"To shine through these thick clouds, the black hole would have to be so bright it would basically blow itself apart," said Harrison, who's principal investigator for the NuSTAR mission. "So what has to be happening is, what we're seeing is these relativistic distortions. And that means that the disk is coming close to the black hole, which means the black hole must be spinning rapidly."
The research team, led by Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics and the Italian National Institute for Astrophysics' Arcetri Observatory, calculated this rotation rate to be 84 percent of that allowed by general relativity.
It's tough to comprehend this figure, since it doesn't translate well into miles per hour. But it's safe to say that the black hole is spinning incredibly fast.
"The analogy of an actual velocity is not quite right," Harrison said. "But what you can say is that spinning black holes twist space-time around them. And if you were standing near the black hole, basically your space-time would be twisted, or dragged, around such that you would have to rotate once every four minutes just to be standing still."
The new study was published online today (Feb. 27) in the journal Nature.
Learning about black hole growth
Astronomers think supermassive black holes acquire most of their spin as they grow, rather than being born with it. So studying their rotation rates can yield insights into how these monsters have evolved over time.
The superfast spin of NGC 1365's black hole, for example, implies that it did not grow via numerous small black-hole mergers, Harrison said, since the odds are very low that many such chaotic events would spin it up in the same direction.
Rather, it's more likely that NGC 1365's central black hole acquired its spin from one major merger, or simply by gobbling material from an accretion disk that has remained stable over the long haul.
The new study represents a first step toward a better understanding of the nature and evolution of supermassive black holes, Harrison said.
"We will make more measurements like this," she said. "Eventually what you'd like to do is have a bigger telescope that can actually measure more distant black holes so we can, using the statistics of the sample, understand how they grow over cosmic time."
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