backOGLE-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
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.