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B2 1023+25 is an extremely radio-loud quasar at z=5.3 which was firstidentified as a likely high-redshift blazar candidate in the SDSS+FIRST quasarcatalog. Here we use the Nuclear Spectroscopic Telescope Array (NuSTAR) toinvestigate its non-thermal jet emission, whose high-energy component wedetected in the hard X-ray energy band. The X-ray flux is ~5.5x10^(-14) ergcm^(-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 isoriented close to the line of sight, with a viewing angle of ~3deg, and hassignificant Doppler boosting, with a large bulk Lorentz factor ~13, whichconfirms the identification of B2 1023+25 as a blazar. B2 1023+25 is the firstobject at redshift larger than 5 detected by NuSTAR, demonstrating the abilityof NuSTAR to investigate the early X-ray Universe and to study extremely activesupermassive black holes located at very high redshift.

NuSTAR detection of the blazar B2 1023+25 at redshift 5.3