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

7.6. Microlensing of Gamma-Ray Bursts

Another method of seeking evidence for compact objects in the mass range 10⁶-10⁸ M is to look for echoes from gamma-ray bursts (on the assumption that these are at cosmological distances). The images can be resolved temporally but not spatially (Paczynski 1987). This effect has been considered by many people (Webster & Fitchett 1986, Krauss & Small 1991, Blaes & Webster 1992, Mao 1992, Gould 1992, Narayan & Wallington 1992). The most recent analysis is that of Nemiroff et al (1993), who find no evidence for echoes in data for 44 bursts discovered by the Gamma Ray Observatory. Using the detection volume technique and theoretical redshifts for the bursts, they infer a limit _C(10^6.5-10^8.1 M) < 1, which is shown in Figure 5. However, it must be stressed that the redshifts of the bursts are quite uncertain (they may not even be cosmological) and, for any particular burst, all one can strictly infer is a constraint on M as a function of redshift.