Missing by more than a mile.

Missing by more than a mile

The center of the Milky Way galaxy has long been known as the source of intense gamma-ray emissions. Now a team from the California Institute of Technology in Pasadena has determined that the bulk of the emissions are coming not directly from the galactic center but from a single source at least 340 light-years away from the center. Until this discovery, most astronomers suspected the galactic nucleus itself as the gamma-ray source.

The researchers, led by Thomas A. Prince, made the discovery using a sensitive, automated gamma-ray camera hanging from a balloon floating 120,000 feet above Alice Springs in Australia. The emissions appear to come from a previously known X-ray object. In the gamma-ray range, this object is only slightly less luminous than Cygnus X-1, the brightest known gamma-ray source in the galaxy. The researchers suspect the object may be a black hole or a neutron star onto which matter is still settling.

Prince and his team plan to continue their gamma-ray observations later this spring. One aim is to see if the galactic center has a gamma-ray source that happened to be off when their initial observations were made.

The researchers will also look for signs of positron annihilation. Matter falling onto a neutron star or black hole produces copious amounts of gamma rays, which in turn generate large numbers of positrons (the antimatter equivalent of electrons). A collision between a positron and an electron destroys the pair, releasing energy in the form of gamma rays of a particular energy. Scientists first detected positron-annihilation emissions from the galactic center in the 1970s, but the signals mysteriously disappeared in 1980. Recently, Crawford J. MacCallum of the Sandia National Laboratories in Albuquerque, N.M., and his collaborators, also using balloon-based observations, found evidence the emissions may be starting again.