Better Astrometric Deblending of Gravitional Microlensing Events by   Using the Difference Image Analysis Method
  Due to the choice of very dense star fields for a higher event rate, the current microlensing searches suffer from large uncertainties caused by blending effect. To measure light variations of microlensing events free from the effect of blending, a newly developed method of Differential Image Analysis (DIA) was proposed for microlensing searches. However, even with the light variation curve obtained by using the DIA method, dramatic reduction of the uncertainty in the determined Einstein time scale is not expected due to the difficulty in determining the baseline flux of a source star.   However, we show in this paper that if the blending effect is investigated by detecting the shift of a source star image centroid, the DIA method will allow one to detect the blending effect with a significantly enhanced efficiency compared the efficiency of the current method based on PSF photometry (PSF method). This is because for a given event the centroid shift measurable by using the DIA method, $\delta\theta_{\rm c,DIA}$, is {\it always} larger than the centroid shift measurable by using the PSF method, $\delta\theta_{\rm c,PSF}$. We find that the ratio $\delta\theta_{\rm c,DIA}/\delta\theta_{\rm c,DIA}$ rapidly increases with increasing fraction of blended light. In addition, for events affected by the same fraction of blended light, the ratio $\delta\theta_{\rm c,DIA}/ \delta \theta_{\rm c,DIA}$ is larger for the event with a lower amplification. Therefore, centroid shift measurements by using the DIA method will be an efficient method to detect the blending effect especially of highly blended events, for which the uncertainties in the determined Einstein time scale are large, as well as of low amplification events, for which the current method is highly inefficient. 
Cheongho Han (Chungbuk National University)