Chandra catches Milky Way monster snacking
Posted: September 6, 2001

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 Sagiattarius A*, a source of radio emission believed to be associated with the black hole at the center of our Galaxy.

This false-color image shows the central region of our Milky Way Galaxy as seen by Chandra. The bright, point-like source at the center of the image was produced by a huge X-ray flare that occurred in the vicinity of the supermassive black hole at the center of our galaxy. This central black hole has about 2.6 million times the mass of our sun and is associated with the compact radio source Sagittarius A. Photo: NASA/MIT/F. Baganoff et al.
"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 ofour 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 Einsteinšs 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 nuclei.

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 flarecould 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 theflare, 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 observation.

Other members of the team are Niel Brandt, George Chartas, Eric Feigelson, Leisa Townsley (Penn State), Yoshitomo Maeda (Insititute 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, AL, manages the Chandra program, and TRW, Inc., Redondo Beach, CA, is the prime contractor for the spacecraft. The Smithsonian's Chandra X-ray Center controls science and flight operations from Cambridge, MA.