The high concentration of X-ray binaries in this small region was a surprise - other observations of a larger region around Sgr A* had suggested that there was only a 20 percent chance that even one X-ray binary would be found within 3 light years of Sgr A*. The observed high concentration of X-ray binaries is strong circumstantial evidence that a dense swarm of 10,000 or more stellar-mass black holes and neutron stars has formed around Sgr A*.
The swarm likely formed as stellar-mass black holes gradually decelerated in their orbits and sank toward the center of the Galaxy. Black holes orbiting the center of the Galaxy at a distance of several light years will pull on surrounding stars, which pull back on the black holes. The net effect of this gravitational action and reaction is to decelerate the black holes, which have masses of about 10 Suns, and speed up the lower-mass surrounding stars.
The black holes spiral inward, and the low-mass stars move out. From the estimated number of stars and black holes in the Galactic Center region, this process, called stellar dynamical friction, could produce a dense swarm of 10,000 or more black holes within 3 light years of Sgr A*. A similar effect could be at work for neutron stars, but to a lesser extent because they have a lower mass.
Once black holes are concentrated near Sgr A*, their intense gravity can induce an ordinary star in a binary system to “change partners” and pair up with the black hole while ejecting its companion. This process and a similar one for neutron stars can account for the observed binary X-ray sources
http://chandra.harvard.edu/photo/2005/gctr_bin/
The net effect is that black holes spiral inward, and the low-mass stars move out. From the estimated number of stars and black holes in the Galactic Center region, dynamical friction is expected to produce a dense swarm of 20,000 black holes within three light years of Sgr A*. A similar effect is at work for neutron stars, but to a lesser extent because they have a lower mass.
Once black holes are concentrated near Sgr A*, they will have numerous close encounters with normal stars there, some of which are in binary star systems. The intense gravity of a black hole can induce an ordinary star to “change partners” and pair up with the black hole while ejecting its companion. This process and a similar one for neutron stars are expected to produce several hundreds of black hole and neutron star binary systems.“If only one percent of these binary systems are X-ray active each year, they can account for the sources we see,” said Eric Pfahl of the University of Virginia in Charlottesville and a coauthor of a paper describing these results that has been submitted to the Astrophysical Journal Letters. “Although the evidence is mostly circumstantial, it makes a strong case for the existence of a large population of neutron stars and stellar-mass black holes within three light-years of the center of our Galaxy.”
The black holes and neutron stars in the cluster are expected to gradually be swallowed by the supermassive black hole, Sgr A*, at a rate of about one every million years. At this rate, about 10,000 black holes and neutron stars would have been captured in a few billion years, adding about 3 percent to the mass of the central supermassive black hole, which is currently estimated to contain the mass of 3.7 million suns.
In the meantime, the acceleration of low-mass stars by black holes will eject low-mass stars from the central region. This expulsion will reduce the likelihood that normal stars will be captured by the central supermassive black hole. This may explain why the central regions of some galaxies, including the Milky Way, are fairly quiet even though they contain a supermassive black hole.
http://chandra.harvard.edu/press/05_releases/press_011005.html
2008-11-08 | achtphasen | 09:12:13 | 
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