Black hole spins super fast

Print edition : March 22, 2013

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THE formation of supermassive black holes is believed to give clues to the formation of a galaxy itself since a fraction of all the matter drawn into the galaxy finds its way into the black hole. Because of this, astronomers are interested in measuring the spin rates of black holes.

A team of scientists has made a definitive measurement of the spin rate of a supermassive black hole for the first time. The two-million-solar-mass black hole, lying at the centre of the spiral galaxy NGC 1365, is spinning so fast that its surface is travelling at nearly the speed of light, close to the maximal rate allowed by Einstein’s theory of relativity.

This is the first time that anyone has accurately measured the spin of a supermassive black hole. The findings, made by the two X-ray space observatories, the National Aeronautics and Space Administration’s (NASA) Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency’s XMM-Newton, solve a long-standing debate about similar measurements in other black holes and will lead to a better understanding of how black holes and galaxies evolve.

The observations also are a powerful test of Einstein’s theory of general relativity, which holds that gravity can bend light and space-time. “We can trace matter as it swirls into a black hole using X-rays emitted from regions very close to the black hole,” said Fiona Harrison, NuSTAR principal investigator at the California Institute of Technology, Pasadena, and co-author of the new study, appearing in the February 28 issue of Nature. “The radiation we see is warped and distorted by the motions of particles, and by the black hole’s incredibly strong gravity.” NuSTAR is designed to detect the highest-energy X-ray light in great detail. The team used NuSTAR to observe X-rays emitted by hot gas in a disc just outside the “event horizon”, the boundary surrounding a black hole beyond which nothing, including light, can escape.

Previous measurements were uncertain because obscuring clouds around the black holes could, in theory, have been confusing the results. By working together with XMM-Newton, NuSTAR was able to see a broader range of X-ray energy, penetrating deeper into the region around the black hole. The new observations ruled out the idea of obscuring clouds, demonstrating that spin rates of supermassive black holes can be determined conclusively.

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