By monitoring X-ray emissions from iron ions (iron atoms with some electrons missing) trapped in the black hole’s accretion disk, the rapidly-rotating inner edge of the disk of hot material has provided direct information about how fast the black hole is spinning.
And by doing this, a long-standing controversy surrounding black hole studies has been laid to rest.
The spinning supermassive black hole lives in the heart of the nucleus of NGC 1365, a nearby galaxy some 56 million light-years away.
X-Ray Fireworks
Accretion disks consist of any material that has drifted too close to the gravitational dominance of a black hole. Gas, dust, even stars succumb to the force inside an active galactic nucleus (AGN). Some material will feed the black hole, whereas a surplus of matter is ejected from the black hole’s poles, blasting into space as jets of material traveling close to the speed of light, generating an intense cosmic fireworks display.
AGNs can be dazzling, shining bright in X-ray radiation -- an indication that the supermassive black hole lurking inside is feeding.
Now, astronomers using data from NASA’s brand new Nuclear Spectroscopic Telescope Array (NuSTAR) -- that was launched into Earth orbit in June 2012 -- and the European observatory XMM-Newton have used this X-ray radiation as a tool to directly measure the spin of NGC 1365’s black hole.
“The accretion disk isn’t hot enough to generate X-rays itself, these X-rays generated in the jet shine down on the disk and reflect off of it, exciting the iron,” Fiona Harrison, professor of physics and astronomy at the California Institute of Technology, Pasadena, Calif., and principal investigator of the NuSTAR mission, told Discovery News. “That’s what enables us to see the accretion disk -- we’re seeing reflected X-rays off the disk.”
“We selected (NGC 1365) because it is bright in X-rays, and previous observations with less powerful satellites suggested that this could be a good candidate for such a study,” said astronomer Guido Risaliti, of the Harvard-Smithsonian Center for Astrophysics, Cambridge, Mass., and the Italian National Institute for Astrophysics, and lead author of research published today (Feb. 27) in the journal Nature.
The environment near the black hole’s event horizon is extreme; the fabric of space-time itself is being warped by the spin of the black hole, dragging the inner edge of the accretion disk with it. As the disk of material rapidly rotates -- like the vortex of a water funnel down a plughole -- it is still emitting X-rays.
The emission from this component of the accretion disk should therefore be stretched, or redshifted, providing astronomers with a means of quantifying how fast the black hole is spinning.
“We’re actually using the rotation of the disk to measure the spin of the black hole,” Harrison added.
However, until now, measurements of the X-ray emission spectrum have been limited to low energies and there were two explanations for the broadening (red-shifting) of the iron emission spectrum.
One theory was that the X-rays were being red-shifted by the extreme relativistic environment near the event horizon of a spinning black hole. The other theory was that the X-rays were being obscured by gas blocking our view of the central black hole, adding complexity to the detected X-ray signal. Through lack of convincing evidence supporting either model, an astrophysical controversy erupted.
NuSTAR, which detects more energetic X-ray emissions, has now definitively ended this controversy. The orbiting X-ray observatory has detected previously undetectable high-energy X-rays and provided conclusive evidence that NGC 1365’s black hole is spinning -- the line broadening is not therefore caused by absorption by intervening clouds of gas.
“It was my expectation, and the main scientific rationale for the project. Of course many colleagues would rather expect absorption as the right explanation ... but the whole project has been conceived to solve this puzzle,” said Risaliti.
“The interesting thing, especially in the system we looked at is that we know there’s partial absorbing clouds -- we see them going in front of the (galactic) nucleus causing time-variable absorption ... it’s not unreasonable to suppose that could be distorting the spectrum in a way that gives you broad lines,” said Harrison. “But when you add the NuSTAR data that can just be ruled out. Yes, there is absorption, but it’s not explaining the iron line.
“What they tell us is that the black hole HAS to be spinning. Now there’s a maximum rate a black hole can spin given by general relativity and that is telling us that this black hole is spinning close to that rate.”
According to Risaliti and Harrison’s team’s research, NGC 1365’s black hole is spinning at a breakneck rate 84 percent of its theoretical maximum.
Shedding Light on Black Hole Evolution
So the first detection of a black hole’s spin has been made, providing observational evidence for something that, until now, has been purely theoretical or inferred. Why is this important?
“Well, first off it’s just cool that we’re seeing the effects of general relativity in the ‘strong field’ regime. Most tests of general relativity are done in the ‘weak field,’” said Harrison, referring to the fact that most tests of general relativity are done in “weak” gravitational fields like Earth’s. NuSTAR is probing the edge of the most extreme gravitational field possible.
Read more at Discovery News
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