A systematic asymmetry in the polarization properties of double radio sources with one jet

It has long been a puzzle why high-luminosity extragalactic radio sources, which are symmetric in many respects, have jets on one side only¹.The jets may be intrinsically asymmetric, because dissipation is stronger on one side or because they flip alternately between the two lobes they supply², or the one-sidedness may be only apparent, the result of Doppler beaming in twin relativistic jets³. So far, attempts to distinguish observationally between these two possibilities have been inconclusive⁴. Following a suggestion by Laing⁵, we report here new observations which establish that almost invariably the jet side depolarizes less rapidly with increasing wavelength than the opposite side. If the jet one-sidedness is intrinsic, this depolarization most probably occurs internally, but if the one-sidedness is due to Doppler beaming the depolarization must occur in a foreground screen. Further detailed polarization observations, which can determine whether the depolarization in such sources is internal or external, should therefore settle whether jets are intrinsically or only apparently one-sided.