Spotting a Black Hole's Point of No Return

TORONTO--Astronomers probing x-ray emissions from distant objects appear to have caught the first glimpse of matter plummeting into a black hole.

The key to the discovery, presented here today at the American Astronomical Society's annual meeting, was understanding that lower levels of energy from a certain class of objects represented experimental evidence of a characteristic of black holes predicted by Einstein's relativity theory. That characteristic is an "event horizon," the point at which light or matter cannot escape a black hole's massive gravitational field.

A team at the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, examined data from Japan's ASCA satellite on nine x-ray novae, objects that intermittently flare up and release greater quantities of x-rays. At the heart of each x-ray nova is presumed to be either a neutron star--superdense matter weighing as much as several suns, but crunched into a sphere roughly 10 kilometers across--or a black hole, matter in its ultimate compressed state. X-ray novae get energy for their bursts from matter ripped from the outer layers of an orbiting stellar companion, a bloated star called a red giant.

The infalling matter sheds energy on both types of x-ray novae. When a neutron star is gobbling up the material, all the energy is radiated, either as it orbits the star or when it hits the surface. But when a black hole lies at the core of an x-ray nova, as much as 99% of the energy disappears beyond the event horizon. The CfA team noted this difference after comparing the energy from the nine x-ray novae. "We think we are seeing, for the first time, direct evidence that the event horizon really exists," says Ramesh Narayan, who made the findings along with Jeffrey McClintock and Michael Garcia.

Other experts are also impressed. "It's terrific stuff, the clearest case by far," says Yale University astronomer Charles Bailyn. "This brings science a little closer to science fiction--it moves some very exotic behavior into observational astronomy," he adds.

The CfA team bases its interpretation on a model of x-ray novae developed by Narayan and Insu Yi of the Institute for Advanced Study in Princeton, New Jersey. The advection-dominated accretion flow model says that during times when infall is low enough, the material is too tenuous to shed its energy efficiently and becomes heated to a trillion kelvins. Such sizzling temperatures cause the infalling matter to form a huge, spherical cloud around the object before making a final plunge. The team was able to measure this heat after it was radiated from neutron stars, but did not observe it coming from objects believed to be black holes.

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