Title: J0906+6930: a radio-loud quasar in the early Universe,
Abstract: Radio-loud high-redshift quasars (HRQs), although only a few of them are
known to date, are crucial for the studies of the growth of supermassive black
holes (SMBHs) and the evolution of active galactic nuclei (AGN) at early
cosmological epochs. Radio jets offer direct evidence of SMBHs, and their radio
structures can be studied with the highest angular resolution using Very Long
Baseline Interferometry (VLBI). Here we report on the observations of three
HRQs (J0131-0321, J0906+6930, J1026+2542) at z>5 using the Korean VLBI Network
and VLBI Exploration of Radio Astrometry Arrays (together known as KaVA) with
the purpose of studying their pc-scale jet properties. The observations were
carried out at 22 and 43 GHz in 2016 January among the first-batch open-use
experiments of KaVA. The quasar J0906+6930 was detected at 22 GHz but not at 43
GHz. The other two sources were not detected and upper limits to their compact
radio emission are given. Archival VLBI imaging data and single-dish 15-GHz
monitoring light curve of J0906+6930 were also acquired as complementary
information. J0906+6930 shows a moderate-level variability at 15 GHz. The radio
image is characterized by a core-jet structure with a total detectable size of
~5 pc in projection. The brightness temperature, 1.9x10^{11} K, indicates
relativistic beaming of the jet. The radio properties of J0906+6930 are
consistent with a blazar. Follow-up VLBI observations will be helpful for
determining its structural variation.