Black holes' spin measured
By Alicia Chang
There's a new spin on supermassive black holes: They're incredibly fast, astronomers say.
It's long been suspected that gigantic black holes lurking in the heart of galaxies rotate faster and grow larger as they feast on gas, dust, stars and matter. But there hasn't been a reliable measurement of the spin rate of a black hole until now.
While black holes are difficult to detect, the region around them gives off telltale X-rays. Using NASA's newly launched NuStar telescope and the European Space Agency's workhorse XMM-Newton, an international team observed high-energy X-rays released by a supermassive black hole in the middle of a nearby galaxy.
They calculated its spin at close to the speed of light – 1078 million km/h.
This is the first "unambiguous measurement of the spin rate" of a supermassive black hole, University of Maryland astronomer Christopher Reynolds, who had no role in the research, wrote in an accompanying editorial.
Behemoth black holes - with masses millions to billions times that of the sun - are thought to reside in every galactic center. They're extremely dense and possess such powerful gravitational tug that not even light can escape.
Scientists are able to pinpoint these monstrous objects from the streams of X-rays emitted during a feeding frenzy. Knowing how fast - or slow - supermassive black holes swirl can help shed light on their growth.
For several days last summer, the two telescopes simultaneously tracked an immense black hole in a spiral galaxy called NGC 1365. The galaxy was chosen because it was 60 million light years away - relatively close by astronomical standards.
Results were published in Thursday's issue of the journal Nature.
So how fast is the black hole at the centre of our Milky Way spinning?
It's tough to know because our galaxy's supermassive black hole isn't as active as the observed one, said lead researcher Guido Risaliti of Italy's Arcetri Astrophysical Observatory.
Aside from occasional flares, hardly any radiation flows from our black hole, making it difficult to calculate its spin, Risaliti said.
Maryland's Reynolds said it's clear that some supermassive black holes rotate very rapidly and there's a need for more powerful X-ray space telescopes.
"We are learning about some of the most exotic and powerful objects in the universe," he said in an email. "This is cool science."