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 {open_quotes}fine structure{close_quotes} 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 {theta}{sub min}/{theta}{sub source}=0.715{plus_minus}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{plus_minus}12R{sub {circle_dot}}, located on the far side of the bulge at {approximately}9kpc. We derive a lens angular velocity, relative to the source, of 21.5{plus_minus}2.9kms{sup {minus}1}kpc{sup {minus}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{sub {minus}0.46}{sup +2.53}M{sub {circle_dot}}, located with 80{percent} probability in the Galactic bulge at a distance of 6.93{sub {minus}2.25}{sup +1.56}kpc. If the lens is a main-sequence star, we can include constraints on the lens luminosity. This modifies our estimates to M{sub lens}=0.53{sub {minus}0.35}{sup +0.52}M{sub {circle_dot}} and D{sub lens}=6.57{sub {minus}2.25}{sup +0.99}kpc. Spectra taken during the event show that the absorption-line equivalent widths of H{alpha} and the TiO bands near 6700 {Angstrom} 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. {copyright} {ital 1997} {ital The American Astronomical Society}
- Research Organization:
- Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
- DOE Contract Number:
- W-7405-ENG-48
- OSTI ID:
- 597190
- Journal Information:
- Astrophysical Journal, Vol. 491, Issue 2; Other Information: PBD: Dec 1997
- Country of Publication:
- United States
- Language:
- English
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