The Hubble Space Telescope (HST) may have, for the first time, provided direct evidence for the existence of black holes by observing the disappearance of matter as it falls through an "event horizon".

An event horizon is the region surrounding a black hole from inside which no light can escape.

Astronomers have seen pulses of ultraviolet light from clumps of hot gas fade and vanish 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.

'Pulse train'

Previous observations with orbiting X-ray telescopes have provided evidence for an event horizon by seeing black hole candidates absorb nearly a hundred times as much energy as they radiate. The observations imply that trillion-degree gas is falling over the brink of an event horizon.

But until Hubble's observations, no-one has ever seen what actually happens to matter as it is pulled into the event horizon.

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

Hubble measured chaotic fluctuations in ultraviolet light from gas trapped in orbit around the black hole. It detected two examples of a so-called "dying pulse train", the rapidly decaying flashes of light from clumps of hot gas falling into the black hole.

Consistent results

Dr Joseph Dolan, of Nasa's Goddard Space Flight Center, 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 clumps of matter forever disappeared from view.

But Dr Dolan cautions that seeing only two infall events means there is a chance that the signature could simply be a statistical fluke.

However, he emphasises the results are consistent with what astronomers would expect to see if matter were really falling into a black hole.

The research was presented to a meeting of the American Astronomical Society in San Diego, California, US.