Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical Gravitational Lensing Experiment. OGLE-1999-BUL-32: the Longest
  Ever Microlensing Event -- Evidence for a Stellar Mass Black Hole?   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.

12
The categories of this paper are: nucl-th
31
The categories of this paper are: ao-sci
34
The categories of this paper are: chem-ph
3
The categories of this paper are: physics
25
The categories of this paper are: patt-sol
8
The categories of this paper are: hep-ex
17
The categories of this paper are: q-fin
11
The categories of this paper are: hep-lat
14
The categories of this paper are: math-ph
15
The categories of this paper are: nucl-ex
33
The categories of this paper are: plasm-ph
37
The categories of this paper are: supr-con
27
The categories of this paper are: comp-gas
19
The categories of this paper are: eess
29
The categories of this paper are: funct-an
9
The categories of this paper are: nlin