Pinball on a Galactic Scale

28 February 2007 (All day)

Sarah Ballantyne

Bank shots.
High-energy protons ejected by the supermassive black hole at the center of the Milky Way follow an erratic course guided by magnetic fields, until collisions with other particles produce gamma-rays.

The center of our Milky Way galaxy crackles with lethal gamma-rays, emitting trillions of electron volts of energy. Yet most astrophysicists consider our corner of the universe a relatively placid place, so the source of all this energy has remained mysterious. Now, members of an international team think they have found the answer: high-energy protons ejected by the supermassive black hole that lies at the heart of the galaxy.

In 1998, scientists confirmed what they had long suspected: The Milky Way's center hosts an immense black hole, equivalent to the mass of about 2.6-million suns. Like all black holes, it pulls in huge amounts of matter to its ultimate doom. But when compared to the hearts of other galaxies, the Milky Way's hole had been considered puny, with not enough mass to generate much radiation in the gamma-ray wavelengths. Nevertheless, in 2004, astronomers found that the black hole was indeed generating gamma-rays, thereby forcing a search for the mechanism that could produce them.

That mechanism appears to be magnetism. Reporting in this week's issue of Astrophysical Journal Letters, a team led by David Ballantyne of the University of Arizona in Tucson concludes that the Milky Way's central black hole is strong enough to generate a powerful but chaotic magnetic field that extends out a distance of about 10 light-years. To reach their conclusions, the researchers used computer programs to calculate the hypothetical trajectories of some 220,000 protons, which are ejected from the Milky Way's black hole and get bounced around by its magnetic field like in a galactic-sized version of a pinball machine. They then compared those paths with recent observations by ground-based instruments of the location of the gamma-rays and found that 69% of the computer-generated trajectories matched what the observed data were showing. The protons were slamming into hydrogen atoms within huge clouds of gas that slowly orbit the black hole about 10 light-years out. These collisions produce the gamma radiation astronomers have observed at the center of the Milky Way.

Co-researcher Fulvio Melia, also of the University of Arizona, says the findings could help astrophysicists understand how the most powerful black holes in the universe likewise produce gamma radiation. "The same particle slinging almost certainly occurs in all black-hole systems," Melia says.

The findings are interesting, but not necessarily conclusive, says astrophysicist Valerie Connaughton of NASA's Marshall Space Flight Center in Huntsville, Alabama. There could be an alternative explanation, such as a recent supernova in the vicinity of the galactic center, she says, which could be accelerating the high-energy protons being detected. Further observations are needed, she says, because "we still don't really understand all of the physics involved."

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