WASHINGTON, D.C.--Starstruck astrophysicists are stunned by the magnitude of an explosion near the center of our galaxy shortly after Christmas. The outburst showered Earth with a more intense pulse of gamma rays and x-rays than ever measured from any other object, even from a distance of tens of thousands of light-years away. The source, scientists believe, was the most exotic kind of neutron star: a dense "magnetar" with a surface tortured and twisted by powerful magnetic fields.
Neutron stars are ultracompact bodies, perhaps 20 kilometers across, left behind by giant stars when they explode as supernovas. For mysterious reasons, some neutron stars possess the strongest known magnetic fields in the universe. These so-called magnetars spew out periodic flares, probably when their surfaces shift and crack. Two magnetars had unleashed giant flares in 1979 and 1998. But the latest flare, on 27 December 2004, was about 100 times more powerful.
Lasting just one-fifth of a second, the flare released as much energy as our sun produces in 250,000 years. About 15 satellites and solar-system probes were bombarded by the gamma rays and x-rays, although the radiation was not strong enough to disable any instruments. The flare also spawned a fireball that radio telescopes can now see blasting into space at 30% of the speed of light, says radio astronomer Bryan Gaensler of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. He and several astrophysicists reported their studies of the giant flare here on 18 February at a NASA press conference. A series of papers also will appear in forthcoming issues of Nature.
Earth's atmospheric shielding absorbed most of the flare's energy. Still, its impact on our planet was startling. The radiation compressed Earth's ionized outer atmosphere tens of kilometers inward from its usual daytime altitude of about 70 kilometers, says atmospheric physicist Umran Inan of Stanford University in California. The extra ionization caused no damage but dramatically affected long-distance radio transmissions. Moreover, the disturbance lingered for an hour. "That's totally unheard of," says Inan, who thinks the radiation temporarily altered chemical reactions that typically restore the ionosphere after solar flares.
The flare's pattern--an intense spike of energy and a fading afterglow--leads researchers to suspect that they caught a miniature gamma ray burst (GRB), a gigantic explosion that astronomers usually see in very distant galaxies. "For all practical purposes, this could be a short GRB in our own galaxy," says astrophysicist Chryssa Kouveliotou of NASA's Marshall Space Flight Center in Huntsville, Alabama. "It's an amazing event, something I never expected to see in my lifetime."