Wolfgang Steffen, Nature 452 (24 April 2008)

Blast off!
Twisted magnetic fields near a spinning supermassive black hole launch giant twin jets of matter and energy.

Follow That Cosmic Jet

Imagine looking up the exhaust blasting out of the biggest jet engine in the world. Enlarge that image hundreds of trillions of times, and you'll get some idea of the size and power of the cosmic jet called a blazar that's shooting out of a black hole some 950 million light-years away. For the first time, astronomers have figured out what causes these monsters and in the process have moved a step closer to observing a black hole.

Cosmic jets are the product of matter and energy trying to squeeze into a small space, drawn in by the enormous gravity of massive stars and black holes. The most powerful jets, called quasars, arise when black holes weighing as much as billions of suns fling infalling matter and energy back out into the galaxy, heating up loads of dust and gas and creating blinding beams of energy. When these jets point directly at Earth, they are called blazars, because their energy is concentrated in a relatively small patch of sky. Astronomers have known about quasars and blazars for some time but couldn't glean much information about them. For example, why is a jet a jet in the first place? And why aren't quasars just very big explosions, as supernovae are?

To find an answer, an international team has spent the past several years using a global array of radio, optical, x-ray, and gamma-ray telescopes to observe a single blazar, called BL Lac. Those observations, the team reports tomorrow in Nature, have yielded unprecedented details about such phenomena.

It turns out that blazars are powered surprisingly like the exhausts of jet engines, albeit on a cosmic scale, says astronomer and lead author Alan Marscher of Boston University. Instead of streaming out of a metal casing and turbine, a blazar's jets "are confined and focused by coiled magnetic fields originating near the [supermassive] black hole." The black hole twists hot, electrically charged gas falling in toward it just as a turbine twists streams of air. The resulting magnetic fields then confine any gas into two jets that shoot out along relatively narrow paths above and below the black hole's spin axis. Marscher says the researchers detected BL Lac's twisted magnetic fields by analyzing a "flare" of visible light, x-rays, and gamma rays from an explosion they watched shoot along the jet's path at near-light speed.

It's a "great" bit of research, says astrophysicist Chris Willott of the University of Ottawa in Canada. "It's remarkable that black holes, which most people think of as relentlessly sucking material in, can expel matter from close to their event horizons at nearly the speed of light," he says. Now, with this latest observation at multiple wavelengths, "we can understand how this material gets launched," Willott says, "and with similar advances in the future, we will get closer to seeing the edge of a black hole."

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