Neutron stars: new link to gamma repeaters.

The heavens often unleash flashes of gamma rays, but nearly every such burst of high-energy radiation occurs just once, vanishing without a trace. Astronomers have found only three sources that flash off and on at irregular intervals.

Scientists have suspected that these recurring emissions, known as soft gamma-ray repeaters because they have lower energy than the standard bursts, might stem from the activity of neutron stars. That's because researchers several years ago found that the location of one of the repeaters coincided with a supernova remnant, the shell of gas and dust ejected when a massive star collapses to form a neutron star. But with a source for only one repeater identified, the origin of these flashes remained uncertain.

A new study now links a second repeater with a supenova remnant, helping clinch the association between this recurring radiation and neutron stars. Shrinivas R. Kulkarni of the California Institute of Technology in Pasadena and Dale A. Frail of the National Radio Astronomy Observatory in Socorro, N.M., report their work in the Sept. 2 NATURE. Some researchers hope the new link may bolster the case that neutron stars produce the standard gamma-ray bursts, which have proved far more mysterious than repeaters (SN: 5/15/93, p.319).

Kulkarni and Frail began their study by searching astronomy catalogs for a radio-emitting counterpart to the soft gamma-ray repeater SGR 1806-20. They found such a match, and an analysis of radio images indicates that the radio-emitting object is almost certainly a young supernova in the Milky Way. Kulkarni cautions that astronomers don't know the location of SGR1806-20 as precisely as that of another repeater, which researchers in 1982 linked with the youthful supernova remnant N49 in a neighboring galaxy, the Large Magellanic Cloud.

Nonetheless, he says, the new study strengthens the link between neutron stars and repeaters. "Taken together, these two associations argue strongly in favor of a neutron-star origon for the repeaters," Frail and Kulkarni note in their report.

In a commentary accompanying the NATURE article, Kevin Hurley of the University of California, Berkeley, notes that our galaxy contains hundreds of young supernova remnants, far outnumbering the three known repeaters. This mismatch, according to Kulkarni and Frail, suggests that only certain neutron stars can produce the gamma signals. These stars would have special properties, such as an unusually intense magnetic field or a particular rotation rate.

Kulkarni also notes an alternative posibility: Perhaps all neutron stars can produce recurring bursts of soft gamma rays, but only during a brief period -- less than 500 years -- of their lifetime. Thus, he speculates, at any given time in our galaxy, only a few neutron stars happen to be at the stage in which they become repeaters. In an upcoming ASTROPHYSICAL JOURNAL, Hurley and a team of French and Russian colleagues suggest that this scenario may best explain the small number of repeaters.

Although standard bursts may originate from far outside our galaxy and differ in several other ways from repeaters, some researchers theorize that neutron stars might explain both phenomena. In the May 1 ASTROPHYSICAL JOURNAL, Fulvio Melia of the University of Arizona in Tucson and Marco Fatuzzo of the University of Michigan in Ann Arbor speculate that it's all a matter of geometry. If a neutron star is oriented so that Earth-bound observers look directly along the star's axis of rotation, they will see a single, energetic burst of high-energy gamma rays. But if observers are not so aligned, they will record a more frequent event: repeated pulses of gamma rays that have lost energy when scattered by charged particles.

Though all neutron stars emit both types of radiation, repeaters from the multitude of distant stars are too weak to be recorded on Earth, Melia suggests.