Probing Quasar-Jet Plasma.

In the worlds of finance and politics, 1 percent doesn't sound like much. But in many fields of science, a 1 percent signal can make or break a theory. A Brandeis University team has scrutinized the radio waves from a distant quasar's jet, and it finds that 1 percent of those waves stand apart because of their circular polarization: their electric fields spiral through space like corkscrews. (By contrast, 10 percent are plane-polarized, slicing through space like knives.) Intriguingly, this deceptively small amount of circular polarization has enabled the scientists to reach fundamental conclusions about the composition of the quasar's fast-moving, energetic jet.

In 1996, John F. C. Wardle and his colleagues used the continent-spanning Very Long Baseline Array of radio telescopes to synthesize images of 3C 279, one of the sky's most luminous and variable quasars. This revealed circular polarization at the base of the quasar's blobby jet. Because the jet is pointed nearly toward us, most of the observed radio waves must pass through the bulk of the jet's constituent plasma en route to Earth. And since those radio waves do not begin their lives with circular polarization, says Wardle, the plasma in the quasar's jet must have imparted the effect upon the waves as they passed through.

According to Wardle, a plasma of electrons and protons is incapable of converting plane-polarized waves to circularly polarized ones, but a plasma of electrons and positrons will do the trick. Thus he concludes that 3C 279's jets are made of electrons and positrons, the latter being the electrons' antimatter counterparts. (Three other powerful quasars also show detectable circular polarization, the Brandeis group has found, but the data on them have yet to be fully analyzed and published.)

The finding's significance is twofold, as the scientists explain in Nature for October 1, 1998. First, astronomers found it difficult to reconcile their observations with a jet of electrons and protons; protons outweigh electrons 1,800 times over, and the kinetic energy of an electron-proton jet would be far greater than required to explain the jet's overall luminosity. An electron-positron jet removes that quandary. Second, the finding sheds light on the energetic processes in the quasar's core, processes that were presumably fueled by the ultrasteep gravity of a supermassive black hole.