The First Spectroscopic Observations of Caustic Crossing in a Binary Microlensing Event

We present the first spectroscopic observations of a binary microlensing event when it was undergoing a caustic crossing with a high magnification of A ≈ 25. The event 96-BLG-3 was identified in real 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 Å and are almost identical, the third one differing only in having an amplitude ≈ 6% lower than the others. By comparison with reference star spectra and by using spectrum synthesis techniques, we infer that the source star is a G0 IV-V star, with an effective temperature of T eff ± 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 ≈ 1.4 −0.4 + 0.6 R ☉ and hence a distance of 6.9 −1.7 + 3.1 kpc, consistent with the source residing in the Galactic bulge. We also determine its heliocentric radial velocity to be v r ± 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 stellar 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.