Grischa Karssen (U of Cologne)
Abstract: With a distance of about 8 kpc, the center of the Milky Way is the closest galactic nucleus to us. Hence, it provides us with a unique opportunity to study a galactic nucleus up close. Longterm observations of stellar kinematics of the Nuclear Star Cluster point to the existence of a super-massive black hole (SMBH) at the position of Sagittarius A* (SgrA*), with a mass of 4 million suns. SgrA* shows flare emission from the millimeter to the X-ray domain. A detailed analysis of the infrared light curves allows us to address the accretion phenomenon in a statistical way. The analysis shows that the near-infrared flare amplitudes are dominated by a single state power law, with the low states in SgrA* limited by confusion through the unresolved stellar background. There are several dusty objects in the immediate vicinity of SgrA*. The source G2/DSO is one of them. Its nature is unclear. It may be comparable to similar stellar dusty sources in the region or may consist predominantly of gas and dust. In this case a particularly enhanced accretion activity onto SgrA* may be expected in the near future. A relativistic model that could explain the flaring nature of SgrA* are hotspots, overdense compact emitting regions, moving inside an accretion flow. To model compact sources orbiting in the immediate vicinity of SgrA*, it is necessary to use the metric for a rotating black hole, the Kerr-metric. There are a couple of relativistic effects on the emission that need to be taken into account, most importantly the gravitational Doppler-shift and gravitational lensing.