Astronomers have made the first reliable measurement of a supermassive black hole's spin, showcasing a technique that could help unravel the mysteries of these monsters' growth and evolution.
The enormous black hole at the center of the spiral galaxy NGC 1365 is spinning about 84 percent as fast as Einstein's general theory of relativity allows it to, researchers determined. The find demonstrates that at least some supermassive black holes are rotating rapidly — a claim previous studies had hinted at but failed to confirm.
"It's the first time that we can really say that black holes are spinning," study co-author Fiona Harrison, of Caltech in Pasadena, told SPACE.com. "The promise that this holds for being able to understand how black holes grow is, I think, the major implication."
Staring at a black hole in X-ray light
Supermassive black holes are almost incomprehensibly huge, with some containing 10 billion or more times the mass of our sun. Scientists think one lurks at the heart of most, if not all, galaxies. [Gallery: Black Holes of the Universe]
NGC 1365, located about 56 million light-years from Earth in the constellation Fornax, does indeed harbor a gigantic black hole — one as massive as several million suns. And this behemoth is blasting out enormous quantities of energy as it gobbles up gas and other nearby matter, making it an intriguing target for astronomers.
In the new study, researchers analyzed observations two X-ray space telescopes — the European Space Agency's XMM-Newton observatory and NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) — made of NGC 1365 in July 2012.
By zeroing in on high-energy light emitted by iron atoms, the telescopes were able to trace the motion of the flat, rotating accretion disk that circles NGC 1365's black hole and funnels gas and dust into its greedy maw.
Astronomers found that the emissions are strongly distorted, suggesting that the inner edge of the accretion disk may be quite close to the black hole — close enough for gravitational effects to wreak havoc with the X-rays streaming from the disk. This in turn implies a rapidly rotating black hole, since general relativity states that the faster a black hole is spinning, the closer its disk can come to it, Harrison said.
But that's just one interpretation. Another holds that such distortion, which has been observed in accretion disk emissions before, could be caused by clouds of gas that hang between a supermassive black hole and the telescopes observing it. [The Strangest Black Holes in the Universe]
"This has been a big controversy — which of the two is going on?" Harrison said.
Pinning down a black hole's spin
The $165 million NuSTAR telescope, which just launched in June 2012, finally cracked the case.
Using NuSTAR's super-sensitive measurements of high-energy X-rays, the astronomers calculated that the purported gas clouds would have to be incredibly thick to produce the observed distortion levels — so thick as to make the whole idea untenable, at least in the case of NGC 1365's black hole.