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Title:
MACHO Alert 95-30: First Real-Time Observation of Extended Source Effects in Graviational Microlensing
Authors:
Alcock, C.; Allen, W. H.; Allsman, R. A.; Alves, D.; Axelrod, T. S.; Banks, T. S.; Beaulieu, S. F.; Becker, A. C.; Becker, R. H.; Bennett, D. P.; Bond, I. A.; Carter, B. S.; Cook, K. H.; Dodd, R. J.; Freeman, K. C.; Gregg, M. D.; Griest, K.; Hearnshaw, J. B.; Heller, A.; Honda, M.; Jugaku, J.; Kabe, S.; Kaspi, S.; Kilmartin, P. M.; Kitamura, A.; Kovo, O.; Lehner, M. J.; Love, T. E.; Maoz, D.; Marshall, S. L.; Matsubara, Y.; Minniti, D.; Miyamoto, M.; Morse, J. A.; Muraki, Y.; Nakamura, T.; Peterson, B. A.; Phillips, M. M.; Pratt, M. R.; Quinn, P. J.; Reid, I. N.; Reid, M.; Reiss, D.; Retter, A.; Rodgers, A. W.; Sargent, W. L. W.; Sato, H.; Sekiguchi, M.; Stetson, P. B.; Stubbs, C. W.; Sullivan, D. J.; Sutherland, W.; Tomaney, A.; Vandehei, T.; Watase, Y.; Welch, D. L.; Yanagisawa, T.; Yoshizawa, M.; Yock, P. C. M.; The Macho; Gman Collaborations
Publication:
Astrophysical Journal v.491, p.436 (ApJ Homepage)
Publication Date:
12/1997
Origin:
APJ
ApJ Keywords:
COSMOLOGY: GRAVITATIONAL LENSING, STARS: FUNDAMENTAL PARAMETERS, STARS: LATE-TYPE
DOI:
10.1086/304974
Bibliographic Code:
1997ApJ...491..436A

Abstract

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 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 a M4 III star of radius 61 +/- 12 Rȯ, 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} Mȯ, 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} Mȯ 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. 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.
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