Brilliant UV light blazes from the
active NGC 6251
in this Hubble image. A black hole
may lurk at its heart, fueling superhot
gas that throws off intense light.
Hubble/NASA
What happens to the space-time fabric that is being dragged into such a hole and is thus being spun around too? Does it get into such a fierce and gnarly twist that it rips, spilling stars, planets, and general-relativity theorists out of the universe and into the cold nonexistence of hyperspace? Probably not. “I think you’ve reached the limits of the fabric analogy,” Begelman says. Space-time is not really a fabric, he explains; it’s a mathematical description of the possible motions of matter and energy. A black hole drags all the possibilities inward. A spinning black hole first drags them around with it for a while. Near the event horizon the drag is so strong that nothing can resist it. The only possible motion is to spin along with the black hole.
The mind reels, begs for a metaphor, a lifeline to a more familiar experience. “If you were to hover just outside the horizon but still under the influence of this twisting of space,” Begelman says, “it would be as if you were riding on—let me think of another analogy—I would say it might be like a whirlpool.”
Like a whirlpool but also like a flywheel, because the tremendous energy stored in that twirling patch of cosmos might actually be extractable.
Images of space-time whirlpools did not immediately pop up on Jörn Wilms’s computer screen when he received his data from MCG-6-30-15. The data came from a satellite telescope called XMM-Newton, operated by the European Space Agency. (X-rays from space don’t penetrate Earth’s atmosphere, so they must be collected in space.) XMM-Newton is on a distended orbit that takes it one-third of the way to the moon; this keeps it out of Earth’s shadow long enough to stay pointed at—and collecting photons from—the same faint object for more than a day.
On June 11, 2000, X-ray photons that had left MCG-6-30-15 during the early Cretaceous period 130 million years ago poured through the open hatch at one end of XMM-Newton. They glanced off gold mirrors, which focused the photons onto a silicon wafer at the other end, 25 feet away. This electronic detector recorded each photon individually. What Wilms received, back in his office in Tübingen, was a long list of several million individual photons posted on a Web site. It included the energy and arrival time of each one. Data do not get much rawer than that.
You don’t see anything in such data without a theory of what you’re looking at and looking for. MCG-6-30-15, the whole galaxy, is not much more than a point in the sky. Yet from previous observations of its spectrum, combined with lots of theoretical calculations, astronomers have sketched a picture of the intense activity in its nucleus. The central black hole, they believe, is girdled by a thin disk of gas that is spiraling inward toward doom. Most of this accretion disk is relatively cool, “which means its temperature is in the millions of degrees,” Wilms says. At that temperature it glows mostly blue and ultraviolet.
The X-rays must come from hotter stuff. The theory says they come from a tenuous, roiling foam of electrons and protons, called the corona, that splashes up from the disk near its center. As blue and ultraviolet photons stream through this billion-degree foam, they ricochet off its high-speed particles and are thereby boosted to X-ray energies—the whole band of X-ray energies, or what astronomers call a continuum spectrum. To be at a billion degrees, the corona must be so close to the black hole that the infalling gas has already converted most of its gravitational energy to heat. And because the corona is small, the X-ray emissions from MCG-6-30-15 can change fast. “We’ve seen its brightness double in 100 seconds,” Andrew Fabian says. “If you looked at it through an X-ray telescope, you would say, ‘Wow!’”
More than a decade ago, Fabian and his colleagues discovered a way of seeing into this shimmering cloud almost to the edge of its black heart. It was exactly this strange, subtle feature of the X-ray spectrum that the Wilms group went looking for. Some of the X-rays from the corona, the Cambridge researchers realized, would shine back onto the accretion disk and excite iron atoms there. And some of those iron atoms would thereupon fluoresce, emitting X-rays of their own—not over the whole band this time but at a single precise line in the energy spectrum: 6.4 kilo-electron volts, which is the energy an electron loses when it falls from one shell in an iron atom to a lower one.
An emission line like that in the hands of an astronomer is like a radar gun in the hands of a cop: It reveals how fast the X-ray-emitting iron atoms are traveling. Because the iron atoms in MCG-6-30-15 are moving, astronomers don’t see the line right at 6.4 kilo-electron volts. Instead the X-rays are Doppler-shifted, like a radar beam bouncing off a speeding car (the radar waves hit the gun more often if the car is moving toward the gun and less often if the car is moving away from the gun). The X-rays are thus shifted toward the blue side of the spectrum, or “blue-shifted” to higher energies, and also intensified on the side of the accretion disk that is moving toward Earth. Likewise, they are “red-shifted” to lower energies on the side that is moving away. When astronomers record a single spectrum for the whole galaxy, the iron line is smeared in both directions by this Doppler effect. At the same time, it is also gravitationally red-shifted because some of the iron atoms are very close to the black hole, where time itself and thus all light waves are stretched.
The net result is that the sharp emission line is smeared into a broad, asymmetrical hump—and the broader the hump, the faster the iron must be moving and the closer it must be to the black hole. Fabian predicted all this in 1989. In 1994, working with Japanese researchers and the Japanese X-ray satellite ASCA, he found evidence for a broad iron line in MCG-6-30-15. Wilms and his colleagues hoped for more conclusive results with the more sensitive XMM-Newton.
“And what we saw right from the beginning was that the iron line was wrong,” Wilms says. “It was much broader than what we thought it should be.” The initial excitement was followed by fretting about whether they understood their own telescope. “Almost monthly, we’d have these panic attacks where someone would raise a calibration problem,” recalls Chris Reynolds of the University of Maryland in College Park, who worked with Fabian on the earlier study and with Wilms on this one. “We would have to do the whole analysis again.”
