NASA's Hubble Space Telescope may have, for the first time, provided direct evidence for the existence of black holes by observing the disappearance of matter as it falls beyond the "event horizon."

Joseph F. Dolan, of NASA's Goddard Space Flight Center in Greenbelt, MD, observed pulses of ultraviolet light from clumps of hot gas fade and then disappear as they swirled around a massive, compact object called Cygnus XR-1. This activity is just as would have been expected if the hot gas had fallen into a black hole.

Illustration of piece of matter falling into black hole Cygnus XR-1. Photo: Ann Feild (STScI)

"We are trying to establish the existence of black holes by obtaining observational evidence that rules out more exotic things, just as previous observations of black hole candidates have ruled out less exotic things," says Dolan, who is presenting his findings today at the American Astronomical Society meeting in San Diego, CA.

An event horizon is the mysterious region surrounding a black hole that forever traps light and matter straying nearby. By definition, no astronomical object other than a black hole can possess an event horizon.

Black holes have been inferred by observing the furious whirlpool motion of trapped gas and estimating how much mass is crammed into the tiny region of space the black hole occupies.

Also, previous X-ray observations have offered evidence for an event horizon by surveying black hole candidates that seem to be swallowing nearly a hundred times as much energy as they radiate. Those results imply that trillion-degree gas is falling over the brink of an event horizon, like water over the edge of a waterfall.

But no one has ever seen what actually happens to a piece of matter swirling into the event horizon, like water down a drain. The secret was tucked away in nearly decade-old Hubble data that took meticulous analysis.

Dolan cautions that his Hubble black hole observations see only two infall events. This means there is a finite chance the signature could simply be a statistical fluke that mimics the behavior of matter near a black hole. But Dolan emphasizes the results are consistent with what astronomers would expect to see if matter were really falling into a black hole.

The discovery comes from a detailed statistical analysis of a 1992 observation of one of the first black holes ever discovered, Cygnus XR-1, which lies 6,000 light-years from Earth in the summer constellation Cygnus the Swan.

Artist's concept of matter falling into a black hole. Photo: Greg Bacon (STScI)

Hubble didn't see the event horizon -- it is too small and too far away -- but instead measured chaotic fluctuations in ultraviolet light from seething gas trapped in orbit around the black hole. Hubble found two examples of a so-called "dying pulse train," the rapidly decaying, precisely sequential flashes of light from a hot blob of gas spiraling into the black hole.

This signature matches theories of what scientists would predict to see when matter is falling so close to the event horizon that its light rapidly dims as it is stretched by gravity to ever-longer wavelengths. Without an event horizon, the blob of gas would have brightened as it crashed onto the surface of the accreting body. Instead, the gas crossed over into a twilight-zone realm when time and space no longer have any practical meaning. Because of the gravitational stretching of light (an effect called redshift), the fragment disappeared from Hubble's view before it ever actually reached the event horizon. The pulsation of the blob - an effect caused by the black hole's intense gravity -- also shortened as it fell closer to the event horizon.

Finding the signature wasn't an easy task. Hubble's high-speed photometer (a very fast light meter) sampled light at the rate of 100,000 measurements per second, during three separate Hubble orbits, each executed in June, July, and August of 1992. The observation yielded 1 billion data-points, which, if printed out on a chart recorder, would stretch 600 miles! Hubble's ultraviolet capability gave it the ability to see the faint flicker of material within 1,000 miles of the event horizon.

Dolan "mined" the enormous database on and off for years. "Looking for the decaying pulse train was like looking for the proverbial needle-in-a haystack," he says. "Put another way, it was like listening for a specific word in a many hours-long transmission of Morse code."

He found two examples of infall events. One event had six decaying pulses; the other had seven pulses. The pulses spanned an interval of merely 0.2 seconds before the blob forever disappeared from view.

Death Spiral
Dynamical models predict that gas from Cygnus XR-1's companion star continuously falls into the black hole. The gas can't directly fall in, but instead swirls into a flattened pancake called an accretion disk. The viscosity in the accretion disk causes the gas to spiral down toward the event horizon. About 1,000 miles above the event horizon (in the case of stellar-mass black holes) the disk vanishes because gas can no longer maintain a stable orbit. This is due to the dragging of space-time by the black hole's intense gravitational field. Instead, blobs of hot gas break off from the inner rim of the disk, like icebergs off an ice shelf. The blob then spirals down to the event horizon. Because of gravitational effects on light near the black hole, the blob appears to pulsate as it makes thousands of orbits around the black hole every second. When it falls inside the accretion disk, the light quickly stretches to longer and longer wavelengths because of the distortion of space-time by the black hole's intense gravity.