Macho alert 95-30: First real-time observation of extended source effects in gravitational microlensing

C. Alcock, W. H. Allen, R. A. Allsman, D. Alves, T. S. Axelrod, T. S. Banks, S. F. Beaulieu, A. C. Becker, R. H. Becker, D. P. Bennett, I. A. Bond, B. S. Carter, K. H. Cook, R. J. Dodd, K. C. Freeman, M. D. Gregg, K. Griest, J. B. Hearnshaw, A. Heller, M. HondaJ. Jugaku, S. Kabe, S. Kaspi, P. M. Kilmartin, A. Kitamura, O. Kovo, M. J. Lehner, T. E. Love, D. Maoz, S. L. Marshall, Y. Matsubara, D. Minniti, M. Mlyamoto, J. A. Morse, Y. Muraki, T. Nakamura, B. A. Peterson, M. M. Phillips, M. R. Pratt, P. J. Quinn, I. N. Reid, M. Reid, D. Reiss, A. Retter, A. W. Rodgers, W. L.W. Sargent, H. Sato, M. Sekiguchi, P. B. Stetson, C. W. Stubbs, D. J. Sullivan, W. Sutherland, A. Tomaney, T. Vandehei, Y. Watase, D. L. Welch, T. Yanagisawa, M. Yoshizawa, P. C.M. Yock

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89 Citations (Scopus)

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 θminSOurce = 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 0.67-0.46+2.53 M located with 80% probability in the Galactic bulge at a distance of 6.93-2.25+1.56 kpc. If the lens is a main-sequence star, we can include constraints on the lens luminosity. This modifies our estimates to Mtens = 0.53-0.35+0.52 M and Dlens = 6.57-2.25+0.99 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.

Original languageEnglish
Pages (from-to)436-450
Number of pages15
JournalAstrophysical Journal
Volume491
Issue number2 PART I
DOIs
Publication statusPublished - 1997

Keywords

  • Gravitational lensing
  • Stars: fundamental parameters
  • Stars: late-type

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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