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 Caltech. "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. "Probably
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. 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 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. The details of the mechanism have yet to be hashed out—and a lot of people are now motivated to work on it. "The theorists have been talking about this kind of process for years," says Reynolds. "But until now there's never really been an observation you can 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 soon dispel the anxiety. Fabian's team has recently
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 last fall Fabian and fellow
astrophysicist Jon Miller of the Massachusetts Institute of Technology
recorded an uncannily similar spectrum from a stellar-mass black hole
in our galaxy. "It looks just the same as MCG-6-30-5," says Fabian.
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," says Begelman. "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," says Wilms. "But I always have big problems imagining it."
Photographs: top to bottom,
courtesy of STSCI/NASA;
courtesy of European Space Agency.
So why have two big observatories? "They are actually quite complementary," says Steve Snowden, an astronomer with the XMM-Newton Guest Observer Facility at NASA's Goddard Space Flight Center in Greenbelt, Maryland. Chandra's four sets of X-ray-reflecting mirrors are smoother and more accurately shaped and aligned than are XMM-Newton's 174 nested mirrors. "It has much clearer vision. It is able to resolve finer objects in the sky," Snowden says, and pick out details in complex structures like supernova remnants and star clusters. But the large surface area of XMM-Newton's mirrors takes in five times the number of X rays and views a larger patch of the sky, "so it is very useful for looking at large structures," adds Snowden.
— Kathy A. Svitil
Check out a tutorial on black holes: www.howstuffworks.com/black-hole.htm.
