Microlensing in the Galactic Bulge: Effects of the Disk behind the Bulge

A large number of microlensing events have been observed in the direction ofthe Galactic bulge, with a measured optical depth in the range 2 - 3 x 10^{-6}.It has been shown that most of these events are due to bulge stars being lensedby other bulge stars or by foreground disk stars. Among the stars observed inthe bulge fields, there should also be disk stars located behind the bulge;here, we consider their effect on the microlensing rates. The optical depth ofbackground disk stars is much higher than that of typical bulge stars, reaching10^{-5} at 6 Kpc behind the bulge. Thus, although background disk stars are avery small fraction of the stars in Baade's window, we find that ~ 5 to 10 % ofthe optical depth should be due to disk stars more than 3 kpc behind the bulge.This fraction is sensitive to the luminosity function of disk stars at largescale-height, to the magnitude cutoff of the survey, and to the amplificationbias effect causing large numbers of ``blended'' events. We consider also theeffect of a warp and flare in the disk at large distances behind the bulge;this could increase the optical depth from the background disk to ~ 20 % of thetotal. Events on background disk stars should on average be longer than otherevents and could be distinguished also by measuring the proper motion ordistance of the stars that have been microlensed. The number of these eventscould be an interesting probe to the structure and stellar population of thefar-side of the Galactic disk.Comment: 23 pages, LaTex, 6 Postscript figures, Published in Ap