  Context: We report new polarization measurements of the variable
near-infrared emission of the SgrA* counterpart associated with the massive
3--4 10^6 solar masses Black Hole at the Galactic Center. Aims: We investigate
the physical processes responsible for the variable emission from SgrA*.
Methods: The observations have been carried out using the NACO adaptive optics
(AO) instrument at the European Southern Observatory's Very Large Telescope
Results: We find that the variable NIR emission of SgrA* is highly polarized
and consists of a contribution of a non- or weakly polarized main flare with
highly polarized sub-flares. The flare activity shows a possible
quasi-periodicity of 20+-3 minutes consistent with previous observations.
Conclusions: The highly variable and polarized emission supports that the NIR
emission is non-thermal. The observations can be interpreted in a jet or
temporary disk model. In the disk model the quasi-periodic flux density
variations can be explained due to spots on relativistic orbits around the
central MBH. Alternative explanations for the high central mass concentration
involving boson or fermion balls are increasingly unlikely.

Astrophysics