Coherent synchrotron emission observed: implications for radio astronomy

Synchrotron radiation by relativistic electrons spiralling in magnetic fields is a mainstay of radio astronomy, accounting for emissions from many objects. Conventional models assume that electrons radiate singly, so power scales with number of electrons. Yet recently jets from active galactic nuclei have shown very high luminosity, inconsistent with plausible single-particle synchrotron emission. We report experiments showing that, by stimulating plasma instabilities with relativistic electron beams, one can induce increases in the synchrotron emission by factors of ∼106. Enhancement presumably arises from coherent bunching of the relativistic electrons as they spiral in an ambient magnetic field. Polarization measurements suggest that electrons radiatively cooperate on scales of ∼6.6 cm. Radio telescope Stokes parameters may be able to reveal such polarization effects in high-brightness sources, a new observing diagnostic.