2020ApJ...904...27A

We report on X-ray measurements constraining the spectral energy distribution (SED) of the high-redshift z = 5.18 blazar SDSS J013127.34-032100.1 with new XMM-Newton and NuSTAR exposures. The blazar's X-ray spectrum is well fit by a power law with Γ = 1.9 and N_H=1.1×10²¹cm^–2, or a broken power law with Γ_l=0.7, Γ_h=1.8, and a break energy E_b=0.7keV for an expected absorbing column density of N_H=3.6×10²⁰ cm^–2, supported by spectral fitting of a nearby bright source. No additional spectral break is found at higher X-ray energies (1-30 keV). We supplement the X-ray data with lower-energy radio-to-optical measurements and Fermi-LAT gamma-ray upper limits, construct broadband SEDs of the source, and model the SEDs using a synchro-Compton scenario. This modeling constrains the bulk Doppler factor of the jets to >=7 and >=6 (90%) for the low- and high-N_H SEDs, respectively. The corresponding beaming implies >=130 (low N_H) or ≥ 100 (high N_H) high-spin supermassive black holes similar to J0131 exist at similar redshifts.