Antimatter Plume Poses Galactic Enigma

Earth's galaxy, the Milky Way, is unleashing a mysterious plume of antimatter - the first such plume ever seen.

Extending 3,000 light years above the Milky Way's core, the newly discovered plume is sending astronomers scrambling to explain why vast quantities of antimatter are present where they weren't expected to be.

"The origin of this new and unexpected cloud is a mystery," acknowledges William Purcell, an astrophysicist at Northwestern University in Evanston, Ill., and a member of the team that announced the discovery yesterday.

Antimatter first appeared on paper in 1928 as a mathematical necessity of quantum mechanics. Five years later, physicists discovered the first antiparticle - the positron - in lab experiments.

Positrons, the antimatter counterparts of electrons, have the same mass and life span as electrons, yet carry an equal but opposite electrical charge. When the two meet, they annihilate each other in a burst of gamma radiation. This "annihilation radiation" carries a unique energy signature that serves as the tip-off that antimatter is - or was - present.

Researchers found the antimatter plume while using this radiation as a proxy for mapping the galaxy's inventory of positrons. The radiation registered on detectors aboard the Compton Gamma Ray Observatory, a NASA satellite.

Oddly, the plume didn't have a counterpart below the nucleus. It's unusual to see a feature extending from a galaxy in only one direction, says James Kurfess, astrophysicist at the Naval Research Laboratory in Washington and lead investigator for the experiment.

The plume presents two puzzles, says Charles Dermer, also of the Naval Research Laboratory. "To have annihilation, you need a source of positrons," he says, "but you also need regular matter in sufficient density" to give the positrons something to annihilate. The Milky Way's plume extends into its halo, a region sparsely populated with matter.

He suggests that supernovas exploding at the center of the galaxy are the likely sources of the matter and antimatter in the plume. Compared with the rest of the galaxy, the nucleus is a hotbed of stellar explosions. In effect, he says, the core is acting like a volcano, sending up a plume of matter and antimatter as massive young stars end their lives in cataclysmic fashion.

Others suggest the plume has its source in a black hole known as the "Great Annihilator," which lies a mere 300 light years from the galactic center. Still others suggest that the plume may be a remnant of a fireball - the merging of two neutron stars, themselves the extremely dense remnants of stars that ended as supernovas.

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