The Milky Way’s pinball wizard

By Jeanna Bryner

Staff Writer

updated 4:09 p.m. ET March 6, 2007

In a cosmic game of pinball, black holes fling high-energy protons into space, where they zigzag around at near light-speeds before smashing into low-energy protons, a new study finds.

Then the collisions send bursts of gamma rays flying out from the center of our galaxy, which explains for the first time the mechanism for the high-energy jets first spotted in 2004.

This proton-slinging could explain more than this cataclysmic light show deep in our galaxy. The scientists suggest other black holes in the universe could rely on the pinball mechanism to produce enormous jets of light.

“Our galaxy's central supermassive object has been a constant source of surprise ever since its discovery some 30 years ago,” said study team member Fulvio Melia, an astrophysicist at the University of Arizona.

“Slowly but surely it has become the best-studied and most compelling black hole in the universe,” Melia said. “Now we're even finding that its apparent quietness over much of the spectrum belies the real power it generates a mere breath above its event horizon — the point of no return.”

Rockin’ rays
In recent years, astronomers have tried to get at the secrets of this gamma-ray light show, which originates from our galactic middle in the neighborhood of a supermassive black hole called Sagittarius A* and boasting 3 million solar masses.

Like all black holes, Sagittarius A* is veiled in a whirlpool of churning spacetime, the outer border of which is called the event horizon. Nothing, not even light, can escape the black hole’s immense gravitation once it passes this perimeter, so astronomers have had a difficult time figuring out what exactly goes on around a black hole.

And also like many black holes, Sagittarius A* emits X-rays as it devours matter crossing the event horizon.

Based on years of theoretical sleuthing, Melia and his colleagues have suggested that chaotic magnetic fields near this event horizon accelerate protons and other particles to high energies.

Play pinball

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To put the theory to an Earthly test, Melia and David Ballantyne, also of the University of Arizona, created a computer model that tracked the trajectories of more than 200,000 protons floating freely in a superheated gas called plasma.

The model found that gravity from Sagittarius A* hurls protons from the magnetized plasma to near light-speeds with energies as high as 100 trillion electron volts. As if shot from a pinball machine’s flippers — the black hole in this case — the particles zigzag along random paths so that it takes thousands of years for each to make it beyond 10 light-years of the black hole.

Once in interstellar space, the protons smash into low-energy protons to form pions. These particles of matter immediately decay into high-energy gamma rays that shoot in all directions.

“So a very high-energy proton can dump its energy into radiation through that mechanism,” Ballantyne told SPACE.com.

The discovery could explain how the most powerful black holes in the universe produce their jets extending over intergalactic proportions, the scientists suggest.

“The same particle slinging almost certainly occurs in all black-hole systems, though with much greater power earlier in the universe,” Melia said.

The findings are detailed in the current issue of Astrophysical Journal Letters.

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