VVV Survey Observations of a Microlensing Stellar Mass Black Hole
  Candidate in the Field of the Globular Cluster NGC 6553
  We report the discovery of a large timescale candidate microlensing event of
a bulge stellar source based on near-infrared observations with the VISTA
Variables in the Via Lactea Survey (VVV). The new microlensing event is
projected only 3.5 arcmin away from the center of the globular cluster NGC
6553. The source appears to be a bulge giant star with magnitude Ks = 13.52,
based on the position in the color-magnitude diagram. The foreground lens may
be located in the globular cluster, which has well-known parameters such as
distance and proper motions. If the lens is a cluster member, we can directly
estimate its mass simply following Paczynski et al. (1996) which is a modified
version of the more general case due to Refsdal. In that case, the lens would
be a massive stellar remnant, with M = 1.5-3.5 Msun. If the blending fraction
of the microlensing event appears to be small, and this lens would represent a
good isolated black hole (BH) candidate, that would be the oldest BH known.
Alternative explanations (with a larger blending fraction) also point to a
massive stellar remnant if the lens is located in the Galactic disk and does
not belong to the globular cluster.
