Joachim Wambsganss, PASA, 18 (2), in press.
Next Section: Quasar Microlensing: Now and
Title/Abstract Page: Microlensing of Quasars
Previous Section: Observational Evidence for Quasar
- Microlensing in individual quasars?
- ``Astrometric Microlensing": Centroid shifts
- Microlensing: here and there!?
Unconventional Considerations on Quasar Microlensing
Microlensing in individual quasars?
There were a number of papers interpreting the variability of individual quasars as microlensing (e.g., Hawkins & Taylor 1997, Hawkins 1998). Although this is an exciting possibility and it could help us detect a population of cosmologically distributed lenses, it is not entirely clear at this point whether the observed fluctuations can be really attributed to microlensing. After all, quasars are intrinsically variable (otherwise we could not measure time delays), and the expected microlensing in single quasars must be smaller than in multiply imaged ones, due to the lower surface mass density. More studies are necessary to clarify this issue.
``Astrometric Microlensing": Centroid shifts
An interesting aspect of microlensing was explored by Lewis & Ibata (1998). They looked at centroid shifts of quasar images due to microlensing. At each caustic crossing, a new very bright image pair emerges or disappears, giving rise to sudden changes in the ``center of light" positions. The amplitude could be of order 100 microarcseconds or larger, which should be observable with the next generation of astrometric satellites, like SIM (Space Interferometry Mission), to be launched in June 2006.
Microlensing: here and there!?
In most cases of quasar microlensing, the surface mass density (or optical depth) is of order unity. In contrast to that, the ``local group" microlensing (Alcock et al. 2000; Aubourg et al. 1999; Udalski et al. 2000) deals with low optical depths, where the action is due to single lenses or physical binaries. Since there are interesting similarities (search for dark matter, i.e. machos - massive compact halo objects) as well differences between these two regimes of microlensing, in Table 1 a few quantities relevant to the two types of microlensing are compared to each other.
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Next Section: Quasar Microlensing: Now and
Title/Abstract Page: Microlensing of Quasars
Previous Section: Observational Evidence for Quasar
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