WILLIAMSBURG, VIRGINIA--Scientists have discovered what appears to be a plume of antiparticles gushing from the center of our galaxy. The finding, announced here today at the Fourth Compton Symposium on Gamma-Ray Astronomy and Astrophysics, suggests that thousands of undetected stellar explosions are rocking the dust-shrouded core of the Milky Way.
Antielectrons, also called positrons, only reveal their existence when they annihilate with electrons, converting both particle and antiparticle into gamma rays at a distinct energy: 511 kiloelectron volts. Detectors lofted by balloon above Earth's gamma-ray absorbing atmosphere in the 1970s picked up this death cry coming from the center of the galaxy. And when NASA launched the Compton Gamma-Ray Observatory satellite in 1991, astrophysicists set out to use the gamma-ray signature to inventory the amount and distribution of positrons around the galactic center, which is thought to harbor giant black holes that spew out antimatter. But last year, the tally turned into an all-out hunt for positrons. One of the satellite's instruments had counted the number of positrons within a broad region, but a second instrument, with finer spatial resolution, couldn't track down about half of these positrons.
The search, led by astrophysicist William Purcell of Northwestern University, has now turned up about 50% of the missing positrons in an unlikely spot--a cloud about 3000 light-years from the galactic center, in a region thought to be almost entirely devoid of matter. "We were very surprised to see this," Purcell says, because the region appears to lack any obvious positron source--either black holes or radioactive atoms forged in stellar explosions. The best explanation, argues team member Charles Dermer of the Naval Research Laboratory in Washington, D.C., is that columns of hot gas, filled with positrons, are erupting from supernovas in "a cauldron of violence" shrouded from telescopes in the galactic core. The plume dumps the positrons into space surrounding the core.
"I like the explanation," says astrophysicist John Mattox of Boston University. Some experts are more skeptical, however. "The physics are a bit contrived," says Roland Diehl of the Max Planck Institute for Extraterrestrial Physics in Munich, Germany, who says his observations have failed to reveal the heavy aluminum that, in the supernova scenario, is the most likely source of the positrons. Other scientists, however, point out that undetected black holes in the cloud, instead of radioactive aluminum, might in fact be generating the positrons. Either way, many of the missing antiparticles now seem accounted for--and the hunt is on for more clouds that harbor the rest.