Distant Star's Radiation Jolts Earth's Atmosphere

WASHINGTON, D.C.--Around 6:22 a.m. EDT on 27 August, a tidal wave of x-ray and gamma ray radiation washed over the Earth, turning night to day in the upper atmosphere and shell-shocking some satellite equipment. The burst, reported at a NASA press conference today, apparently got its start 20,000 light-years away, where a super-dense, super-magnetized neutron star suffered a massive "star-quake." The formidable burst is "about the wildest thing in 30 years since we've been monitoring these things," says astrophysicist Kevin Hurley at the University of California, Berkeley.

The intensity of the 5-minute pulse when it reached the Earth "was about a tenth of a dental x-ray dose," Hurley estimates, "but it's a hell of a lot of radiation for a source that far away." None of it penetrated the atmosphere, but the deluge was enough to trigger the momentary shut-down of x-ray equipment on the NEAR and Konus-Wind spacecraft as well as NASA's Rossi X-ray Timing Explorer. It was also enough to leave its fingerprints on Earth's atmosphere.

Umran Inan, a Stanford University physicist, and colleagues used a string of 50 radio antennas to monitor low-frequency Navy radio broadcasts that bounce between the Earth and the ionosphere--the blanket of charged particles that surrounds the Earth. The thicker the ionosphere gets, the more it tends to act as a large pillow, weakening radio signals as they strike it. Inan and colleagues found that the strength of Navy radio signals plummeted at the time the pulse swept over the Earth, implying that the ionosphere's inner edge briefly plunged to 60 kilometers, about where it sits when the full force of daylight sun ionizes molecules in the atmosphere.

The burst's source appears to be the neutron star SGR 1900+14. This dense ember of a burned out star could have inherited and compressed the magnetic field of its parent star to an enormous strength. The magnetic field of such a "magnetar" would periodically tear apart the star's hard crust of heavy elements, relieving stress as in an earthquake. Particles shot upwards in the quake would be accelerated by the magnetic field, producing a strong wave of radiation.

Could a nearby magnetar threaten the human race? "Yeah, I did that calculation," Hurley says. To seriously damage the atmosphere, the explosion would have had to occur as close as the comet belts that girdle the solar system. A magnetar hiding there would have been sniffed out long ago, he says.