Sbarrato, T.3; Tagliaferri, G.4; Ghisellini, G.4; Perri, M.5; Puccetti, S.5; Balokovic´, M.15; Nardini, M.7; Stern, D.15; Boggs, S. E.8; Brandt, W. N.16; Christensen, Finn Erland1; Giommi, P.5; Greiner, J.10; Hailey, C. J.17; Harrison, F. A.15; Hovatta, T.15; Madejski, G. M.12; Rau, A.10; Schady, P.10; Sudilovsky, V.10; Urry, C. M.13; Zhang, W. W.18
1 National Space Institute, Technical University of Denmark2 Astrophysics, National Space Institute, Technical University of Denmark3 Università dell'Insubria4 National Institute for Astrophysics5 ASI Science Data Center6 California Institute of Technology7 University of Milano Bicocca8 University of California at Berkeley9 Pennsylvania State University10 Max Planck Institute11 Columbia University12 SLAC National Accelerator Laboratory13 Yale University14 NASA Goddard Space Flight Center15 California Institute of Technology16 Pennsylvania State University17 Columbia University18 NASA Goddard Space Flight Center
B2 1023+25 is an extremely radio-loud quasar at z = 5.3 that 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 ~5.5 × 10-14 erg cm-2 s-1 (5-10 keV) and the photon spectral index is ΓX ≃ 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 ~3°, 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.