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New class of celestial bursters.

New class of celestial bursters

Up to now astronomers have known two classes of bursting sources of high-energy radiation in the sky, gamma-ray bursters and X-ray bursters. Gamma-ray bursters tend to burst once and not again. Nobody is sure whether they are in our galaxy or out of it. X-ray bursters repeat and seem to be clustered in the bulge of our galaxy.

Now there is a third class, represented so far by one object, called by its discoverers Soft Gamma Repeater 1806-20.

In two papers in the Sept. 15 ASTROPHYSICAL JOURNAL, 17 scientists from France, the United States and the Soviet Union report observations of SGR 1806-20 since it was first noticed on Jan. 7, 1979. The accumulated data indicate that although the shape of the object's pulses in time resembles that of a gamma-ray burster, it repeats like an X-ray burster. It has repeated 100 times between Aug. 13, 1978 and June 27, 1986. Its spectral characteristics lie between the other two classes.

Gamma-ray bursters may lie outside our galaxy, and some theorists have suggested exotic explanations for them--for example, that they are vibrating cosmic strings (SN: 5/30/87, p.345). X-ray bursters are probably in our galaxy and may be stars that suffer periodic nuclear explosions on their surfaces. If the Soft Gamma Repeater source lies on the galactic bulge, where the X-ray bursters are believed to be, its discoverers calculate that it would have to be 1,500 times as bright as they are. However, certain of its characteristics suggest it may be much closer, perhaps as near as 6 parsecs (20 light-years).
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Title Annotation:repeating gamma-ray burster
Publication:Science News
Date:Oct 3, 1987
Words:268
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