Galaxy's Black Hole: X Rays Mark Spot.

An X-ray outburst from the center of our galaxy is providing compelling new evidence that a monster black hole lurks at the Milky Way's core.

The high velocity of stars orbiting near the galactic center had already convinced astronomers of a supermassive black hole. To hold these stars in their orbits, the center must harbor an object equal in mass to 2.6 million suns in a region smaller than the radius of the solar system (SN: 10/7/00, p. 232).

But even that is still 30,000 times larger than the black hole's radius as predicted by Einstein's general theory of relativity. According to the theory, the size of a black hole is defined by its event horizon--the region that represents a point of no return. Any object, even a photon, that comes closer would be unable to resist the hole's gravitational grasp and would therefore vanish.

The X-ray emissions, recorded with the Chandra X-ray Observatory, come from a region at the very fringes of the event horizon. In contrast, stars whose motions astronomers have tracked lie some 1,500 times farther from the galactic core. Frederick K. Baganoff of the Massachusetts Institute of Technology and his team describe their work in the Sept. 6 NATURE.

Last October, Chandra recorded a flare from the Milky Way's center that lasted about 3 hours. The brightness of the flare dropped dramatically, to one-fifth its previous value, over 10 minutes.

The rapidity of that drop provides a powerful new estimate of the black hole's size. For the intensity of the X-ray flare to vary so quickly, the event horizon can be no larger than the distance light travels in 10 minutes. That's 150 million kilometers, or about the distance between Earth and the sun.

The data therefore indicate that Chandra has for the first time observed material "that is as close to the black hole as the Earth is to the sun," notes study coauthor Gordon P. Garmire of Pennsylvania State University in State College. That's about 20 times the theoretical radius of the black hole's event horizon.

Because the location of the flare dramatically limits the size of the region that the massive central object can inhabit, Baganoff and his collaborators have solidified the case that the object might be an ultradense body--a black hole, asserts Fulvio Melia of the University of Arizona in Tucson.

Baganoff's team says it's unlikely that an X-ray-emitting binary star, for example, could be the source of the flare rather than material falling into the black hole. Observations of the radio-emitting source Sagittarius A*, which lies at the galactic center and is considered to mark the location of the black hole, bolster that assertion. Just before the X-ray flare erupted, Sagittarius A* increased its intensity. The team suggests that the timing of the flare and the upswing in radio emissions indicate that these observations are linked. Thus, the flare probably represents black hole activity.

"The combination of stellar [motions], radio observations, and now the X-ray data makes the galactic center the best and most compelling case that such massive black holes do exist," says Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany.

Chandra's sharp optics picked out the relatively dim flare, which could have been dwarfed by other X-ray activity in our galaxy's crowded core. The flare's dimness indicates that the black hole isn't dining voraciously. A shock wave from a supernova that exploded near the galactic center about 10,000 years ago may have swept away most of the material that the black hole would otherwise have gobbled, suggests Baganoff.

Even if the black hole isn't eating much, the extremely low level of radiation from the region just outside its event horizon is still puzzling. Baganoff speculates that material may be falling straight into the beast. For many other black holes, material would orbit in a disk before falling in. The more direct route of infall would yield much less radiation.