2008A&A...479..625E

Stellar dynamics indicate the presence of a supermassive 3-4x10⁶M_☉ black hole at the Galactic Center. It is associated with the variable radio, near-infrared, and X-ray source Sagittarius A* (Sgr A*). The goal is the investigation and understanding of the physical processes responsible for the variable emission from Sgr A*. The observations have been carried out using the NACO adaptive optics (AO) instrument at the European Southern Observatory's Very Large Telescope (July 2005, May 2007) and the ACIS-I instrument aboard the Chandra X-ray Observatory (July 2005). We find that for the July 2005 flare the variable and polarized NIR emission of Sgr A* occurred synchronous with a moderately bright flare event in the X-ray domain with an excess 2-8keV luminosity of about 8x10³³erg/s. We find no time lag between the flare events in the two wavelength bands with a lower limit of ≤10min. The May 2007 flare shows the highest sub-flare to flare contrast observed until now. It provides evidence for a variation in the profile of consecutive sub-flares. We confirm that highly variable and NIR polarized flare emission is non-thermal and that there exists a class of synchronous NIR/X-ray flares. We find that the flaring state can be explained via the synchrotron self-Compton (SSC) process involving up-scattered X-rays from the compact source component. The observations can be interpreted in a model involving a temporary disk with a short jet. In the disk component the flux density variations can be explained by spots on relativistic orbits around the central supermassive black hole (SMBH). The profile variations for the May 2007 flare can be interpreted as a variation of the spot structure due to differential rotation within the disk.