On the Baseline Flux Determination of Microlensing Events Detectable with the Difference Image Analysis Method

To improve photometric precision by removing blending effect, a newly developed technique of difference image analysis (DIA) is adopted by several gravitational microlensing experiment groups. However, the principal problem of the DIA method is that, by its nature, it has difficulties in measuring the baseline flux $F_0$ of a source star, causing degeneracy problem in determining the lensing parameters of an event. Therefore, it is often believed that the DIA method is not as powerful as the classical method based on the PSF photometry in determining the Einstein time scales $t_{\rm E}$ of events. In this paper, we demonstrate that the degeneracy problem in microlensing events detectable from the searches by using the DIA method will not be as serious as it is often worried about. This is because a substantial fraction of events will be high amplification events for which the deviations of the amplification curves constructed with the wrong baseline fluxes from their corresponding best-fit standard amplification curves will be considerable even for a small amount of the fractional baseline flux deviation $\Delta F_0/F_0$. With a model luminosity function of source stars and under realistic observational conditions, we find that $\sim 30%$ of detectable Galactic bulge events are expected to have high amplifications and their baseline fluxes can be determined with uncertainties $\Delta F_0/F_0\leq 0.5$.