Theoretical work on quasar microlensing

Right after the discovery of the first multiply imaged quasar, Chang & Refsdal (1979) suggested that the flux of the two quasar images can be affected by stars close to the line of sight. Gott (1981) suggested that a heavy halo made of low mass stars ``should produce fluctuations of order unity in the intensities of the QSO images on time scales of 1-14 years." Young (1981), Paczynski (1986), Kayser, Refsdal, & Stabell (1986) and Schneider & Weiss (1987) used different techniques to explore microlensing light curves and magnification distributions. The first observational evidence for quasar microlensing in the quadruple quasar Q2237+0305 was presented by Irwin et al. (1989). Such fluctuations could be explained by the lensing action of ordinary stars and be used to put a limit on the quasar size (Wambsganss, Paczynski, & Schneider 1990). Later, Witt (1993) and Lewis et al. (1993) developed a new technique for the investigation of microlensing, More recently, Lewis et al. (1998) showed that spectroscopic monitoring of multiple quasars can be used to probe the broad line regions. Fluke & Webster (1999) explored analytically caustic crossing events for a quasar. Wyithe et al. (2000a, 2000b) explored and found limits on the quasar size and on the mass function in Q2237+0305. In the last two years new techniques to recover the one-dimensional brightness profile of a quasar were developed, based on earlier work by Grieger et al. (1988, 1991). Agol & Krolik (1999) showed that by frequent monitoring of a caustic crossing event in many wave bands (they had of order 40 data points in eleven filters over the whole electromagnetic range), one can recover a map of the frequency-dependent brightness distribution of a quasar. Yonehara et al. (1998) in a similar approach explored the effect of microlensing on various accretion disk models.

The early papers on microlensing made four predictions concerning the scientific success. With microlensing we should be able to 1) determine the effects of compact objects between the observer and the source, 2) determine the size of quasars, 3) determine the two-dimensional brightness profile of quasars, 4) determine mass (and mass distribution) of lensing objects. At this moment, it can be stated that 1) has been achieved. Some limits on the size of quasars have been obtained, so 2) is partly fulfilled. We are still (far) away from solving promise 3), and concerning point 4) it is fair to say that it was shown that the results are consistent with certain mass ranges.

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