Stellar dynamics in the innermost region

A HKL colour composite of the Galactic Centre region. The central black hole is located in the centre of the box which marks the area shown in the images above and below.

Stellar dynamics in the innermost region

Time resolved astrometry over a time span of now already 12 years allows a description of the proper motions of the Galactic Centre stars. The observations clearly show, that some stars in the immediate vicinity of Sgr A* - i.e. in distances up to around 30 light days - move on Keplerian orbits around the central mass. From the shape of these orbits, the distance between earth and Sgr A* and the mass of Sgr A* could be calculated.

Radio and X-ray emission from Sgr A*

The supermassive black hole in the centre of the Milky Way was discovered as a bright non-thermal radio source in the 1970s and termed Sagittarius A* (Sgr A*). Potential X-ray radiation by Sgr A* was detected with the X-ray observatory Rosat in the 1990s. A reliable identification of X-rays from Sgr A* was finally possible with the new X-ray satellites Chandra and XMM, with their high spatial resolution and sensitivity. The radio emission of SgrA* only varies slowly on time scales of several days to a few hundred days and generally with an amplitude <10%. However, in the X-ray regime, SgrA* was found to exhibit two different states. On the one hand, in the quiescent state, weak X-ray emission appears to come from a slightly extended area around the black hole that appears to be evidence of hot accreting gas in the environment of SgrA*. On the other hand, SgrA* shows X-ray flares with a period of about one per day. During these flares, the emission rises by factors up to 100 during several tens of minutes and a distinctive point source becomes visible at the location of SgrA*. The short rise-and-decay times of the flares suggest that the radiation must origin from a region within less than 10 Schwarzschild radii of a 3.6 million solar mass black hole.