By David Shiga
Thousands of superdense neutron stars and midget black holes lurk near the center of our galaxy, according to new X-ray studies of the sky. The stellar-mass black holes are each just 10 times the mass of the sun, much smaller than the supermassive black hole known to inhabit the Milky Way’s center. That central black hole has an estimated mass of 3.7 million suns. Each neutron star is about the mass of the sun.
Using the orbiting Chandra X-ray Observatory, Michael P. Muno of the University of California, Los Angeles (UCLA) and his colleagues found four dramatically flickering X-ray sources within 3 light-years of the Milky Way’s supermassive black hole, known as Sagittarius A. The large fluctuations in these objects’ brightness suggest that they are black holes and neutron stars stealing matter from companion stars. Stellar-mass black holes and neutron stars both form from the collapsed cores of stars that died in supernova explosions.
Team member Mark R. Morris, also of UCLA, theorized in 1993 that 10,000 stellar-mass black holes resided within 3 light-years of our galaxy’s center. In the process he invoked, known as dynamical friction, frequent close encounters among objects fling the more massive objects toward the center of the crowded environment and less massive objects outward. During the several billion years that our galaxy has been around, Morris proposed, this effect should have caused black holes and neutron stars, which are more massive than typical stars, to migrate toward Sagittarius A.
A small fraction of these migrating objects would pair up by chance with stars near the galactic center and start pulling in matter from these accidental partners. Such transfers of matter would trigger copious X-ray emissions detectable by Chandra.
The researchers calculate that without dynamical friction, there would be only a 20 percent chance of finding one of these X-ray sources within 3 light-years of Sagittarius A. Finding four of them suggests that such objects are 20 times as common in the galaxy’s inner 3 light-years as scientists would expect without the concentration effect. “This is the first evidence that [Morris’] prediction is actually true,” Muno says. The researchers presented their new data on Jan. 10 in San Diego at a meeting of the American Astronomical Society.
“The objects observed by Muno are undoubtedly either stellar-mass black holes or neutron stars,” says Milos Milosavljevic of the California Institute of Technology in Pasadena, who studies interactions of objects near the galaxy’s center. But he says that the objects could have formed where they are rather than migrated in from farther out.
Figuring out exactly where the black holes came from means figuring out where the short-lived, massive stars that gave birth to them originated, says Morris. He’s planning to determine the motions of the current generation of such stars by making further observations that provide clues to their origins.
