Chandra Discovers Light Echo from the Milky Way's Black Hole (Forwarded)

Steve Roy
Marshall Space Flight Center, Huntsville, Ala.
(Phone: 256/544-6535)

Megan Watzke
Chandra X-ray Center, Cambridge, Mass.
(Phone: 617/496-7998)

For Release: January 10, 2007

CXC RELEASE 02-07

Chandra Discovers Light Echo from the Milky Way's Black Hole

Like cold case investigators, astronomers have used NASA's Chandra X-ray
Observatory to uncover evidence of a powerful outburst from the giant
black hole at the Milky Way's center.

A light echo was produced when X-ray light generated by gas falling into
the Milky Way's supermassive black hole, known as Sagittarius A*
(pronounced "A-star"), was reflected off gas clouds near the black hole.
While the primary X-rays from the outburst would have reached Earth about
50 years ago, the reflected X-rays took a longer path and arrived in time
to be recorded by Chandra.

"This dramatic event happened before we had satellites in space that could
detect it," said Michael Muno of the California Institute of Technology in
Pasadena. "So, it's remarkable that we can use Chandra to dig into the
past and see this monster black hole's capacity for destruction."

Previously, scientists have used Chandra to directly detect smaller and
more recent outbursts from the black hole. This latest outburst revealed
by the X-ray echo was about 1,000 times brighter and lasted well over
1,000 times longer than any of the recent outbursts observed by Chandra.

Theory predicts that an outburst from Sagittarius A* would cause X-ray
emission from the clouds to vary in both intensity and shape. Muno and his
team found these changes for the first time, thus ruling out other
interpretations. The latest results corroborate other independent, but
indirect, evidence for light echoes generated by the black hole in the
more distant past.

Scientists have long known that Sagittarius A*, with a mass of about 3
million suns, lurked at the center for Milky Way. However, the black hole
is incredibly faint at all wavelengths, especially in X-rays.

"This faintness implies that stars and gas rarely get close enough to the
black hole to be in any danger," said co-author Frederick K. Baganoff of
the Massachusetts Institute of Technology in Cambridge. "The huge appetite
is there, but it's not being satisfied."

During the outburst, the area close to the black hole would have been
about 100,000 times brighter than it is currently. If such an outburst had
occurred more recently, it likely would have been detected by an X-ray
instrument, or would have produced similar features in other nearby
clouds.

"Our data show it has been 50 years or so since the black hole had its
last decent meal," said Muno. "This is nothing like the feasting that
black holes in other galaxies sometimes enjoy, but it gives unique
knowledge about the feeding habits of our closest supermassive black
hole."

The details of how Sagittarius A* feeds remain unclear. For example, one
possibility is that the black hole grows by pulling in matter from the
winds of nearby young stars. Also, if there is a disk of material swirling
around Sagittarius A*, it might be unstable in such a way that material
migrates toward the black hole's edge in clumps, emitting X-rays before
disappearing from the universe forever. The theoretical work is still
being developed.

Studying this light echo is also important because it illuminates and
probes the poorly understood molecular clouds near the center of the
galaxy. In particular, it gives information about the dense cores of these
clouds where new stars may be forming.

Variability in the X-ray emission between three Chandra observations in
2002, 2004 and 2005 argues against an alternate source for the light echo,
which is that it came from a neutron star or black hole pulling matter
away from a binary companion. This explanation is not favored because the
data show the outburst would have been unusually long and bright for such
a binary.

These results were presented at the American Astronomical Society meeting
in Seattle, Wash., and will appear in an upcoming issue of The
Astrophysical Journal Letters. NASA's Marshall Space Flight Center,
Huntsville, Ala., manages the Chandra program for the agency's Science
Mission Directorate. The Smithsonian Astrophysical Observatory controls
science and flight operations from the Chandra X-ray Center, Cambridge,
Mass.

Additional information and images are available at:

http://chandra.harvard.edu/photo/2007/gcle/
and
http://chandra.nasa.gov


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