— Matter can stably orbit closer to a spinning black hole, right, than a non-spinning one, left (Illustration: NASA/CXC/M Weiss)
A black hole has been found to be spinning faster than ever seen before, a new analysis suggests. The finding supports the idea that only fast-spinning stars can collapse to create powerful explosions called long gamma-ray bursts.
To measure the spin of black holes, astronomers measure the size of the discs of matter that orbit them. A spinning black hole drags space-time around with it as it spins, boosting the speed of matter in orbit around it. That allows the matter to orbit closer in without getting sucked into the black hole itself so the faster a black hole spins, the closer matter can stably orbit around it.
But the innermost edge of this disc is too small to see directly. So previous measurements of black hole spins have had to make assumptions about properties such as the tilt of the disc to Earth's line of sight.
Now, astronomers have measured the spin of a black hole with a new method that requires fewer assumptions. The team was led by Jeffrey McClintock of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US.
McClintock's team examined a black hole in our galaxy called GRS 1915+105, which lies about 36,000 light years away. Matter gets hotter as it gets closer to the black hole, so the team used X-ray observations from NASA's Rossi X-ray Timing Explorer to measure the temperature of the gas in the disc.
They found the innermost stable orbit around GRS 1915 is so close that the black hole must be spinning at nearly 1000 times per second the fastest ever recorded.
"The application of this to understanding black holes and black hole physics are really quite important," McClintock told New Scientist. "Its the most exciting thing I've worked on."
But a second study of GRS 1915 suggests that the spin could be lower, according to an analysis of the same RXTE data by Matthew Middleton of the University of Durham, UK, and his colleagues.
Chris Done, a member of Middletons team, says their analysis suggests the spin is substantial but not extreme. They argue that X-rays scattering off of electrons in the disc make the temperatures appear higher than they really are. This gives the illusion of a closer-in disc, and therefore a faster spin for the black hole, they say.
But if McClintock's team is right, the black hole is spinning at 98% of the theoretical maximum rate, which is calculated by how fast stars can spin before they collapse to form black holes.
The observation provides support for the idea that gamma-ray bursts fleeting but powerful explosions are produced by fast-spinning stars.
In this scenario, a black hole forms at the centre of such a fast-spinning star and some of the remaining stellar material forms a disc that spirals into the black hole.
The interaction of the black hole and the disc produces jets, which emit copious amounts of gamma rays. But the star has to be spinning very quickly when it collapses for this disc to form, and some astronomers have expressed doubt that stars would be spinning fast enough at this stage in their lives.
The new research may quell some of those doubts. "It says sometimes stars do find some path for dying with a huge amount of rotation in their middle," says Stanford Woosley of the University of California in Santa Cruz, US, who is not a member of the team.
Christopher Fryer of the Los Alamos National Laboratory in New Mexico, US, who is also not on the team, agrees. He says it is "strong evidence that nature can get the high spin rates in stars to produce gamma-ray bursts".
McClintock says he hopes that analysing similar observations for other systems will allow them get spin rates for half a dozen more black holes within the next two years. "We're going to apply it as widely as we can," he says.
Journal references: The Astrophysical Journal (vol 652, p 518)
Monthly Notices of the Royal Astronomical Society (DOI: 10.1111/j.1365-2966.2006.11077.x)