NOAO >   Observing Info >   Approved Programs >   2012A-0520

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Proposal Information for 2012A-0520


PI: Kailash Sahu, Space Telescope Science Institute, ksahu@stsci.edu
Address: 3700 San Martin Drive, Baltimore, MD 21218, USA

CoI: Stefano Casertano, Space Telescope Science Institute
CoI: Howard E. Bond, Space Telescope Science Institute
CoI: Tom Brown, Space Telescope Science Institute
CoI: Martin Dominik, University of St. Andrews
CoI: Harry Ferguson, Space Telescope Science Institute
CoI: Andrzej Udalski, Warsaw University
CoI: Michael Albrow, University of Canterbury

Title: Discovery of Isolated Black Holes and Neutron Stars through Astrometric Microlensing

Abstract: To date, BH and NS masses have been directly measured only in binaries; no isolated BH has been detected unambiguously within our Galaxy. We have underway a large, 3-year \HST program (192 orbits) designed to detect microlensing events caused by non-luminous isolated black holes (BHs) and neutron stars (NSs) in the direction of the Galactic bulge. We expect to detect ~120 microlensing events, of which ~30 will be due to BH and NS lenses. The \HST observations will allow detection of the astrometric shift of the source during the event, thus leading to a direct measurement of the angular Einstein radii. The distance to the source can be determined from the color- magnitude diagram obtained with HST. The remaining unknown needed to determine the lens mass is the distance to the lens. This can be determined from the parallactic distortion of the microlensing light curve due to the orbital motion of the Earth during the event. Since the HST observations must cover a large time-span of 3 years, they have a 2-week cadence, which is insufficient to determine this parameter. We propose Gemini imaging of our bulge field with a ~3-day cadence in order to obtain this crucial parameter. This will allow us, for the first time, to determine the mass function of isolated BHs and NSs.


National Optical Astronomy Observatory, 950 North Cherry Avenue, P.O. Box 26732, Tucson, Arizona 85726, Phone: (520) 318-8000, Fax: (520) 318-8360



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