HOW FAST ARE THE LARGE-SCALE JETS IN QUASARS - CONSTRAINTS ON BOTH DOPPLER BEAMING AND INTRINSIC ASYMMETRIES

We analyse the jet-to-counterjet flux ratios measured from deep VLA im ages of 13 3CR quasars made by Bridle et al., in order to derive const raints on the degree of Doppler boosting (and hence the characteristic jet speeds on kiloparsec scales) and on how much intrinsic asymmetry between jets and counterjets is permitted. The main result is that the large jet-counterjet brightness ratios measured by Bridle et al. must be attributed almost entirely to the Doppler effect, requiring charac teristic jet speeds greater than or equal to 0.6c on kiloparsec scales . Modest intrinsic asymmetries are permitted, but do not reduce the ch aracteristic speed significantly, while large asymmetries are ruled ou t by VLBI data. The data also require that orientations close to the p lane of the sky are excluded (theta(max) < 75 degrees), so at least so me quasars masquerade as galaxies. For theta(max) in the range 45 degr ees to 60 degrees, as suggested in certain unified schemes, the best-f itting jet speeds are in the range 0.6c to 0.7c. Analysis of jet-to-lo be ratios for a complete sample of 3CR quasars leads to an upper limit on jet speed of < 0.95c, demonstrating that there is at least some de celeration between parsec and kiloparsec scales.