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Optical Gravitational Lensing Experiment. OGLE-1999-BUL-32: the Longest Ever Microlensing Event -- Evidence for a Stellar Mass Black Hole?

Shude Mao·Martin C. Smith·P. Wozniak·A. Udalski·M. Szymanski M. Kubiak·G. Pietrzynski·I. Soszynski·K. Zebrun
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Abstract

We describe the discovery of the longest microlensing event ever observed, OGLE-1999-BUL-32, also independently identified by the MACHO collaboration as MACHO-99-BLG-22. This unique event has an Einstein radius crossing time of 641 days. The high quality data obtained with difference image analysis shows a small but significant parallax signature. This parallax effect allows one to determine the Einstein radius projected onto the observer plane as rE^hat ~ 29.2AU. The transverse velocity projected onto the observer plane is about 79km/s. We argue that the lens is likely to be have a mass of at least a few solar masses, i.e., it could be a stellar black hole. The black hole hypothesis can be tested using the astrometric microlensing signature with the soon-to-be installed Advanced Camera for Surveys on board the Hubble Space Telescope. Deep X-ray and radio images may also be useful for revealing the nature of the object.

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Shude Mao, Martin C. Smith, P. Wozniak, A. Udalski, M. Szymanski M. Kubiak, G. Pietrzynski, I. Soszynski, K. Zebrun. 2001-08-20. Optical Gravitational Lensing Experiment. OGLE-1999-BUL-32: the Longest Ever Microlensing Event -- Evidence for a Stellar Mass Black Hole?. https://doi.org/10.1046/j.1365-8711.2002.04986.x

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