2004MNRAS.351..727A

A simple integral relation is obtained for the distribution functions of jet speeds and jet-counterjet flux asymmetries of classical double radio sources on the basis of a simple model in which it is assumed that the jets are intrinsically symmetric and that the asymmetries are attributed to relativistic beaming. Analytic expressions relating the mean jet speed and the mean jet flux asymmetry, and their variances are derived. The results are considered in the light of orientation-based unified schemes, and an analytical equation for the critical angle separating quasars and radio galaxies is derived. The mean jet speeds of classical double radio sources are estimated using the asymmetry of jet-counterjet flux densities taken from the 3CRR and B2 samples. For FRI radio sources the mean jet speed is ∼(0.54±0.03)c, while for FRII low-redshift radio galaxies and intermediate-redshift quasars the values found are ∼(0.4±0.06)c and ≳ 0.6c respectively.