Some black holes actively produce X-rays, while others are relatively quiet. A new study may help explain why. Our galaxy contains many black holes that weigh the equivalent of a few Suns. They feed off the gas from companion stars, which swirls into a flat disk before disappearing into the black hole. Just before it goes in, though, the gas is sped up to a significant fraction of the speed of light.
The gas becomes so hot that it glows with X-rays. In similar systems, a neutron star replaces the black hole. Neutron stars are made entirely of neutrons and are also very dense, though not as dense as a black hole.
Some of these binary systems, as they are called, 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 of Southampton University led a team of researchers who looked at the visible light from the gas disc of a quiescent black-hole and binary star system called 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, the researchers figure.
"We have yet to observe visible and X-ray flares simultaneously," Hynes said. "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 other observations with the Gemini South Telescope, the team found even more rapid variations in flaring in other systems. They saw the visible brightness of one system increase by 25 percent in about 60 seconds.
"These are the most rapid variations of these faint, quiescent black holes that anyone has found so far," Hynes said. "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."
The study was presented earlier this month at the UK the National Astronomy Meeting.
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