As we noted in the Introduction, direct imaging of the central accretion flow will be impossible for the foreseeable future because of the small size scales involved. Reverberation mapping in the X-rays may provide one way of probing the accretion flow with the next generation of high-throughput X-ray satellites such as XMM and Constellation-X. In addition, Nature has provided us with a cosmic telescope that can optically perform this feat, at least in one source. The line of sight to quasar Q2237+0305 passes right through the central core of a foreground, face-on spiral galaxy. This galaxy lenses the quasar into four images that make up the "Einstein Cross" (Huchra et al. 1985). Because of the large surface density of stars in the galaxy's central region, microlensing is observed in the images (Irwin et al. 1989; Pettersen 1990; Corrigan et al. 1991; Racine 1991; Nadeau et al. 1991; Racine 1992; Østensen et al. 1996). The size of microlensing variations implies that the quasar emission region must be smaller than the scale of variations in the caustic network formed by the foreground stars. This scale is typically given by the Einstein radius ~ 1017M1/2 cm, where M is the mass of the lensing star in solar masses. If a rapid microlensing event is resolved in time, even more stringent constraints can be placed on the source size. The most rapid observed event in image "A" (Irwin et al. 1989) constrains the size of the optical emission region to be 2 × 1015 cm (Wambsganss, Schneider, & Paczynski 1990). This is far below any length scale that can be probed by other techniques.
The small size of the optical emission region places strong constraints on accretion disk models. Rauch & Blandford (1991) found that to explain both the observed luminosity and the microlensing size scale, an emissivity in excess of the blackbody limit was required. In other words, a thermal accretion disk is too large (by about a factor 3) to fit within the microlensing size constraint. Somewhat different modeling by Jaroszynski, Wambsganss, & Paczynski (1992) found that accretion disks were consistent with the microlensing constraint. Part of the difference between these two papers was that Jaroszynski et al. considered accretion disks around Kerr holes, whereas Rauch & Blandford considered only Schwarzschild holes. This allowed the former authors to consider more compact disks, as well as producing a greater concentration of luminosity in a hot spot produced by beaming of radiation toward the observer by the orbiting emitting plasma.
However, an even more important difference between the two papers is how the SED problem was handled. As we have pointed out here in this review, luminous accretion disks produce SEDs that are too blue compared to observations. To get around this, Rauch & Blandford modified the radial emissivity of the disk to be flatter than equation (1) above, i.e., allowing more of the accretion power to be dissipated at larger radii. This is one way of explaining the red observed SEDs of AGNs but of course leads to larger optically emitting regions than the standard disk. Jaroszynski et al., on the other hand, kept the standard radial emissivity and assumed instead that there was another source of red light in the source that arises from farther out and is not microlensed. Both approaches are ad hoc, although there are other reasons for believing that the radial emissivity profile that Rauch & Blandford assume is correct: optical/UV variability suggests that it arises from reprocessing of radiation from the inner disk. Czerny, Jaroszynski, & Czerny (1994) have explored the problem of irradiated disks and again find accretion disks to be consistent with the microlensing data. However, they found it necessary to consider accretion luminosities half that of Eddington, so a slim disk (see Section 5.2 below) really needs to be considered. The microlensing problem is clearly intimately tied to the SED problem, and continued monitoring is crucial to tighten the observational constraints.
While the microlensing observations have often been presented as a challenge for accretion disk models, it is perhaps worthwhile pointing out that they do in fact prove that the optical emission in at least this quasar originates very close to the central black hole. This is at least in qualitative agreement with the idea that the central accretion flow is responsible for the BBB.