A team of scientists led by Harvard astronomer Guido Risaliti recounts its findings in the latest issue of Nature. The researchers accomplished the feat by measuring electromagnetic radiation emanating from the center of spiral galaxy NGC 1365. There — not unlike the center of our own Milky Way — a spherical region of spacetime more than 2 million miles in diameter whirls violently, its gravity so strong it actually schleps surrounding space along with it. Any matter that trespasses beyond the black hole’s event horizon spirals inward and collects in what’s known as an accretion disc, where it is subjected to so much friction it emits X-rays.
Thanks to a joint effort by the ESA’s XMM-Newton and NASA’s recently launched NuSTAR (both X-ray observatories, positioned in Earth orbit), Risaliti and his colleagues were able to locate the inner boundary of the accretion disc. Sometimes known as the Innermost Stable Circular Orbit, the position of this accretion disc “edge” depends on the speed of the black hole’s overall rotation. The astronomers used this relationship to calculate the spin rate of the black hole’s surface, which they estimate is is traveling at nearly the speed of light — about 84% as fast, to be exact.
In a statement, Risaliti says that it is “the first time anyone has accurately measured the spin of a supermassive black hole,” but insists that even more important is what his team’s findings can tell us about this black hole’s past, and the developmental history of its surrounding galaxy.
The spin of a black hole is thought to be affected by the way it pulls in matter. It stands to reason, for example, that a black hole that subsumes gas and stars at random is more likely to fetter its angular momentum than add to it. According to Risaliti and his team, that the supermassive black hole at the center of NGC 1365 is spinning at speeds approaching the cosmic speed limit would suggest it acquired mass through ordered accretion, as opposed to multiple random events.
For more details, visit SPACE.com, where Mike Wall has a great overview of the role that NASA’s NuSTAR (launched in July of last year) has played in resolving a longstanding debate over the implications of X-ray emission patterns emanating from black holes.
“It’s the first time that we can really say that black holes are spinning,” said study co-author Fiona Harrison in an interview with Wall. “The promise that this holds for being able to understand how black holes grow is, I think, the major implication.”
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