attributed to a larger angle between the jet and the line of sight, while the similar X-ray luminosities lead to the suggestion that the X-ray beam is broader than the radio and optical beams (Maraschi et al 1986; Padovani & Urry 1990; Sambruna et al 1996). Kollgaard (1994) argued that the different properties of XBLs and RBLs can be explained in terms of the accelerating jet model (Ghisellini & Maraschi 1989) where the X-rays arise from the region of the jet closer to the core than that of the radio emission. The X-rays are subject to less beaming and so are detected over a wider range of angle than that of the radio emission. This accelerating model has gained support from the obtained Lorentz factors [FORMULA] (Padovani & Urry 1990) and [FORMULA] (Urry et al. 1991) and has been used to discuss the differences between RBLs and XBLs in luminosities, spectral indices, and the multifrequency correlations.

Recently, from the spectral energy distribution, Sambruna et al. (1996) proposed that the homogeneous and inhomogeneous jet models cannot explain the different energy distribution. It follows that the orientation effect alone is not sufficient to turn an XBL into a RBL. Instead, the full range of observed spectral energy distribution can be accounted for by a change of intrinsic parameters, such as magnetic field, jet size, and the maximum electron energy. But this argument does not imply that the average beaming factor and viewing angles of XBLs and RBLs should be the same. In fact, the beaming factor itself maybe an additional intrinsic difference between RBLs and XBLs (Sambruna et al. 1996).

Since the beaming factor may be an additional intrinsic difference between XBLs and RBLs, and the beaming effect has been used to discuss the difference between XBLs and RBLs in luminosities, spectral indices, and the multiwavelength correlations, we propose to use it to discuss the difference in polarization between RBLs and XBLs.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998