X-ray constraints on the spectral energy distribution of the $z=5.18$ blazar SDSS J013127.34-032100.1

Abstract

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 $\Gamma=1.9$ and $N_{\rm H}=1.1\times10^{21}\rm \ cm^{-2}$, or a broken power law with $\Gamma_l=0.5$, $\Gamma_h=1.8$, and a break energy $E_b=0.7$ keV for an expected absorbing column density of $N_{\rm H}=3.6\times 10^{20}\rm \ cm^{-2}$, 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 $\ge$7 and $\ge$6 (90%) for the low- and high-$N_{\rm H}$ SEDs, respectively. The corresponding beaming implies $\ge$130 (low $N_{\rm H}$) or $\ge$100 (high $N_{\rm H}$) high-spin supermassive black holes similar to J0131 exist at similar redshifts.