Astronomers have conclusively measured the spin of a black hole for the first time by detecting the mind-bending relativistic effects that warp space-time at the very edge of its event horizon -- the point of no return, beyond which even light cannot escape.

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.

ANALYSIS: NuSTAR Spies Ravenous Black Holes

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.”

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“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.

This chart illustrates the basic model for determining the spin rates of black holes. The three artist's concepts represent the different types of spin: retrograde rotation, where the disk of matter falling onto the hole, called an accretion disk, moves in the opposite direction of the black hole; no spin; and prograde rotation, where the disk spins in the same direction as the black hole.NASA/JPL-Caltech

Astrophysical Controversy

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.

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“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.”

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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, although previous X-ray observatories have detected this iron line broadening for low-energy X-rays, NuSTAR's data of high-energy X-rays conclusively shows that the broadening is caused by relativistic effects, thereby proving it's a measure of the black hole’s spin. 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.

This chart depicts the electromagnetic spectrum, highlighting the X-ray portion. NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton telescope complement each other by seeing different colors of X-ray light.NASA/JPL-Caltech

Extreme general relativity to one side, what does this mean for black hole studies?

“Where did the black hole get its spin? It could have some spin when it was born, but most of it, particularly at these fast rates, must be accumulated as the black hole grows,” said Harrison. “It can accumulate because the accretion disk -- as the matter swirls onto the black hole it can add spin. And also the process by which black holes merge – when two galaxies merge, their black holes also merge – this can add, or subtract, spin from the black hole.

“If we can measure the spins of a large number of black holes, we can begin to say things very concrete about how they grew.”

This research isn’t restricted to NGC 1365, however.

“(NGC 1365’s) properties are pretty "normal", so we expect to find similar line broadening in other (maybe most) supermassive black holes,” said Risaliti.

“Actually, it is even better than this: we already have measurement of line broadening for many (~20-30) supermassive black holes. Until now however we could not be sure about the uniqueness of the interpretation. So our result for one black hole also validates previous studies on many others.”

Knowing how black holes grow and evolve is critical to the understanding of galactic development. AGNs (driven by the engine of a ravenous black hole) can dramatically impact star formation in a galaxy. The more we know about black hole spin rates, the more we may be able to understand about the life cycle of an entire galaxy.

We may even be able to use these data to do some galactic archaeology and ask whether a given galaxy was actually the result of a galactic merger.

“What excites me is the fact that we are able to do this for the very massive black holes at the centers of galaxies but we can also make the same measurement for black holes in our galaxy ... black holes that resulted from the explosion of a star ... The fact we can extend this from billions of solar masses to 10 solar masses is pretty cool,” Harrison concluded.