The magnetic fields of the Milky Way cause electrons with nearly the speed of light to rotate and to emit radio waves. As consequence, this radiation should also "rotate" in some way, i.e. the polarization of the radiation will change. This very weak circular polarization of the Milky Way, however, has not been observed so far. Researchers at the Max Planck Institute for Astrophysics and colleagues have now predicted some properties of this polarization and created a "wanted poster" to allow targeted searches. A measurement of the circular polarization would provide important insights into the structure of the galactic magnetic fields and confirm that electrons - and not positrons - are the source of this radio emission in the Milky Way.
Information about both the amount of electrons and positrons with almost the speed of light and the transverse component of the magnetic field is given by the radio map of the Milky Way (Fig. 1). In general, it is assumed that this emission is generated mainly by electrons with only a small contribution by positrons.
1. Torsten A. Enßlin, Sebastian Hutschenreuter, Valentina Vacca, and Niels Oppermann
The Galaxy in circular polarization: all-sky radio prediction, detection strategy, and the charge of the leptonic cosmic rays
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