6.2.5 Relation of X-Ray- and Radio-Selected BL Lac Objects
The difference between XBL and RBL is surely a fundamental issue in understanding the BL Lac phenomenon. Our beaming results indicate that radio-selected BL Lac objects constitute a smaller fraction of FR I radio galaxies than do X-ray-selected BL Lacs (Table 3). This can lead to a higher fitted Lorentz factor in the radio calculation, since is inversely proportional to the normalization of the beamed luminosity function. The values from our published calculations are < r > ~ 7 (Urry et al. 1991a) and x ~ 3 (Padovani and Urry 1990), in effect suggesting the radio emission is more collimated than the X-ray.
This result was exciting because it suggested XBL and RBL might represent slightly different orientations of the same underlying relativistic jet. Problems with this interpretation have recently emerged, as discussed below, but first we summarize the original argument. The apparent difference in Lorentz factors was interpreted (Urry et al. 1991a) in terms of an accelerating jet model (suggested for other reasons by Ghisellini and Maraschi 1989) in which the X-ray emission from the most compact region would have a smaller than the more extended radio-emitting region. In addition to explaining the apparent difference in number densities of XBL and RBL, the accelerating jet explained naturally the lower variability, polarization, and luminosities of XBL (Sec. 6.2.2, and references therein).
The original number density argument was actually independent of the Lorentz factors obtained from beaming calculations. Because XBL and RBL have similar X-ray luminosities, X-ray selection should be relatively unbiased (Maraschi et al. 1986). Since X-ray surveys detected very few known RBL, they must be relatively rare, exactly as expected if the radio emission is more beamed than the X-ray. This was supported by the X-ray number counts of RBL and XBL (Urry et al. 1991a). If, as our beaming results implied, RBL are viewed within ~ 12° of the jet axis while XBL are between ~ 12° and 30°, then RBL are a factor of ~ 7 less numerous than XBL (Table 3).
An alternative explanation of the same data was that the physical collimation itself increased along the jet, so that the narrower radio beams made it less likely that our line of sight would intercept their emission. This was commensurate with the X-ray and radio luminosity functions of RBL and XBL (Celotti et al. 1993). For this kind of fan beam, the luminosity function calculation (Celotti et al. 1993) implied a Lorentz factor that was quite high, = 29 (with c, r = 2 / 4°, c, x = 13°, fr = 6 x 10-3 and fx = 5 x 10-3, assuming no evolution for both XBL and RBL) but it could be reduced if, for example, only 10% of FR Is were parents of BL Lacs ( = 10, c, r 11°, c, x = 41°, fr = 0.2 and fx = 0.5).
This simple and satisfying picture based on jet collimation is now in doubt. Most significantly, the accelerating jet and the fan-beam jet are both contradicted by analysis of the multiwavelength spectra of complete samples of XBL and RBL (Sambruna 1994; Sec. 7.2.2). In addition, the fitted for RBL can actually be pushed to lower values, reducing the apparent difference between XBL and RBL (Sec. 6.3 and Table 3; but note that the fraction of beamed objects still remains smaller for RBL than for XBL). Finally, it is important to remember that the X-ray unification calculation is more poorly constrained than the radio calculation because the FR I samples contain fewer objects with X-ray data and because of the less extensive imaging information; thus interpreting any difference in fitted Lorentz factors was premature.