Deborah Halber
Massachusetts Institute of Technology
Cambridge, MA
Phone: 617-258-9276
Email: dhalber@mit.edu
Steve Roy
Marshall Space Flight Center
Huntsville, AL
Phone: 256-544-6535
Dr. Wallace Tucker
Chandra X-ray Observatory Center
Cambridge, MA
Phone: 617-496-7998
Barbara Kennedy
Penn State PIO
University Park, PA
Phone: 814-863-4682
Email: science@psu.edu
CXC PR: 00-04
Chandra Discovers X-ray Source at the Center of Our Galaxy
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, 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 satelllite,” 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 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 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.
To follow Chandra’s progress or download images visit the Chandra sites at
http://chandra.harvard.edu/photo/cycle1/0204/index.html
AND
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.
IMAGE CAPTIONS:
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* (white dot at the center of the
image). 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.
(Credit: NASA/MIT/PSU)
BACKGROUND DEFINITIONS
SAGITTARIUS A*:
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.
X-RAY ASTRONOMY:
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.
