Annu. Rev. Astron. Astrophys. 1992. 30: 311-358 Copyright © 1992 by Annual Reviews. All rights reserved

6.3 Exploitation of Time Delays

The occurrence of time delays among multiple images of a lensed source can provide unique opportunities to study transient phenomena. For instance, were a supernova to go off in a multiply-imaged region of an arc-galaxy, the explosion will be seen three or more times with delays (Kovner & Paczynski 1988). If a reasonable estimate of the time delay is available (say from a model), one could wait and observe one of the later incarnations of the event in much greater detail than would be possible without prior warning. (However, this will not provide a measurement of H₀ as the cluster potential is unlikely to be understood well enough.) The same principle could be used with any unusual event in the multiply-imaged quasars, e.g. transient flaring of the optical or X-ray emission, or ejection of a radio VLBI blob.

The BATSE experiment on the Compton Gamma Ray Observatory (Meegan et al. 1992) has shown that most gamma-ray bursts are isotropically distributed on the sky and have non-Euclidean source counts, which suggests that they lie at cosmological distances (e.g. Prilutski & Usov 1975, Mao & Paczynski 1992). If so, gamma-ray bursts constitute a fairly homogeneous population of near point sources with (model-dependent) angular sizes ~ 10^-20 radians. Gamma-ray telescopes have only primitive angular resolving power, but sub millisecond temporal resolution, and so this hypothesis can be tested by seeking multiple bursts from the same source separated in time by ~ 10^-5 (M / M) s where M is the lens mass (Paczynski 1986b, 1987b). Known galaxies will create repeating bursts with median time delays ~ 1 month with a probability ~ 10^-3 (Mao 1992), and examples ought to be seen after ~ 3 yr of full operation of BATSE. Microlensing by individual stars will probably not be resolvable, but a cosmologically significant density of compact masses with M 100M could be detected. Using the observed time delays and magnification ratios, the masses of the individual lenses could be determined with good accuracy (Narayan & Wallington 1992). Interestingly, for a lens with mass 10¹⁸ g, the Fresnel length will be smaller than the Einstein radius and energy-dependent diffraction effects are predicted (Bliokh & Minakov 1975, Gould 1992).

6.4 Quasar Absorption Lines

Multiply-imaged quasars provide unique opportunities to probe the intervening intergalactic medium between us and the source. Since the geometry of the two ray-paths is known, the presence or absence of common absorption lines in the multiple spectra leads to powerful constraints on the transverse sizes of Lyman- clouds and metal-line systems. (Weymann et al. 1979, Krolik & Kwan 1979, Young et al. 1981c, Weymann & Foltz 1983, Foltz et al. 1984, Thomas & Webster 1990, Steidel & Sargent 1990, Smette et al. 1992). Sizes of 5-25 kpc have been deduced from Lyman- absorption lines observed in Q0957+561 and Q2345+007. Conversely, the redshift distribution of the absorption systems in the images can be used as an argument for, or against, multiple imaging (Duncan 1991, Steidel & Sargent 1991).

For most models of the clouds responsible for the Lyman- forest, no significant gravitational lensing is expected by the clouds themselves (Ikeuchi & Turner 1991).