Annu. Rev. Astron. Astrophys. 1997. 35: 445-502
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3.2. Microlensing as a Possible Cause of Quasar Variability

Variability of distant quasars could result from microlensing by compact bodies in intervening galaxies. The cosmological implications would be very important: If there were such a lens along all sight lines to high-z AGN, then Omega in compact objects would be close to one (Press & Gunn 1973, Blandford & Narayan 1992). (This potential cause of variability does not pertain to the nearby, rather faint AGN discussed in Section 2, as they have a low probability of having a lensing object in the line of sight.)

For the ESO/SERC 287 sample, the main predictions of microlensing for variability are borne out by the observations (Hawkins 1996). The smoothness of the light curves constrains the quasar emission region to be commensurate with the Einstein radius of the lenses. With a typical variability time scale of ~ 2 years and for a transverse velocity of 600 km s-1, the Einstein radius is ~ 8 × 10-4 pc, corresponding to a Jupiter mass (Hawkins 1996).

Another view is that the quasar light curves give an upper limit to the lensing effect because some or all of the quasar variability can be intrinsic. Recently Schneider (1993), following an idea of Canizares (1982), made a theoretical analysis of the characteristics of the light curves in Hawkins' sample and calculated the properties of the lenses that can reproduce them. He can set an upper limit to the mass of dark matter in the form of compact objects in the specific mass range DeltaM 0.001-0.03 Msun. This limit corresponds to Omega(DeltaM) < 0.1.

At present, the results of quasar variability studies appear to be consistent with microlensing. It is difficult to disprove that quasar variability is dominated by microlensing because the properties of the lenses - in particular, their space density and their Einstein radii - are free parameters.

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