Difference Image Analysis of Galactic Microlensing. I. Data Analysis

This is a preliminary report on the application of Difference Image Analysis(DIA) to galactic bulge images. The aim of this analysis is to increase thesensitivity to the detection of gravitational microlensing. We discuss how theDIA technique simplifies the process of discovering microlensing events bydetecting only objects which have variable flux. We illustrate how the DIAtechnique is not limited to detection of so called ``pixel lensing'' events,but can also be used to improve photometry for classical microlensing events byremoving the effects of blending. We will present a method whereby DIA can beused to reveal the true unblended colours, positions and light curves ofmicrolensing events. We discuss the need for a technique to obtain the accuratemicrolensing time scales from blended sources, and present a possible solutionto this problem using the existing HST colour magnitude diagrams of thegalactic bulge and LMC. The use of such a solution with both classical andpixel microlensing searches is discussed. We show that one of the major causesof systematic noise in DIA is differential refraction. A technique for removingthis systematic by effectively registering images to a common airmass ispresented. Improvements to commonly used image differencing techniques arediscussed.Comment: 18 pages, 8 figures, uses AAS LaTEX 4.0, To appear in Astrophysical Journa