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.
"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.
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.
