The MACHO project: Microlensing detection efficiency

The MACHO project is a search for dark matter in the form of massive compac t halo objects (MACHOs). The project has photometrically monitored over for ty million stars in the Large Magellanic Cloud (LMC), Small Magellanic Clou d (SMC), and Galactic bulge in search of rare gravitational microlensing ev ents caused by these otherwise invisible objects. In 5.7 yr of observations toward the LMC some 13-17 microlensing events have been observed by the MA CHO survey, allowing a direct estimate of the optical depth of tau = 1.2(-0 .3)(+0.4) x 10(-7) to be measured. A critical component of this measurement is an accurate determination of the survey's detection efficiency. The det ection efficiency is a complicated function of temporal sampling, stellar c rowding (the luminosity function), image quality, photometry, time-series a nalysis, and criteria used to select the microlensing candidates. Here we d escribe the details of a Monte Carlo used to calculate the efficiency prese nted in the MACHO 5.7 yr LMC results. A similar calculation was performed f or MACHO's 1 yr and 2 yr results. Several shortcomings of these past determ inations are corrected, including the addition of fainter source stars, an up-to-date luminosity function for the LMC, and many other improvements. We find the MACHO detection efficiency peaks at 40%-50% for durations between 100 < <(t)over cap> < 300 days, depending upon the selection criteria empl oyed.