7.2.3 BL Lac Objects as Gravitationally Micro-Lensed FSRQ
The main differences between BL Lacs and FSRQ are, to first order, that the FSRQ are more distant, more luminous, and have stronger emission lines. Ostriker and Vietri (1985, 1990) have suggested that gravitational micro-lensing by stars in a foreground galaxy could turn an optically violently variable (OVV) quasar (i.e., an FSRQ, in our terminology) into a BL Lac. Specifically, a distant FSRQ with a compact optical/ultraviolet continuum region and an extended optical/ultraviolet emission-line region, whose line of sight passes nearly through the center of a foreground galaxy, would have the continuum preferentially amplified by micro-lensing. A factor of 10 enhancement could, Ostriker and Vietri estimated, reduce the equivalent widths of the BL Lac emission lines sufficiently. In addition, the BL Lac would appear to lie in the (approximate) center of the (nearby) lensing galaxy. Its redshift, in practice often inferred from the ``host'' galaxy spectrum, would refer to the lensing galaxy rather than the FSRQ, so the derived luminosity of the BL Lac object would be artificially low.
In this picture, BL Lacs and FSRQ are intrinsically the same objects, and there is no separate low-luminosity blazar class to be unified with FR I radio galaxies. Note that the relativistic jet is probably still required to explain the rapid variability, polarization, and other blazar characteristics of FSRQ, although micro-lensing could account for some low-amplitude, approximately achromatic, variability in BL Lac objects (Urry et al. 1993; Edelson et al. 1995).
There are a number of arguments against the micro-lensing hypothesis, albeit none definitive. These include the following.
(1) The optical amplification factors derived from numerical simulations under the micro-lensing hypothesis, basically comparing the optical continuum fluxes of BL Lacs and FSRQ in the 1 Jy and 2 Jy samples, respectively, are much smaller than the factor of 10 required by Ostriker and Vietri to swamp the emission lines of quasars (Padovani 1992b).
(2) The observed optical number counts of BL Lacs (Padovani and Urry 1991; Hawkins et al. 1991) do not appear to flatten at B ~ 16-18, as predicted by Ostriker and Vietri, although the faint counts are still quite uncertain.
(3) When BL Lac ``host'' galaxies are detected (e.g., Abraham et al. 1991), their nuclei are almost always well-centered on the galaxy (see Stocke et al. 1995 for evidence of de-centering in MS 0205.7+3509, an EMSS BL Lac), whereas the micro-lensing scenario would allow the background FSRQ to be well off the center of the micro-lensing galaxy. Existing limits on de-centering may already be sufficient to rule out the presence of even a small number of micro-lensed sources (Merrifield 1992) and HST imaging of complete samples have the potential to decide this issue finally.
(4) At least five of the low-redshift 1 Jy RBL have emission lines and absorption lines at the same redshift (Stickel et al. 1993), indicating that in a substantial number of cases, the absorption line redshifts are from the host galaxy rather than the lensing galaxy.
(5) The systematic differences in VLBI polarization structure for BL Lac objects and FSRQ, notably the 90° difference in mean magnetic field orientation, should not exist if one is simply an amplified version of the other (Gabuzda et al. 1992). Micro-lensing changes the image position randomly with respect to the direction of polarization (which is unchanged), since the lens mass distribution does not ``know'' about the orientation of the background jet. Thus the VLBI polarization in BL Lacs should be relatively uncorrelated with jet morphology compared to FSRQ, whereas they are parallel and perpendicular to the jet, respectively.
(6) Micro-lensing should cause the radio-to-optical continuum of the ``low-luminosity'' BL Lac objects (basically, the HBL) to be flatter than that of FSRQ, which is more or less as observed (Stocke et al. 1985; Sambruna 1994); however, the observed difference is larger than expected from micro-lensing. To shift the wavelength of the peak synchrotron emission in FSRQ, generally in the infrared, to the ultraviolet/EUV/soft X-ray, would require amplifying the short-wavelength continuum by several orders magnitude (see Fig. 15).
The evidence for micro-lensing in a few individual cases remains unclear. One of the most promising micro-lensing candidates, 0235+164 (Stickel et al. 1988a), which has multiple absorption systems (Yanny et al. 1989), a de-centered foreground group of galaxies (Abraham et al. 1993), and an extremely high superluminal velocity (a ~ 90; Bååth 1984), is almost certainly weakly amplified by macro-lensing (Abraham et al. 1993) but no one has calculated the effects of micro-lensing by stars in the foreground lens. (Abraham et al. 1993 argued that by analogy to a normal quasar, micro-lensing could cause significant variability only on time scales of years; however, the relevant source-lens velocity for FSRQ is the superluminal jet velocity rather than the typical stellar velocity in a galaxy core [Gopal-Krishna and Subramanian 1991]. Therefore, the variability time scales can be much shorter, quite comparable to those observed in 0235+164.) In the case of PKS 0537-441, another lensing candidate, Falomo et al. (1992) failed to detect the foreground galaxy the presence of which had been suggested by Stickel et al. (1988b).
In summary, there are a number of arguments against micro-lensing in large numbers of BL Lac objects, and there are no clear cases of micro-lensing in any one BL Lac. Still, there are no data that actually falsify the micro-lensing hypothesis. We suggest a clear and incontrovertible test. As the HST Key Project on quasar absorption lines has demonstrated (Bahcall et al. 1993), the ultraviolet spectrum of a distant FSRQ should show multiple absorption lines due to intervening Ly clouds. Suppose we have a BL Lac object that appears to lie in an elliptical galaxy at a redshift of, say, z = 0.25, and we want to test whether it is actually a quasar at a redshift of, say, z = 1.9 (or less) that is micro-lensed by that galaxy. With an ultraviolet spectrum from ~ 1500 to 3500 Å (with sufficient signal-to-noise ratio to detect absorption lines with equivalent widths of a few hundred milliangstroms) the presence or absence of unidentified (i.e., Ly) absorption features answers this question unambiguously. This project is ideal for HST.