Shude Mao, Martin C. Smith, P. Wozniak, A. Udalski, M. Szymanski M. Kubiak, G. Pietrzynski, I. Soszynski, K. Zebrun MNRAS, 2002, 329, 349
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≈29.2 AU. The transverse velocity projected onto the observer plane is about 79 km/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.
The paper is available as astro-ph/0108312 from arXiv preprint archive.