23 November 2006

Black hole spins at the limit

Cosmos Online
A black hole close to our Solar System spins at close to the maximum theoretical speed and has far-reaching implications for high-energy events in our universe, according to new U.S. study.
Black hole spins at the limit

Artist's impression of a swirling disc of gas orbiting a black hole. Credit: David A. Aguilar/CfA

SYDNEY: A black hole close to our Solar System spins at close to the maximum theoretical speed and has far-reaching implications for high-energy events in our universe, according to new U.S. study.

“We have measured the spin frequency of a famous black hole which goes by the name of GRS 1915+105,” said Ramesh Narayan from the Harvard-Smithsonian Centre for Astrophysics in the U.S. “According to theory, it can have a maximum spin rate of 1,150 times per second. Our measurements indicate that the hole is spinning between 950 and 1,150 times per second – that is, it is spinning quite close to the maximum.”

When any mass, such as a star, becomes more compact than a certain limit, its own gravity becomes so strong that the object collapses to a singular point called a black hole.

A black hole has so few properties that it is completely described by only two numbers: its mass and its spin. Astronomers have been successful in measuring the mass of about 30 black holes, but the second fundamental property has been much more elusive.

“It has been a major goal of the field to estimate black hole spins but nobody had succeeded until now,” said Narayan, who is co-author of the paper published in the Astrophysical Journal.”Our work is, for the first time, beginning to provide information on the spin distribution of real black holes in nature.”

GRS 1915+105 lies in our galaxy, the Milky Way, approximately 35,000 light-years from Earth. Gas from a nearby orbital star gets pulled off by the immense gravitational field and spirals onto the black hole, forming a disc of gas around it.

As the gas spirals in towards the black hole, getting closer to the centre, it heats up to millions of degrees. At a certain distance from the centre, the gas becomes so hot that it begins to emit X-rays with only slightly less energy than gamma rays. The specific types of X-rays produced depend on how close the gas is to the centre.

According to general relativity, inside a certain radius the gas falls into the black hole much too quickly to radiate any energy. This critical radius depends on the black hole spin, so measuring the radius provides a direct estimate of the spin. The researchers measured the radius by analysing X-rays emitted using NASA’s Rossi X-ray Timing Explorer satellite data.

“This result has major implications for … modelling possible sources of gamma-ray bursts, and for the detection of gravitational waves,” Narayan said.

Gamma-ray bursts are short, powerful explosions, thought to be produced when the core of a star collapses into a black hole. For gamma-ray bursts to occur, a star would have to be spinning incredibly fast – fast enough for the remaining stellar materials to form a disc around the black hole.

Many astronomers doubted that these spin rates could be reached. This study confirms, for the first time, that these spin-rates are in fact possible. Accurate, measurable spin-rates of black holes are also crucial for the detection of gravitational waves.

Predicted by general relativity, gravitational waves are caused by the movement of massive bodies that disturb the space-time surrounding them, causing ripples of the waves to radiate outwards. Plans for a space-based antenna to detect these waves are being developed, and the antenna is expected to launch in 2015.

A major source of gravitational waves is the merging of two black holes, according to Narayan. The real values of spin provided by this study will significantly narrow the type of signals scientists will look for when trying to detect gravitational waves.


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