The extremely high redshift ( z =5.3 ) radio source SDSS J102623.61+254259.5 ( J1026+2542 ) is among the most distant and most luminous radio-loud active galactic nuclei ( AGN ) known to date .
Its one-sided radio jet structure on milli-arcsecond ( mas ) and \sim 10-mas scales typical for blazars was first imaged at 5 GHz with very long baseline interferometry ( VLBI ) in 2006 .
Here we report on our dual-frequency ( 1.7 and 5 GHz ) imaging observations performed with the European VLBI Network ( EVN ) in 2013 .
The prominent jet structure allows us to identify individual components whose apparent displacement can be detected over the time span of 7.33 yr .
This is the first time when jet proper motions are directly derived in a blazar at z > 5 .
The small values of up to \sim 0.1 mas yr ^ { -1 } are consistent with what is expected in a relativistic cosmological model if redshift is a measure of distance .
The apparent superluminal jet speeds , considered tentative because derived from two epochs only , exceed 10 c for three different components along the jet .
Based on modeling its spectral energy distribution , J1026+2542 is known to have its X-ray jet oriented close to the line of sight , with significant Doppler boosting and a large bulk Lorentz factor ( \Gamma \approx 13 ) .
The new VLBI observations , indicating \sim 2.3 \times 10 ^ { 12 } K lower limit to the core brightness temperature , are consistent with this picture .
The spectral index in the core region is -0.35 .