MACHO Alert 95-30 : First Real-Time Observation of Extended Source Effects in Gravitational Microlensing
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.
Keywords
Cite
@article{arxiv.astro-ph/9702199,
title = {MACHO Alert 95-30 : First Real-Time Observation of Extended Source Effects in Gravitational Microlensing},
author = {C. Alcock and W. H. Allen and R. A. Allsman and D. Alves and T. S. Axelrod and T. S. Banks and S. F. Beaulieu and A. C. Becker and R. H. Becker and D. P. Bennett and I. A. Bond and B. S. Carter and K. H. Cook and R. J. Dodd and K. C. Freeman and M. Gregg and K. Griest and J. B. Hearnshaw and A. Heller and M. Honda and J. Jugaku and S. Kabe and S. Kaspi and P. M. Kilmartin and A. Kitamura and O. Kovo and M. J. Lehner and T. E. Love and D. Maoz and S. L. Marshall and Y. Matsubara and D. Minniti and M. Miyamoto and Y. Muraki and T. Nakamura and B. A. Peterson and M. R. Pratt and P. J. Quinn and I. N. Reid and M. Reid and D. Reiss and A. Retter and A. W. Rodgers and W. L. W. Sargent and H. Sato and M. Sekiguchi and P. B. Stetson and C. W. Stubbs and D. J. Sullivan and W. Sutherland and A. Tomaney and T. Vandehei and Y. Watase and D. L. Welch and T. Yanagisawa and M. Yoshizawa and P. C. M. Yock},
journal= {arXiv preprint arXiv:astro-ph/9702199},
year = {2009}
}
Comments
32 pages including 6 tables, and 15 figures; Uses AASTeX 4.0; submitted to The Astrophysical Journal