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