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.

Table 1: A few properties for the two regimes of microlensing in search of ``machos'' are compared to each other: local group microlensing and quasar microlensing. (The last three lines are very rough estimates.)

�	�	�
Lensing galaxy:	Milky Way	Lens in Q0957+561
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distance to Macho known?	no	yes
velocity of Macho known?	no	(no)
mass?	???	???
optical depth?	$\approx 10^{-7}$	$\approx 1$
Einstein angle (1 M $_\odot$ )?	$\approx$ 1 milliarcsec	$\approx$ 1 microarcsec
time scale?	hours to years	weeks to decades
event?	individual/simple	coherent/complicated
default light curve?	smooth	sharp caustic crossing
when/who proposed?	Paczynski 1986	Gott 1981
first detection?	EROS/MACHO/OGLE	Schild 1996 (Irwin et al. 1989)
�	1993	�
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No. people involved	$\sim 100$	$\sim 20$
Telescope hours	$\sim$ 40000	$\sim 1000$
CPU years	$\sim$ 500	$\sim 10$
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