Black Holes 1050.1750.150.1250.10.0750.050.02500.05 0.025 –0.0252001200519922010
19952002SgrA*RA-offset (arcsec)S2 (S02)0 – 0.05 –0.075Figure 5.6Orbit of the star S2 moving around Sgr A* [10]. Copyright 2010 by the American
Physical Society. (See plate section for color version.)
a very strong gravitational field of a practically pointlike source of mass 4. 4 × 106 푀⊙.
The best measured orbit of the star푆2 which has completed a full orbit since the
beginning of 1992 is shown in Figure 5.6 [10].
There is a hot accreting zone around the event horizon where Sgr A∗is accreting
from the winds of surrounding stars at the outer boundary. Typically the radiated
power from the accretion zone feeds the luminosity which is very weak both in radio
and X-ray emission, but hot, thermal X-ray emission associated with Sgr A∗has been
claimed. It appears that there is a dynamically important magnetic field near the black
hole, determining the structure of the accretion flow. If this field is accreted down to
the event horizon it provides enough magnetic flux to explain the observed emission.
At the time of writing, an enigmatic small nonself-gravitating gas cloud is seen
falling almost exactly onto Sgr A∗at a nominal pericenter distance of only 2200
Schwarzschild radii (푟c= 136 km). Observations over the next decade should enlighten
its fate and allow us to explore general relativity.