The discovery of this halo, thousands of light-years in diameter, was announced today at a meeting of the American Astronomical Society in Estes Park, Colorado. Astrophysicists don't know what the source is yet, but they have no shortage of theories. Gamma rays, the most intense form of radiation, are usually produced only by violent astronomical events, such as the explosion of a star. The first gamma-ray map of the sky, generated 2 years ago by a space telescope called the Compton Gamma Ray Observatory, was dominated by gamma rays coming from the center of the Milky Way, where intense star formation occurs, and by a few bright sources outside our galaxy. Dave Dixon, a physicist at the University of California, Riverside, made a remarkable find when he subtracted from this image the core glow of the Milky Way and other known gamma-ray sources.
Bright spots are sources such as active galaxies or pulsars.
Other theories, no doubt, will be proposed. To find this accelerator in the sky, astronomers may have to wait for the next generation of gamma-ray telescopes, due to be launched a decade from now.