Page last updated at 10:00 GMT, Thursday, 18 September 2008 11:00 UK

X-ray pulse seen in biggest holes

Black hole (Aurore Simonnet, Sonoma State University)
Gas falling on to the hole is heated, emitting X-rays in pulses

Scientists have found what they describe as a missing link between the behaviour of the smallest and the biggest black holes.

Star-sized black holes often pulse X-rays as they pull gas into themselves and tear it apart.

Durham University researchers say they have now witnessed this same pulsing signature in the gargantuan black holes that reside at the cores of galaxies.

The team reports its observations in the journal Nature.

The researchers believe the phenomenon could be used to measure the mass of far-distant super-massive black holes.

Their observations were made with Europe's Newton-XMM space telescope.

Question of gravity

Black holes are incredibly dense points of matter, where an intense gravitational field prevents even light from escaping.

The scientists saw the strong X-ray pulse emanating from the giant black hole at the centre of the REJ1034+396 galaxy, which lies some 500 million light-years from Earth.

X-ray pulses are common among smaller black holes - the type that result when a big star collapses in on itself in a supernova explosion.

The pulses are seen, in particular, in so-called binary systems where a black hole pulls gas off a companion star into a disc of matter that then sweeps around and into the hole.

Binary system (Nasa/Esa)
In a binary system, a black hole feeds off a companion star

"We've known about the X-rays coming from black holes for a long time and we've known that this emission varies - that it goes up and down in strength over time," explained Professor Martin Ward from Durham University.

"But this is a periodic type of variability, like a tuning fork; that's the interesting thing."

These quasi-periodic oscillations occur on timescales of just fractions of seconds for the smallest black holes but on the order of an hour for the huge hole at the centre of REJ1034+396.

The scientists believe that as gas swirls around the hole, instabilities build up in the disc. These become apparent in variable X-ray emissions as the clumps of gas get super-heated.

Time signal

The Durham research is said to be the first definitive observation of quasi-periodic oscillations at a super-massive black hole.

Most galaxies are believed to contain such objects at their centres. Our own Milky Way has just such a hole at its core, sited in the southern sky in the Sagittarius constellation some 26,000 light-years from Earth. It has the mass of nearly four million suns.

Scientists have been able to work out the hole's size by studying the orbits of the stars that sweep around its gravity field, some of them pulled along at several thousand kilometres per second.

But for far-distant super-massive black holes that are feeding on discs of gas, studying their X-ray behaviour may also provide information about their mass.

"These periodic oscillations tell us a little bit about how the disc works, how the material is going in," said Professor Ward.

"In the binary stars, they happen in fractions of a second, whereas the one we've found takes an hour. And that's one of the ways we hope we can measure the mass, by scaling up from the ones we know."

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