Frederick K. Baganoff and colleagues from Pennsylvania State University, University Park, and the University of California, Los Angeles, will present their findings today in Atlanta at the 195th national meeting of the American Astronomical Society.
Baganoff, lead scientist for the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer (ACIS) team's "Sagittarius A* and the Galactic Center" project and postdoctoral research associate at MIT, said that the precise positional coincidence between the new X-ray source and the radio position of a long-known source called Sagittarius A* "encourages us to believe that the two are the same."
Sagittarius A* is a point-like, variable radio source at the center of our galaxy. It looks like a faint quasar and is believed to be powered by gaseous matter falling into a supermassive black hole with 2.6 million times the mass of our sun.
Chandra's remarkable detection of this X-ray source has placed astronomers within a couple of years of a coveted prize: measuring the spectrum of energy produced by Sagittarius A* to determine in detail how the supermassive black hole that powers it works. "The race to be the first to detect X-rays from Sagittarius A* is one of the hottest and longest-running in all of X-ray astronomy," Baganoff said. "Theorists are eager to hear the results of our observation so they can test their ideas."
But now that an X-ray source close to Sagittarius A* has been found, it has taken researchers by surprise by being much fainter than expected. "There must be something unusual about the environment around this black hole that affects how it is fed and how the gravitational energy released from the infalling matter is converted into the X-ray light that we see," Baganoff said. "This new result provides fresh insight that will no doubt stir heated debates on these issues.
"Chandra's sensitivity is 20 times better than achieved with the best previous X-ray telescopes," said Gordon Garmire, the Evan Pugh Professor of Astronomy and Astrophysics at Penn State University and head of the team that conceived and built Chandra's Advanced CCD Imaging Spectrometer (ACIS) X-ray camera, which acquired the data. "This sensitivity, plus the superior spatial resolution of Chandra's mirrors, make Chandra the perfect tool for studying this faint X-ray source in its crowded field."
Sagittarius A*, which stands out on a radio map as a bright dot, was detected at the dynamical center of the Milky Way galaxy by radio telescopes in 1974.
More recently, infrared observations of the movements of stars around Sagittarius A* has convinced most astronomers that there is a supermassive black hole at the center of our galaxy and that it is probably associated with Sagittarius A*. A black hole is an object so compact that light itself cannot escape its gravitational pull. A black hole sucks up material thrown out by normal stars around it.
Because there are a million times more stars in a given volume in the galactic center than elsewhere in the galaxy, researchers cannot yet say definitively that Sagittarius A* is the newly detected source of the X-rays. "We need more data to clarify our observations," Baganoff said.
If Sagittarius A* is powered by a supermassive black hole, astronomers expected that there would be a lot of matter to suck up in a crowded place like the galactic center. The faintness of the source may indicate a dearth of matter floating toward the black hole or it may indicate that the environment of the black hole is for some reason rejecting most of the infalling material.