| Abstract
| - Aims. We investigate the frequency-dependent radio properties of the jet of the luminous high-redshift ( $z=3.2$) radio quasar PKS 1402+044 (J1405+0415) by means of radio interferometric observations. Methods. The observational data were obtained with the VLBI Space Observatory Programme (VSOP) at 1.6 and 5 GHz, supplemented by other multi-frequency observations with the Very Long Baseline Array (VLBA; 2.3, 8.4, and 15 GHz) and the Very Large Array (VLA; 1.4, 5, 15, and 43 GHz). The observations span a period of 7 years. Results. We find that the luminous high-redshift quasar PKS 1402+044 has a pronounced “core-jet” morphology from the parsec to the kilo-parsec scales. The jet shows a steeper spectral index and lower brightness temperature with increasing distance from the jet core. The variation of brightness temperature agrees well with the shock-in-jet model. Assuming that the jet is collimated by the ambient magnetic field, we estimate the mass of the central object as ~ $10^9~M_odot$. The upper limit of the jet proper motion of PKS 1402+044 is 0.03 mas yr -1 (~ $3c$) in the east-west direction.
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