On the Doppler boosting in the compact radio jet of the distant blazar
  J1026+2542 at z=5.3
  Based on its broad-band spectral energy distribution, and the X-ray spectrum
in particular, the radio-loud active galactic nucleus (AGN) SDSS
J102623.61+254259.5 (J1026+2542) has recently been classified as a blazar. The
extremely high redshift of the source, z=5.3, makes it one of the most distant
and most luminous radio-loud AGN known to date. From published 5-GHz very long
baseline interferometry (VLBI) imaging data obtained in 2006, the source has a
typical blazar appearance on mas scales, with a prominent one-sided jet
extending to ~20 mas. We estimate the brightness temperature of J1026+2542 and
find no strong evidence for Doppler boosting. The jet viewing angle is possibly
at least ~20 deg. The bulk Lorentz factor and the viewing angle of the jet
could reliably be determined in the near future from multi-epoch VLBI
observations.
