Magnitude offset between lensed stars and observed stars: A new probe of the structure of the galactic bar

We propose a new method that can be used to constrain the properties of the Galactic bar (bulge). If the majority of the lensing objects are in the Galactic bar, then we predict a systematic offset in the apparent magnitude between lensed stars and all observed stars. Using OGLE color-magnitude diagram data we model this effect in the region of the diagram dominated by bulge red clump stars and find that for some models of the Galactic bar the expected offset in the apparent magnitude may be as large as $0.2 mag$. About 100 lensed stars in the red clump region of the color-magnitude diagram is needed to unambiguously detect this effect, a number within the reach of current microlensing projects. We find a good correlation between the extent of the bar along the line of sight and the expected magnitude offset. We also obtain a constraint for the extent of the bar along the line of sight using the observed luminosity function for the red clump stars.Comment: 10 pages, 3 figures, revised version accepted for the publication in ApJL, uses aaspp.sty macro, PostScript figures and PostScript version of the paper available through anonymous ftp from astro.princeton.edu, directory stanek/bar_lens, or at http://astro.princeton.edu/~library/prep.htm