The 3--53 keV Spectrum of the Quasar 1508+5714: X-Rays from Z = 4.3

We present a high-quality X-ray spectrum in the 3-53 keV rest frame band of the radio-loud quasar 1508+5714, which is by far the brightest known X-ray source at z > 4. A simple power-law model with an absorption column density equal to the Galactic value in the direction of the source provides an excellent and fully adequate fit to the data; the measured power-law photon index Γ = 1.42^+0.13_-0.10. Upper limits to Fe Kα line-emission and Compton-reflection components are derived. We offer evidence for both X-ray and radio variability in this object and provide the first contemporaneous radio spectrum (α = -0.25). The data are all consistent with a picture in which the emission from this source is dominated by a relativistically beamed component in both the X-ray and radio bands.