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

Detection Efficiency

What do the microlensing events mean for the dark matter

question? In order to answer, we need to know the efficiency with

which our system can detect microlensing. This is a non-trivial

calculation. In order to calculate our efficiencies, we add simulated

stars to real images, and then artificially brighten them. We run the

photometry code on these simulated images and find what the

photometry code returns when a star brightens under different

atmospheric and crowding conditions. These results are

incorporated into a large Monte Carlo in which simulated

microlensing events are added to our actual (non-microlensing)

data and fed into the same time-series analysis and selection

procedure which produced the 3 LMC microlensing candidates.

Thus we have explicitly taken into account inefficiencies caused by

bad weather and system down time, our analysis and selection

procedure, as well as blending of the underlying stars due to

crowding, and systematic errors in our photometry. Since in order

to calculate the expected number of events, we need to integrate a

theoretical microlensing rate over our measured efficiency, we need

efficiency as a function of . The function can be found in

references [37, 38]. Once is calculated, the number of expected

events is , where our total exposure

star-years, and is a differential

microlensing rate which can be calculated given a model of the

dark halo [49, 50].

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