We present the results of a near-infrared monitoring of Sgr A*,
the super-massive black hole at the center of our Galaxy.
Sgr A* was observed continuously for about four hours on 28 May 2008,
using the near-infrared camera CIAO and the 36-element AO system on
the Subaru telescope.
In this observation, three flares have been detected.
The first flare had a moderate peak flux, but had a long duration of
more than 80 min; the second had a strong peak followed by a plateau,
and a duration of about 40 min; the last had a very narrow peak, and its
rise/decay time was about 6.5 min. This short rise/decay time suggests
that the flare event occurred very close to Sgr A*, on a scale of 10
Schwarzschild radii.
In spite of these flare events, we were unable to find significant
evidence for quasi-periodic modulation of about 20 min,
which has been suggested in previous observations.
Clear variations in the degree and position angle of polarization have
been detected: an increase of the degree of polarization of about 20 %,
and a swing of the position angle of about 60 degree in the declining
phase of the flares. These correlation between the flux and the degree
of polarization suggests that the flare emission comes from hot spot(s)
orbiting Sgr A*.