The MACHO Project: Microlensing Detection Efficiency

The MACHO project is a search for dark matter in the form of massive compacthalo objects (MACHOs). The project has photometrically monitored tens ofmillions of stars in the Large Magellanic Cloud (LMC), Small Magellanic Cloud(SMC), and Galactic bulge in search of rare gravitational microlensing eventscaused by these otherwise invisible objects. In 5.7 years of observationstoward the LMC some 13-17 microlensing events have been observed by the MACHOsurvey, allowing powerful statements to be made about the nature of the darkpopulation in the halo of our Galaxy. A critical component of these statementsis an accurate determination of the survey's detection efficiency. Thedetection efficiency is a complicated function of temporal sampling, stellarcrowding (the luminosity function), image quality, photometry, time-seriesanalysis, and criteria used to select the microlensing candidates. Such acomplex interdependence is most naturally solved using a Monte Carlo approach.Here we describe the details of the Monte Carlo used to calculate theefficiency presented in the MACHO 5.7-year LMC results. Here we correct severalshortcomings of past determinations, including (1) adding fainter source stars(2.5 magnitudes below our faintest detected "stars"), (2) an up-to-dateluminosity function for the LMC, (3) better sampling of real images in bothstellar density and observing conditions, (4) an improved scheme for addingartificial microlensing onto a random sample of real lightcurves, and manyother improvements. [Abridged]Comment: 32 pages, Latex with 16 postscript figures, submitted to ApJ