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

7. GRAVITATIONAL LENSING EFFECTS

One of the most useful signatures of baryonic dark matter candidates is undoubtedly their gravitational lensing effects. Indeed, it is remarkable that lensing could permit their detection over the entire mass range of 10^-7 M to 10¹² M. All sorts of astronomical objects can serve as lenses (Blandford & Narayan 1992) but the crucial advantage of Population III objects is that they are compact and spherically symmetric, which makes their effects very clean. To search for them, one requires sources that are numerous, small, bright, and have predictable intrinsic variations (Nemiroff 1991a). The most useful sources to date have been quasars, galaxies, radio jets, gamma-ray bursts, and stars; all of these are discussed below. Other possibilities include radio sources (Blandford & Jarosynski 1981), supernovae (Schneider & Wagoner 1987, Linder et al 1988, Rauch 1991), and pulsars (Krauss & Small 1991). There are two distinct lensing effects and these probe different but nearly overlapping mass ranges: macrolensing (the multiple-imaging of a source) can be used to search for objects larger than 10⁴ M, while microlensing (modifications to the intensity of a source) can be used for objects smaller than this. The current constraints on the density _c of compact objects in various mass ranges are brought together in Figure 5.

Figure 5. Macrolensing and microlensing constraints on thc density parameter for compact objects of mass M. Current limits are shown by shaded lines and potential ones by broken lines. (a) VLA limit of Hewitt (1986); (b) optical and HST limit of Surdej et al (1993); (c) VLBI limit of Kassiola et al (1991); (d) and (e) potential speckle interferometry and VLBA limits; (f) region required to explain the quasar variations claimed by Hawkins (1993); (g) Dalcanton et al (1994) quasar line-continuum limit; (h) gamma-ray burst limit of Nemiroff et al (1993), assuming these are at a cosmological redshift; (i) corresponds roughly to the range of values required to explain the MACHO and EROS microlensing results; (j) potential limit associated with the null results from the EROS CCD study.