A new high-resolution study of the hot, charged gas spouting from an enormous black hole provides the most direct evidence yet that such plasma jets are powered by corkscrew-shaped magnetic fields. Researchers say the finding helps clarify the inner workings of blazars, extremely energetic galaxies that flare up unpredictably, driven by central black holes millions of times more massive than the sun.
Researchers believe that large galaxies such as the Milky Way contain supermassive black holes in their cores that drag dust and gas toward them in a disk and fling it back out via jets of ionized gas or plasma moving at up to 99.9 percent of the speed of light. If that jet points toward Earth, researchers call it ablazar, and it is "one of the most impressive high-energy natural laboratories" in the universe, says astronomer Alan Marscher of Boston University's Institute for Astrophysical Research.
Among the consequences of these near light-speed or relativistic jets are flashlightlike beams of high-energy x-rays and gamma rays as well as the illusion of superluminal (faster-than-light) speeds when viewed straight on [click below to listen to "Superluminal Lover," a "gamma-rated" (not X-rated) ode to plasma jets performed by Marscher].
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The leading model holds that blazars result from magnetic field lines—think of arcs formed by iron filings near a magnet—poking out from the accretion disk. If the black hole twisted the field lines together like a handful of twist ties, they would store energy similar to that of a twisted spring, Marscher says. Occasionally, material from the disk would get sucked into the magnetic tangle [see animation below].
To figure out exactly what's going on, researchers need to study plasma jets close to the black hole. In their new study, Marscher and his colleagues used the Very Long Baseline Array—a far-flung network of radio telescopes controlled from Socorro, N.M.—to probe deep into the core of BL Lacertae, a blazar 950 million light-years away. They captured a series of radio images revealing a "knot" of plasma shooting out along the blazar's single visible jet during a flare-up late in 2005.
BL Lacertae flared twice during the knot's migration—once as it was building speed and once when it passed through what researchers believe was a standing shock wave farther out in the jet ["X" shape depicted below]. Theoretical models predict that a twisted magnetic field near the black hole would accelerate outgoing plasma along a trajectory shaped like a stretched-out Slinky.
True to the model, radiation from the blazar cycled in its spatial orientation (polarization) during the first flare-up, indicating that the plasma in the knot moved circularly. "This is what the jet-formation theorists predicted," Marscher says. "Our observations lift the veil a bit on the mystery of jet formation and, more definitively, tell us where in the jet the outbursts occur."
If they hold up, the results are "very intriguing," says astrophysicist John Hawley of the University of Virginia in Charlottesville, who specializes in simulations of accretion disks around black holes. Simulations have shown that plasma jets happen naturally in corkscrew-shaped magnetic fields, he says [see example below: cutaway view of a jet (orange) close to an accreting black hole].
Hawley says that true verification will only come when three-dimensional modeling can confirm that the corkscrew shape does not unwind due to some unexpected effect, adding that if astronomers keep mining for data, "hopefully we'll meet somewhere in the middle."