On the feasibility of characterizing lens stars in future space-based microlensing surveys

If a light-emitting star is responsible for a gravitational microlensing event, the lens can be characterized by analyzing the blended light from the lens. In this paper we investigate the feasibility of characterizing lenses by using this method in future space-based lensing surveys. To judge the feasibility of the method, we estimate the portions of events whose blended flux F-b can be firmly noticed, and most of it can be attributed to the lens by carrying out detailed simulations of Galactic bulge lensing events considering various blending sources, including the lens, background stars, and binary companions to the lens and source. From this, it is estimated that among the events to be detected from a survey using a 1 m space telescope, similar to 27% will have blending fractions of F-b/F >= 10%, and the blended flux of half of these events will be contaminated more than (F-b - F-L)/F-b 20% by the flux from blending sources other than the lens, implying that the contamination of the blended flux will be substantial. Although the contamination by the background stars can be reduced by using an instrument with a higher resolution, it is estimated that the blended flux of more than one-third of the events will still be contaminated ( mostly by binary companions) even using a telescope equivalent to the Hubble Space Telescope, assuming 50% binary frequency. We, therefore, conclude that caution and consideration of the blending contaminants are required in applying the lens-light analysis method. Additional astrometric information of the source star image centroid motion would be crucial to resolving the contamination.