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ATLANTA -- Culminating 25 years of searching by astronomers, researchers at Massachusetts Institute of Technology say that a faint X-ray source, newly detected by NASA's Chandra X-ray Observatory, may be the long-sought X-ray emission from a known supermassive black hole at the center of our galaxy.
Frederick K. Baganoff and colleagues from Pennsylvania State University, University Park, and the University of California, Los Angeles (UCLA), 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."
"The luminosity of the X-ray source we have discovered already is a factor of five fainter than previously thought, based on observations from an earlier X-ray satellite," Baganoff said. "This poses a problem for theorists. The galactic center is a crowded place. If we were to find that most or all of the X-ray emission is not from Sagittarius A*, then we will have shown conclusively that all current models from Sagittarius A* need to be rethought from the ground up."
Astronomers believe that most galaxies harbor massive black holes at their centers. Many of these black holes are thought to produce powerful and brilliant point-like sources of light that astronomers call quasars and active galactic nuclei. Why the center of our galaxy is so dim is a long-standing puzzle.
ONE SOURCE STANDING OUT IN A CROWD
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.
CHANDRA'S POWERFUL VISION
Optical telescopes such as the Hubble Space Telescope cannot see the center of our galaxy, which is enshrouded in thick clouds of dust and gas in the plane of the galaxy. However, hot gas and charged particles moving at nearly the speed of light produce X-rays that penetrate this shroud.
Only a few months after its launch, Chandra accomplished what no other optical or X-ray satellite was able to do: separate the emissions from the surrounding hot gas and nearby compact sources that prevented other satellites from detecting this new X-ray source. Mark Morris of UCLA, who has studied this region intensely for 20 years, called Chandra's data "a gold mine" for astronomers.
"With more observing time on Chandra in the next two or three years, we will be able to build up a spectrum that will allow us to rule out various classes of objects and either confirm or deny Sagittarius A* as the origin of the X-ray emission," Baganoff said. "If we show that the emission is from a supermassive black hole, we will then be set to begin a detailed study of the X-ray emission from the nearest analog of a quasar or active galactic nucleus."
Chandra's ACIS detector, the Advanced CCD Imaging Spectrometer, was conceived and developed for NASA by Penn State University and MIT under the leadership of Penn State Professor Gordon Garmire.
NASA's Marshall Space Flight Center in Huntsville, Ala., manages the Chandra program. TRW Inc., Redondo Beach, Calif., is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, Mass.
Sagittarius A is the brightest radio source in the constellation Sagittarius. It is composed of two large features: Sagittarius A East, a large shell-shaped radio source on the eastern side of the structure that is thought to be the result of a recent supernova explosion; and Sagittarius A West, a bright spiral-shaped region of ionized gas falling towards or orbiting around the center of our galaxy. The asterisk in the name Sagittarius A* is used to indicate the point-like, variable radio source within Sagittarius A West that is thought to be powered by a known supermassive black hole at the dynamical center of our galaxy.
The study of objects in the universe using X-rays rather than visible light or other wavelengths of electromagnetic radiation. The X-rays can be imaged with grazing incidence mirrors which must be polished with extreme accuracy to reflect short-wavelength X-rays. An X-ray detector is placed at the focal plane of the telescope. The ACIS detector is a sophisticated version of the CCD detectors commonly used in video cameras or digital cameras.
CHANDRA X-RAY OBSERVATORY:
The latest in NASA's series of Great Observatories. Chandra is the "X-ray Hubble," launched in July 1999 on the Space Shuttle Columbia and then sent into a deep-space orbit around the Earth. Chandra carries a large X-ray telescope to focus the X-rays from objects in the sky. An X-ray telescope cannot work on the ground because the X-rays are absorbed by the Earth's atmosphere.
ACTIVE GALACTIC NUCLEUS:
The centers of some galaxies are unusually bright and variable in radio, infrared, optical, and X-ray light. These point-like sources are thought to be powered by gaseous matter falling into a supermassive black hole containing millions or billions of times the mass of our sun. Active galactic nucleus is a classification astronomers use to describe any bright, variable, and point-like source at the center of a nearby galaxy. Looking farther away, astronomers see sources which are even more luminous than the nuclei of nearby active galaxies. These sources are called quasars.