"X-ray Flashes" Puzzle Astronomers

HUNTSVILLE, ALABAMA--It may sound like a feeble joke, but astronomers say they have discovered a new kind of gamma ray burst: one without gamma rays. At the 5th Huntsville Gamma Ray Burst Symposium here, astronomer John Heise of the Space Research Organization Netherlands announced yesterday that the Dutch-Italian satellite BeppoSAX has spotted strange outbursts of x-rays that look exactly like the byproducts of a gamma ray burst; they just don't come with gamma rays.

A mystery for decades, gamma ray bursts have recently been linked to some of the most energetic events in the cosmos (Science, 15 October, p. 395): vast supernova explosions that spawn black holes in the far reaches of the universe. Invisible from Earth because the atmosphere absorbs gamma radiation, they are spotted about once a day by Earth-orbiting satellites like BeppoSAX. BeppoSAX also detects flashes of x-rays, which have accompanied every gamma burst in the field of view of its x-ray cameras so far, and which also emanate from astronomical objects such as flare stars and x-ray novae. Now Heise is reporting still another kind of x-ray burst.

Sifting through the BeppoSAX data, he found nine x-ray explosions that resemble the x-ray signal from normal gamma ray bursts in every respect--they last tens of seconds, are scattered randomly across the sky, and have comparable spectra--except that no gamma rays were seen. "These are certainly not normal flare stars or other known objects," says Heise. "It could be a completely new class of events."

But the "x-ray flashes," as Heise prefers to call them, could be related to gamma ray bursts after all, says Stan Woosley of the University of California, Santa Cruz. Woosley has developed theoretical models for what he calls "dirty" gamma ray bursts: If a cosmic explosion takes place where the interstellar gas is relatively dense, the gamma rays could easily be absorbed by the gas while some x-rays escape, he says. However, Woosley's model predicts a different kind of x-ray spectrum than the ones BeppoSAX has measured.

Quick follow-up observations made immediately after a burst to look for an "afterglow" at other wavelengths, including optical and radio, could reveal the true nature of Heise's mysterious flashes. And those could come quickly: Immediately after Heise's announcement, Luigi Piro of the Istituto di Astrofisica Spaziale in Rome, the mission scientist for BeppoSAX, decided that x-ray flashes will from now on get the same treatment as gamma ray bursts: BeppoSAX's narrow-field x-ray cameras will pinpoint their exact location as soon as possible, to tell optical and radio telescopes where to look for an afterglow.