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A Monster in the Middle

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a radio map of Galactic centerFebruary 29, 2000 -- The core of the Milky Way galaxy is a busy place. It's filled with giant molecular clouds, the remnants of exploding stars, and mysterious filaments hundreds of light years long. At the center of this menagerie lies an object radio astronomers call Sagittarius A*. Sag A* is a radio source that looks like a faint quasar. Scientists have long suspected that it is powered by a supermassive black hole with 2.6 million times the mass of our Sun.

Right: This beautiful high-resolution radio image covers a 4x4 degree region around the galactic center. It was constructed from one-meter wavelength radio data obtained by telescopes of the Very Large Array near Socorro, New Mexico, USA. The galactic center itself is at the edge of the extremely bright object labeled Sagittarius (Sgr) A, suspected of harboring a million solar mass black hole. [larger image]

If Sag A* really does harbor a black hole, it ought to shine as an X-ray source. The X-rays would come from hot gas in an accretion disk swirling into the dense gravitational field of the hole. Previous X-ray satellites lacked the combined resolution and sensitivity to make this basic test. But now, thanks to the Chandra X-ray Observatory, astronomers finally have the data they've been waiting for.

A group of researchers led by Frederick K. Baganoff and colleagues from Pennsylvania State University announced last month that a faint X-ray source, newly detected by Chandra, may be the long-sought X-ray emission from a supermassive black hole at the center of the Milky Way.

"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."

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.

As the high energy X rays stream away from the vicinity of the black hole, they heat the blanketing gas to temperatures of a few million degrees, and the blanket absorbs some of the X rays from the central source. This produces dark stripes, or absorption lines in the X-ray spectrum. Bright stripes or emission lines due to emission from the blanketing gas are also present. Since each element has its own unique structure, these lines can be read like a cosmic bar code to take inventory of the gas. The team was able to determine what atoms the gas contains and how many, the number of electrons each atom has retained in the hostile environment of the black hole, and how the gas is moving there. They found lines from eight different elements including carbon, nitrogen, oxygen, and iron. The amount of this gas was found to be about 100 times greater than that found with optical and ultraviolet observations.

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 image of the galactic centerLeft: Chandra X-ray image of the innermost 10 light years at the center of our galaxy. The image has been smoothed to bring out the X-ray emission from an extended cloud of hot gas surrounding the supermassive black-hole candidate Sagittarius A* (larger white dot at the very center of the image- a little to the left and above the smallest white dot). This gas glows in X-ray light because it has been heated to a temperature of millions of degrees by shock waves produced by supernova explosions and perhaps by colliding winds from young massive stars.

"Chandra's sensitivity is 20 times better than achieved with the best previous X-ray telescopes," said Penn State's Gordon Garmire, 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.

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.

X-ray Vision

Artist's superposition of Chandra and an active X-ray sourceOptical telescopes such as the Hubble Space Telescope cannot see the center of our galaxy, which is enshrouded in thick clouds of dust and gas. 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 the University of California at Los Angeles, 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 concluded. "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."

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.

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.

Web Links

Chandra home page -from Harvard

Chandra News -from NASA

Black Holes -a tutorial about black holes and accretion disks

X-Rays - Another Form of Light - the basics of X-rays from the Chandra home page at Harvard