Their observations are helping to build up a picture of precisely where X-rays are generated in the gas as it heats up to extreme temperatures and swirls around under incredible gravitational forces before cascading into the black hole itself.

On Friday 12 April, Dr Robert Hynes of Southampton University will tell the National Astronomy Meeting in Bristol about detailed observations of flares lasting a few hours, made with the William Herschel Telescope on the island of La Palma, and even more recent observations made with the brand new Gemini South telescope in Chile of the shortest such flares ever spotted from a quiescent black hole, lasting only a matter of minutes, or less.

The best evidence astronomers have for the existence of black holes within our own galaxy comes from X-ray binary stars where a black hole or neutron star is fed gas by an ordinary star in orbit around it.

The gas becomes so hot that it glows with X-rays. Some of these binaries have a 'quiescent' state in which the X-rays they emit are more than a million times less powerful than normal.

It is believed that less gas is falling onto the black hole or neutron star at these times, but quiescent systems with black holes appear even fainter than the ones with neutron stars.

This might be because energy is disappearing past the black hole's event horizon - the point of no return beyond which energy is irretrievably lost. But to be sure, astronomers need to know more about how the dribble of gas flows onto the black hole during the quiescent period.

To investigate this, Robert Hynes, collaborating with Professor Phil Charles also at Southampton, Dr Carole Haswell of the Open University and Cristina Zurita and Dr Jorge Casares of the Instituto de Astrofisica de Canarias on Tenerife, has used the William Herschel Telescope to look at the visible light from the gas disc of a quiescent black-hole X-ray binary star (V404 Cygni).

The glow from the disc varied by a large amount - during flares lasting for a few hours, gas all around the black hole was lit up, most likely by X-rays shining on it. 'We have yet to observe visible and X-ray flares simultaneously,' says Dr Hynes, 'but if this explanation for the visible flares is correct, we can use them to pinpoint more accurately where the X-rays are coming from.'

In the most recent observations with the Gemini South Telescope, the team have found even more rapid variations. They saw the visible brightness of one system increase by 25% in about one minute. 'These are the most rapid variations of these faint, quiescent black holes that anyone has found so far,' says Dr Hynes. 'They are far from being the dormant objects we imagined, and there must still be dramatic activity going on where gas falls onto the black hole.'

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