Annu. Rev. Astron. Astrophys. 1994. 32: 531-590 Copyright © 1994 by Annual Reviews. All rights reserved

7.2. Microlensing in Macrolensed Quasars

Even if a lens is too small to produce resolvable multiple images of a source, it may still induce detectable intensity variations. In particular, one can look for microlensing in quasars that are already macrolensed. This possibility arises because, if a galaxy is suitably positioned to image-double a quasar, then there is also a high probability that an individual halo object will traverse the line of sight of one of the images (Gott 1981); this will give intensity fluctuations in one but not both images. Although the effect would be observable for objects bigger than 10^-4 M, the timescale of the fluctuations is around 40(M / M)^1/2y, and this would exceed a decade for M < 0.1 M.

There is already evidence of this effect for the quasar 2237+0305 (Irwin et al 1989). This has four images at a redshift of 1.7 and the lens is a galaxy at redshift 0.04. The brightest image brightened by 0.5 magnitudes from September, 1987 to August, 1988 and then dimmed by 0.15 magnitudes by September, 1988. There was no variation in the other images, even though the difference in light-travel time is only hours. The observed timescale for the variation indicates a mass in the range 0.001 M to 0.1 M, although Wambsganss et al (1990) argue that it might be as high as 0.5 M, the mass where a standard IMF gives the dominant contribution. (The variable image is almost exactly aligned with the center of the lensing galaxy, where the density should be dominated by ordinary stars). Analysis of more extensive data (Corrigan et al 1991) has strengthened the evidence for microlensing with a mass below 0.1 M (Webster et al 1991).