THE 3-53 KEV SPECTRUM OF THE QUASAR-1508- X-RAYS FROM Z=4.3(5714 )

We present a high-quality X-ray spectrum in the 3-53 keV rest frame ba nd of the radio-loud quasar 1508+5714, which is by far the brightest k nown X-ray source at z > 4. A simple power-law model with an absorptio n column density equal to the Galactic value in the direction of the s ource provides an excellent and fully adequate fit to the data; the me asured power-law photon index Gamma = 1.42(-0.10)(+0.13). Upper limits to Fe K alpha line-emission and Compton-reflection components are der ived. We offer evidence for both X-ray and radio variability in this o bject and provide the first contemporaneous radio spectrum (alpha = -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.