The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 7H.

Advantages of Having Many Events

There are two main advantages of having several times more events

than we originally thought we would have. First, we can do

statistical tests on the data. For example, simple geometry predicts a

specific distribution of maximum amplifications. Basically, every

lens/line-of-sight impact parameter should be equally likely, so the

distribution of 's should be uniform (taking into account that

our efficiency for detecting high magnification (low ) events is

larger). We have performed a Kolmogorov-Smirnov (K-S) test on

the bulge events and find good consistency with the microlensing

hypothesis. Thus the microlensing interpretation of these events is

greatly strengthened.

The second advantage of having many events, is that rare events

can be found. For example events of high magnification or long

duration should occur occasionally. For some types of rare events

additional important information concerning the Macho

mass/velocity/distance can be extracted. For example, in reference

[64] we show an event which lasted about 1/2 year, during which

time the Earth had a chance to travel part way around the Sun.

This gave our telescope two different perspectives on the lens,

resulting in a parallax event. Thus the lightcurve does not fit the

naive amplification formula presented earlier. Including the Earth's

motion, we find a good fit, and discover that the Macho was moving

with a projected transverse velocity of km/sec. The Macho

mass is determined by a combination of this velocity, the event

duration, and the distance to the Macho, so for such parallax events

there is a one-to-one relationship between the Macho mass and

distance. In this case the Macho could be either a brown dwarf star

in the galactic bulge, an M-dwarf star at a distance of 2 to 6 kpc, or

a more massive star quite nearby.

Another rare type of microlensing event is shown in reference [47,

40]. This lightcurve is characteristic of lightcurves formed by binary

lenses. This particular event was first seen by the OGLE group [34],

and detailed analysis will again give information as to the lens

masses, distances and velocities. An exciting aspect of such a binary

Macho detection, is the possibility of detecting planets around

Machos. Given that some of the lenses we observe are in fact low

mass stars, it is possible to observe caustic crossing such as

mentioned above, for planets even down to Earth mass [65, 66].

Thus microlensing may well be the best way to discover and get

statistics on extra-solar planets.

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