The MACHO Project: First Detection of the Parallax Effect" in Gravitational Microlensing
Session 17 -- Macho Projects
Display presentation, Monday, 9, 1995, 9:20am - 6:30pm

[17.04] The MACHO Project: First Detection of the Parallax Effect" in Gravitational Microlensing

K. Cook, C. Alcock (LLNL), R. Allsman (ANUSF), T. Axelrod, K. Freeman, B. Peterson, P. Quinn, A. Rodgers (MSSSO), D. Bennett, S. Perlmutter (CfPA/UC Berkeley), K. Griest (CfPA/UCSD), S. Marshall, M. Pratt (CfPA/UCSB), C. Stubbs (CfPA/U Washington), W. Sutherland (Oxford)

The MACHO project has recently completed the analysis of 24 Galactic bulge fields containing $1.3\times 10^7$ stars observed for 190 days. Approximately 45 microlensing events have been detected in this data set, and one of these events shows clear evidence of a "parallax" effect. That is, the effect of the earth's motion around the Sun can clearly be seen in the light curve. The event displayed an Einstein ring crossing time of $\sim 108$ days and a peak amplification of $\sim$10x. The "parallax" induced asymmetry in the light curve enables us to determine a component of the velocity of the microlensing object with respect to the line connecting the Sun with the source star. The transverse component of this velocity projected to the position of the Sun is determined to be $v_t \sim 54\pm 5 {\rm km/sec}$. This value and the timescale of the event can be used to show that the lensing event is probably due to lensing by a low mass star in the galactic disk or a brown dwarf in the galactic bulge. High resolution spectroscopy and multi-color photometry have been obtained and will provide constraints on the lens's distance and kinematic population.