RELATIVISTIC POSITRONS IN NONTHERMAL RADIO-SOURCES

We describe a procedure for measuring the contribution of relativistic positrons to radio synchrotron radiation. The method relies on the fa ct that synchrotron radiation from particles of one sign (e.g., electr ons) is circularly polarized by a small but measurable amount. If, on the other hand, there are equal numbers of relativistic positrons and electrons, the net circular polarization is zero. The method is illust rated through high-accuracy mapping of the circular polarization of th e Crab Nebula at 610 MHz. No significant circular polarization was det ected: a very conservative limit is 0.05%, and a more realistic one is 0.03%. We calculate the degree of circular polarization expected if o nly electrons are present, allowing for the reduction in polarization resulting from nonuniformities in the magnetic held along the line of sight and across the telescope beam. This reduction due to held nonuni formity is estimated from measurements of the degree of linear polariz ation at optical and high radio frequencies with similar angular resol ution to the circular polarization measurements. We find that the obse rved upper limit on the degree of circular polarization is comparable to or below that expected if only electrons radiate. Various explanati ons of this result are discussed, including (1) a weaker than assumed magnetic field, (2) a held preferentially nearly perpendicular to the line of sight, (3) a held structure of such a type that nonuniformitie s reduce the degree of circular polarization by more than they reduce the degree of linear polarization, and (4) the presence of relativisti c positrons. Although explanation 1 is implausible, possibilities 2 an d 3 cannot be excluded. If future observations establish that the degr ee of circular polarization at 610 MHz is less than 0.01%, a contribut ion from positrons would be strongly favored.