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

doi:10.1086/304974

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C. Alcock et al. 1997 ApJ 491 436 doi:10.1086/304974

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

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C. Alcock^1,2, W. H. Allen³, R. A. Allsman⁴, D. Alves^1,5,6, T. S. Axelrod⁷, T. S. Banks⁸, S. F. Beaulieu⁹, A. C. Becker^2,10, R. H. Becker^1,5, D. P. Bennett^1,2,11, I. A. Bond¹², B. S. Carter³, K. H. Cook^1,2, R. J. Dodd³, K. C. Freeman⁷, M. D. Gregg^1,5, K. Griest^2,13, J. B. Hearnshaw¹⁴, A. Heller¹⁵, M. Honda¹⁶, J. Jugaku¹⁷, S. Kabe¹⁸, S. Kaspi¹⁵, P. M. Kilmartin^14,19, A. Kitamura¹², O. Kovo¹⁵, M. J. Lehner^2,13, T. E. Love^14,19, D. Maoz¹⁵, S. L. Marshall¹, Y. Matsubara¹², D. Minniti¹, M. Miyamoto²⁰, J. A. Morse²¹, Y. Muraki¹², T. Nakamura²², B. A. Peterson⁷, M. M. Phillips²³, M. R. Pratt^2,10,24, P. J. Quinn²⁵, I. N. Reid²⁶, M. Reid⁸, D. Reiss¹⁰, A. Retter¹⁵, A. W. Rodgers⁷, W. L. W. Sargent²⁶, H. Sato²², M. Sekiguchi²⁰, P. B. Stetson²⁷, C. W. Stubbs^2,7,10,24, D. J. Sullivan⁸, W. Sutherland²⁸, A. Tomaney¹⁰, T. Vandehei¹³, Y. Watase¹⁸, D. L. Welch²⁹, T. Yanagisawa¹², M. Yoshizawa²² and P. C. M. Yock¹⁹

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¹ Lawrence Livermore National Laboratory, Livermore, CA 94550
² Center for Particle Astrophysics, University of California, Berkeley, CA 94720
³ Carter National Observatory, Wellington, New Zealand
⁴ Supercomputing Facility, Australian National University, Canberra, ACT 0200, Australia
⁵ Department of Physics, University of California, Davis, CA 95616
⁶ Visiting Astronomer, Cerro Tololo Inter-American Observatory
⁷ Mount Stromlo and Siding Spring Observatories, Australian National University, Weston, ACT 2611, Australia
⁸ Department of Physics, Victoria University, Wellington, New Zealand
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
¹⁰ Departments of Astronomy and Physics, University of Washington, Seattle, WA 98195
¹¹ Department of Physics, University of Notre Dame, IN 46556
¹² Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya 464-01, Japan
¹³ Department of Physics, University of California, San Diego, CA 92093
¹⁴ Department of Physics and Astronomy, University of Canterbury, Christchurch, New Zealand
¹⁵ School of Physics and Astronomy and Wise Observatory, Tel-Aviv University, Tel Aviv 69978, Israel
¹⁶ Institute for Cosmic Ray Research, University of Tokyo, Tokyo 188, Japan
¹⁷ Research Institute of Civilization, Tokai University, Hiratsuka 259-12, Japan
¹⁸ National Laboratory of High Energy Physics (KEK), Tsukuba 305, Japan
¹⁹ Faculty of Science, University of Auckland, Auckland, New Zealand
²⁰ National Astronomical Observatory, Mitaka, Tokyo 181, Japan
²¹ Center for Astrophysics and Space Astronomy, University of Colorado, Campus Box 389, Boulder, CO 80309
²² Department of Physics, Kyoto University, Kyoto 606, Japan
²³ Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
²⁴ Department of Physics, University of California, Santa Barbara, CA 93106
²⁵ European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748, Garching, Germany
²⁶ Astronomy Department 105-24, California Institute of Technology, Pasadena, CA 91125
²⁷ Dominion Astrophysical Observatory, 5071 West Saanich Road, RR 5, Victoria, BC V8X 4M6, Canada
²⁸ Department of Physics, University of Oxford, Oxford OX1 3RH, UK
²⁹ Visiting Astronomer, Cerro Tololo Inter-American Observatory

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Abstract References Cited By

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 θ_min/θ_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 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 M_lens=0.53^{+ 0.52}_−0.35 M and D_lens=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.

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