Color Change Measurements of Gravitational Microlensing Events By Using the Difference Image Analysis Method

Detecting color changes of a gravitational microlensing event induced by the limb-darkened extended source effect is important to obtain useful information both about the lens and source star. However, precise measurements of the color changes are hampered by the blending effect, which also causes color changes of the event. In this paper, we show that although the color change measured from the subtracted image by using the recently developed photometric method of the ``difference image analysis'' (DIA) differs from the color change measured by using the conventional PSF method, the color-change curve constructed by using the DIA method (DIA color-change curve) enables one to obtain the same information about the lens and source star, but with significantly reduced uncertainties due to the absence of the blending effect. We investigate the patterns of the DIA color-change curves for both single-lens and binary-lens events by constructing color-change maps