Extragalactic double radio sources with asymmetric jets

If the apparent asymmetry in the jets seen in several double radio sources is due to relativistic beaming, then the direction of the jet can be used to identify the approaching outer component which should appear further from the core than the receding one, due to the finite light-travel time across the source. It is found, however, that in the majority of a sample of double sources with asymmetric jets, the approaching component lies nearer to the core, and in these sources this component tends to have a more luminous compact feature compared to the receding lobe. Possible explanations of these trends are discussed in terms of gradients in the density of the intergalactic medium and non-collinear ejection of the outer components. (author)