Galactic Flare Sets a Record

WASHINGTON, D.C.--An hours-long pulse of gamma rays from a distant galaxy is the most powerful ever seen from a celestial source, astronomers said here today at a meeting of the American Physical Society. The biggest surprise is not the sheer power of the signal but its safe passage through intergalactic space, where astronomers had expected that a haze of background photons would weaken it.

Astronomers at the Whipple Observatory on Mount Hopkins, Arizona, detected the signals last May, when their telescope picked up a blue glow excited as gamma rays at energies of 6 trillion electron volts crashed into the atmosphere. The source was Markarian 421, an elliptical galaxy about 400 million light-years away that was already known to emit gamma rays, probably generated as matter plunges into a black hole at its center. After a year-long analysis of atmospheric radiation caused by the gamma rays, the Whipple confirmed that the flare-up must have been unusually large. "It's the biggest gamma-ray signal seen from any source, period," said Jeffrey Zweerink, a team member from Iowa State University.

The arrival of extremely energetic gamma rays has astronomers amazed. Gamma rays at such high energies tend to destroy themselves when they collide with other photons, like the infrared photons radiated by hot gas and dust in intergalactic space. Over distances as large as that from Markarian 421 to Earth, few high-energy gamma photons should survive their trip, but somehow they did. "It was a surprise," says team member Trevor Weekes, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

"This has big implications," says Charles Dermer, an astronomer at the Naval Research Laboratory in Washington, D.C. He thinks that the safe arrival of high-energy gamma rays could mean that intergalactic space has fewer infrared photons than expected. And because this paucity conflicts with some of astronomers' ideas about the density and evolution of galaxies, they may have to revise their models. "It's a really exciting result," says Dermer.