  Context: The supermassive black hole at the Galactic center, SgrA*, shows
frequent radiation outbursts, often called 'flares'. In the near-infrared some
of these flares were reported as showing intrinsic quasi-periodicities. The
flux peaks associated with the quasi-periodic behavior were found to be highly
polarized. Aims: The aim of this work is to present new evidence to support
previous findings of the properties of the polarized radiation from SgrA* and
to again provide strong support for the quasi-periodicity of ~18+-3 min
reported earlier. Methods: Observations were carried out at the European
Southern Observatory's Very Large Telescope on Paranal, Chile. We used the
NAOS/CONICA adaptive optics/near-infrared camera instrument. By fitting the
polarimetric lightcurves with a hot-spot model, we addressed the question of
whether the data are consistent with this model. To fit the observed data we
used a general relativistic ray-tracing code in combination with a simple
hot-spot/ring model. Results: We report on new polarization measurements of a
K-band flare from the supermassive black hole at the Galactic center. The data
provide very strong support for a quasi-periodicity of 15.5+-2 min. The mean
polarization of the flare is consistent with the direction of the electric
field vector that was reported in previous observations. The data can be
modeled successfully with a combined blob/ring model. The inclination i of the
blob orbit must be i > 20 deg on a 3sigma level, and the dimensionless spin
parameter of the black hole is derived to be a > 0.5.
