MACHO Alert 95‐30: First Real‐Time Observation of Extended Source Effects in Gravitational Microlensing
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abstract
We present analysis of MACHO Alert 95-30, a dramatic gravitational
microlensing event towards 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 kpc distant. We find a lens angular impact
parameter theta_{min}/theta_{source} = 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 an M4 III star of radius 61 +/- 12 Rsun, located on the far side of the
bulge at 9 kpc. We derive a lens angular velocity, relative to the source, of
21.5 +/- 4.9 km/s/kpc, where the error is dominated by uncertainty in the
source radius. Likelihood analysis yields a median lens mass of
0.67{+2.53}{-0.46} Msun, 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
M_lens = 0.53{+0.52}{-0.35} Msun and D_lens = 6.57{+0.99}{-2.25} kpc. Spectra
taken during the event show that the absorption line equivalent widths of H
alpha and the TiO bands near 6700 A vary, as predicted for microlensing of an
extended source. This is most likely due to center-to-limb variation in the
stellar spectral lines. These data demonstrate the feasibility of using
microlensing limb crossings as a tool to probe stellar atmospheres directly.
