For the first time, a rapid X-ray flare has been observed from the
direction of the supermassive black hole that resides at the center of our
galaxy. This violent flare captured by NASA’s Chandra X-ray Observatory has
given astronomers an unprecedented view of the energetic processes
surrounding this supermassive black hole.

A team of scientists led by Frederick K. Baganoff of MIT detected a
sudden X-ray flare while observing Sagittarius A*, a source of radio
emission believed to be associated with the black hole at the center of our

“This is extremely exciting because it’s the first time we have seen
our own neighborhood supermassive black hole devour a chunk of material,”
said Baganoff. “This signal comes from closer to the event horizon of our
Galaxy’s supermassive black hole than any that we have ever received before.
It’s as if the material there sent us a postcard before it fell in.”

In a just few minutes, Sagittarius A* became 45 times brighter in
X-rays, before declining to pre-flare levels a few hours later. At the peak
of the flare, the X-ray intensity dramatically dropped by a factor of five
within just a 10-minute interval. This constrains the size of the emitting
region to be no larger than about 20 times the size of the “event horizon”
(the one-way membrane around a black hole) as predicted by Einstein’s theory
of relativity.

The rapid rise and fall seen by Chandra are also compelling evidence
that the X-ray emission is coming from matter falling into a supermassive
black hole. This would confirm the Milky Way’s supermassive black hole is
powered by the same accretion process as quasars and other active galactic

Dynamical studies of the central region of our Milky Way Galaxy in
infrared and radio wavelengths indicate the presence of a large, dark
object, presumably a supermassive black hole having the mass of about 3
million suns. Sagittarius A* is coincident with the location of this object,
and is thought to be powered by the infall of matter into the black hole.
However, the faintness of Sagittarius A* at all wavelengths, especially in
X-rays, has cast some doubt on this model.

The latest precise Chandra observations of the crowded galactic
center region have dispelled that doubt, confirming the results of the
dynamical studies. Given the extremely accurate position, it is highly
unlikely that the flare is due to an unrelated contaminating source such as
an X-ray binary system.

“The rapid variations in X-ray intensity indicate that we are
observing material that is as close to the black hole as the Earth is to the
Sun,” said Gordon Garmire of Penn State University, principal investigator
of Advanced CCD Imaging Spectrometer (ACIS), which was used in these
observations. “It makes Sagittarius A* a uniquely valuable source for
studying conditions very near a supermassive black hole.”

The energy released in the flare corresponds to the sudden infall of
material with the mass equivalent to a comet. Alternatively, the scientists
speculate that this flare could have been caused by the reconnection of
magnetic field lines just outside the event horizon, similar to phenomenon
responsible for solar flares but on a tremendous scale.

In either scenario, the energy released would be accompanied by
shock waves that accelerated the electrons near the black hole to nearly the
speed of light, leading to an outburst of X-rays. A longer-term increase in
radio emission was also observed beginning around the time of the flare,
indicating that the production of high-energy electrons was increasing.

“It’s truly remarkable that we could identify and track this flare
in such a crowded region of space,” said Mark Bautz of MIT. “This discovery
would not have been possible without the resolution and sensitivity of
Chandra and the ACIS instrument.”

The team first observed Sgr A* with ACIS on September 21, 1999, and
again on October 26-27, 2000. The X-ray flare was detected in the second

Other members of the team are Niel Brandt, George Chartas, Eric
Feigelson, Leisa Townsley (Penn State), Yoshitomo Maeda (Institute of Space
and Astronautical Science, Japan), Mark Morris (UCLA), George Ricker (MIT),
and Fabian Walker (CalTech).

The ACIS instrument was developed for NASA by Penn State and MIT
under the leadership of Garmire. NASA’s Marshall Space Flight Center in
Huntsville, Ala., manages the Chandra program, and 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,

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