MACHO Alert 95-30: First Real-Time Observation of Extended Source Effects in Gravitational Microlensing

We present analysis of MACHO Alert 95-30, a dramatic gravitational microlensing event toward the Galactic bulge whose peak magnification departs significantly from the standard point-source microlensing model. Alert 95-30 was observed in real time by the Global Microlensing Alert Network (GMAN), which obtained densely sampled photometric and spectroscopic data throughout the event. We interpret the light-curve "fine structure" as indicating transit of the lens across the extended face of the source star. This signifies resolution of a star several kiloparsecs distant. We find a lens angular impact parameter θmin/θsource = 0.715 ± 0.003. This information, along with the radius and distance of the source, provides an additional constraint on the lensing system. Spectroscopic and photometric data indicate the source is a M4 III star of radius 61 ± 12 R☉, located on the far side of the bulge at ~9 kpc. We derive a lens angular velocity, relative to the source, of 21.5 ± 2.9 km s-1 kpc-1, where the error is dominated by uncertainty in the angular size of the source star. Likelihood analysis yields a median lens mass of 0.67+ 2.53−0.46 M☉, located with 80% probability in the Galactic bulge at a distance of 6.93+ 1.56−2.25 kpc. If the lens is a main-sequence star, we can include constraints on the lens luminosity. This modifies our estimates to Mlens=0.53+ 0.52−0.35 M☉ and Dlens=6.57+ 0.99−2.25 kpc. Spectra taken during the event show that the absorption-line equivalent widths of Hα and the TiO bands near 6700 Å vary, as predicted for microlensing of an extended source. This is most likely due to center-to-limb variation in the stellar spectral lines. The observed spectral changes further support our microlensing interpretation. These data demonstrate the feasibility of using microlensing limb crossings as a tool to probe stellar atmospheres directly.