  B2 1023+25 is an extremely radio-loud quasar at z=5.3 which was first
identified as a likely high-redshift blazar candidate in the SDSS+FIRST quasar
catalog. Here we use the Nuclear Spectroscopic Telescope Array (NuSTAR) to
investigate its non-thermal jet emission, whose high-energy component we
detected in the hard X-ray energy band. The X-ray flux is ~5.5x10^(-14) erg
cm^(-2)s^(-1) (5-10keV) and the photon spectral index is Gamma_X=1.3-1.6.
Modeling the full spectral energy distribution, we find that the jet is
oriented close to the line of sight, with a viewing angle of ~3deg, and has
significant Doppler boosting, with a large bulk Lorentz factor ~13, which
confirms the identification of B2 1023+25 as a blazar. B2 1023+25 is the first
object at redshift larger than 5 detected by NuSTAR, demonstrating the ability
of NuSTAR to investigate the early X-ray Universe and to study extremely active
supermassive black holes located at very high redshift.
