The analysis consisted in building, brick by theoretical brick, a model of MCG-6-30-15 that would explain the data they got from the real galaxy. The researchers started with a model that included only continuum X-rays from the corona; they found that it produced too many X-rays at low energies and not enough at high ones. They added a cloud of warm haze a few light-years from the black hole to absorb some of those low-energy X-rays. (That haze really seems to exist; it’s what makes MCG-6-30-15 look dull in visible light.) Finally they added a fluorescent iron line, amazingly bright and red-shifted so strongly that it had to be coming from iron atoms streaking just over the event horizon at near-light speed. Bingo.
“It was almost like a blazing ring right around the black hole,” Reynolds says.
For the iron atoms to get that bright so close to the black hole in MCG-6-30-15, the hole has to be rotating rapidly. By dragging space-time around with it, a rotating hole allows gas to orbit closer to the event horizon without falling in. And if the iron atoms are fluorescing that brightly, it means something is wrong with the standard model of black-hole accretion disks. In that view the disk is lit up only by gravitational energy, which is converted into heat and light by means of friction. But it’s hard to generate blazing rings that way. “The gravitational energy is released gradually, so the glowing region of the accretion disk is fairly extended,” Reynolds says.
“There is no way you can produce more energy, say, by throwing the stuff down the black hole faster,” Wilms adds. “You really need some other mechanism.”
“If what we’re seeing is what we think we’re seeing,” Begelman says, “then it’s very significant.”
The new mechanism for getting energy out of a black hole is not actually new. Roger Blandford and Roman Znajek of Cambridge University proposed it in 1977. And the reason you can get energy out of a black hole, that swallower of all things, is that the energy you detect never really got into the black hole to begin with—it’s associated with the space-time whirlpool created outside the event horizon by the black hole’s rotation.
Magnetic fields, Blandford and Znajek realized, could convert that rotational energy to electricity. The accretion disk is made of charged particles, and when the particles move, they generate a magnetic field. From then on, the field lines and the gas tend to stick together and move together. When the gas plunges into the black hole, it follows the magnetic field lines. In Blandford and Znajek’s theory, these lines protrude from the event horizon like quills from a porcupine. Passing first through the space-time whirlpool, they continue far beyond it into quieter realms. The whirlpool keeps whipping these magnetic field lines around.
Magnetic field lines protrude from the event horizon like quills from a porcupine.
It was Michael Faraday who discovered what happens when a magnetic field moves through an electrical conductor, or vice versa—though he certainly didn’t have the ionized gas of an accretion disk in mind. “Faraday said changing magnetic flux generates an electromotive force—a voltage, if you like,” says Blandford, who is now at Stanford University. “That’s the basis of simple generators. It’s the same thing here. We’ve got a black hole that’s spinning, so it’s moving magnetic fields around it, and that creates voltages. This time, though, the voltages can be prodigiously large.” In theory the voltage difference between the black hole’s poles and its equator can be billions of trillions of volts.
You can think of the magnetic field lines as wires in a titanic electric circuit, with the black hole as the generator, or you can think of them as elastic bands that literally fling electrically charged particles into distant space as they themselves are whipped around by the rotating black hole. The black hole acts like a flywheel. As matter falls into it and increases its spin, it stores energy; it releases energy again and slows down a bit as the magnetic field lines accelerate charged particles. “What may happen is that you twist up the field lines by a certain amount, and then they snap back,” Begelman speculates. “Then you twist them up again, and they snap back again. This would happen in an unsteady and somewhat unpredictable way, and as a result you would extract the energy in fits and starts.”
That sort of pulsing certainly goes on in cosmic jets, which are what Blandford and Znajek invented their theory to explain. Jets are narrow streams of gas that emerge from the cores of some galaxies, travel at more than 99 percent the speed of light, and penetrate as much as several million light-years into intergalactic space before fanning out into broad, luminous lobes. How might a black-hole whirlpool generate such a pair of waterspouts? Swirling bundles of magnetic field lines, flinging particles outward from the poles of the hole, provide a natural explanation. It would be nice, though, to have some direct observational evidence for the theory. Blandford has been waiting more than a quarter century for that.
MCG-6-30-15, unfortunately, has no jets. For an active galaxy it is relatively quiet. But it does seem to have that blazing ring right around the black hole—and at the moment, say Wilms and his colleagues, the most plausible source of that light is some type of electromagnetic generator powered by the rotation of the black hole. Though the details of the mechanism have yet to be hashed out, a lot of people are now motivated to work on it. “The theorists have been talking about this kind of process for years,” Reynolds says. “But until now there’s never really been an observation you could point to and say, ‘We think we have hard facts.’”
How hard are those facts? There is no doubt that the observation Wilms and his colleagues made was hard in another sense—“at the limits of our current technology,” as Begelman puts it. The only easily recognizable thing on their Figure 1 is the little spike at the summit of the spectrum: That’s the iron line, right where it should be, at around 6.4 kilo-electron volts. But it’s the unshifted iron line, made by slow-moving iron atoms far from the black hole. The broad iron line, the feature they were looking for, is so broadened that it is almost horizontal, an extra stratum laid over the continuum X-rays from the corona. So it is hardly rude to be skeptical. “It’s very tricky telling what’s the feature and what’s the continuum,” says Julian Krolik of Johns Hopkins University, one of the theorists now trying to figure out how magnetic fields could convert a black hole’s spin energy into light. “We’re all a little anxious about this.”
More data may dispel the anxiety. Fabian’s team has observed MCG-6-30-15 again with XMM-Newton—watching it for three times as long as the Wilms team did—during which time it got twice as bright. They, too, found a broad iron line. And Fabian and fellow astrophysicist Jon Miller of the University of Michigan recorded an uncannily similar spectrum from a stellar-mass black hole in our galaxy. “It looks just the same as MCG-6-30-15,” Fabian says.
To get a clearer view of the iron-line emission from close to the black hole, both European and U.S. teams are proposing a next-generation super X-ray telescope. Should one or both proposals be approved, a new telescope could be launched as early as 2018.
Perhaps the remarkable thing is that there should be any observational evidence at all for so outlandish a phenomenon. “We’re testing some of the most exotic predictions of the theory of black holes,” Begelman says. “Even beyond the idea that they themselves can exist—the idea that a black hole can actually grab on to space and twist it around, forcing everything in the vicinity to spin.” Einstein himself couldn’t accept the first idea, even as a matter of theoretical principle; now scientists are on the verge of actually measuring the second one. Which doesn’t mean they find it any easier than the rest of us to imagine a space-time whirlpool.
“I can do the math, and it pops out,” Wilms says. “But I always have big problems imagining it.”
