THE FIRST SPECTROSCOPIC OBSERVATIONS OF CAUSTIC CROSSING IN A BINARY MICROLENSING EVENT

We present the first spectroscopic observations of a binary microlensi ng event when it was undergoing a caustic crossing with a high magnifi cation of A approximate to 25. The event 96-BLG-3 was identified in re al time by the MACHO collaboration, in the Baade's window field toward the Galactic bulge. Three spectra were taken consecutively, spanning the light-curve peak of the caustic crossing, each integration lasting 30 minutes. The spectrograms covered the wavelength range 3985-6665 A ngstrom and are almost identical, the third one differing only in havi ng an amplitude approximate to 6% lower than the others. By comparison with reference star spectra and by using spectrum synthesis technique s, we infer that the source star is a GO IV-V star, with an effective temperature of T-eff = 6100 +/- 150 K, a metallicity in the range [M/H ] = +0.3 to +0.6, and a logarithmic surface gravity of log g = 4.25 +/ - 0.25. Using theoretical evolutionary tracks, we derive a radius of a pproximate to 1.4(+0.6)(-0.4) R. and hence a distance of 6.9(+3.1)(-1. 7) kpc, consistent with the source residing in the Galactic bulge. We also determine its heliocentric radial velocity to be nu(r) = 122 +/- 3 km s(-1). Caustic-crossing microlensing events such as 96-BLG-3, if they are observed with 8-10 m class telescopes, can resolve the stella r surface of distant sources with a resolution of 10(10) cm or better. This permits a detailed study of the center-to-limb variation of the stellar surface and the intrinsic properties of the lensed source.