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We report deep Karl G. Jansky Very Large Array (VLA) observations of theoptically ultraluminous and radio-quiet quasar SDSS J010013.02 + 280225.8(hereafter J0100+2802) at redshift $z=$6.3. We detected the radio continuumemission at 1.5 GHz, 6 GHz, and 10 GHz. This leads to a radio power-lawspectral index of $\alpha = -0.52\pm0.18$ ($S \propto \nu^{\alpha}$). The radiosource is unresolved in all VLA bands with an upper limit to the size of$0.2^{\prime \prime}$ (i.e., $\sim$ 1.1 kpc) at 10 GHz. We find variability inthe flux density (increase by $\sim 33\%$) and the spectral index (steepened)between observations in 2016 and 2017. We also find that the VLA 1.5 GHz fluxdensity observed in the same year is 1.5 times that detected with the Very LongBaseline Array (VLBA) in 2016 at the same frequency. This difference suggeststhat half of the radio emission from J0100+2802 comes from a compact corewithin 40 pc, and the rest comes from the surrounding few kpc area which isdiffuse and resolved out in the VLBA observations. The diffuse emission is fourtimes brighter than that would be expected if driven by star formation. Weconclude that the central active galactic nucleus is the dominant power engineof the radio emission in J0100+2802.

Exploring the radio spectral energy distribution of the ultraluminous radio-quiet quasar SDSS J0100+2802 at redshift 6.3