Difference image analysis of galactic microlensing. I. Data analysis

This is a preliminary report on the application of Difference Image Analysi s (DIA) to Galactic bulge images. The aim of this analysis is to increase t he sensitivity to the detection of gravitational microlensing. We discuss h ow the DIA technique simplifies the process of discovering microlensing eve nts by detecting only objects that have variable flux. We illustrate how th e DIA technique is not limited to detection of so-called "pixel lensing" ev ents but can also be used to improve photometry for classical microlensing events by removing the effects of blending. We will present a method whereb y DIA can be used to reveal the true unblended colors, positions, and light curves of microlensing events. We discuss the need for a technique to obta in the accurate microlensing timescales from blended sources and present a possible solution to this problem using the existing Hubble Space Telescope color-magnitude diagrams of the Galactic bulge and LMC. The use of such a solution with both classical and pixel microlensing searches is discussed. We show that one of the major causes of systematic noise in DIA is differen tial refraction. A technique for removing this systematic by effectively re gistering images to a common air mass is presented. Improvements to commonl y used image differencing techniques are discussed.