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Out of the center: gamma-ray redux.

Out of the center: Gamma-ray redux

The center of the Milky Way galaxy, 25,000 light-years from Earth, is again producing large quantities of radiation in the form of gamma-rays of a specific energy, 511 kilo-electron volts. These emissions, detected by a sophisticated, balloon-borne gamma-ray telescope, result from collisions between electrons and positrons (the antimatter equivalent of electrons), which destroy the particles and generate gamma-rays. Scientists first detected positron annihilation from the galactic center in the 1970s, but the signals abruptly dimmed, then disappeared in the early 1980s (SN: 1/21/89, p.44). Such a sharp change indicates the radiation source must be compact.

Now, Marvin Leventhal of AT&T Bell Laboratories in Murray Hill, N.J., and Jeffrey E. McClintock of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., suggest the gamma-ray signals may be coming not from the galactic center but from an unusual X-ray source known as GX1+4, in the same region of the sky. The source appears to be a binary system consisting of a neutron star and a nearby red giant (SN: 7/2/88, p.6). Leventhal and McClintock note that changes in the intensity of X-ray emissions from GX1 + 4 appear to mimic the positron source's fluctuating behavior. Such a correlation suggests that GX1 + 4 itself may be the positron source.

New gamma-ray instruments that provide a narrower field of view would settle the question by pinpointing the source's true location, Leventhal says. Until that measurement is done, most astronomers still favor the presence of a black hole at the galactic center as the explanation for the gamma-rays.
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Title Annotation:Physics; GX1 + 4 positron source
Author:Peterson, Ivars
Publication:Science News
Date:May 13, 1989
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