The Discovery of an X-Ray Selected, Radio-Loud Quasar at z=3.9

A flux-limited catalog of radio sources in the northern sky has been used to select ~600 weak x-ray sources from a database of ~10^5^ 2-4 σ fluctuations derived from Einstein Observatory imaging proportional counter x-ray images. Optical spectroscopy of only six of these sources resulted in the discovery of a quasar with an emission-line redshift of 3.87, the highest redshift x-ray source yet discovered. Subsequent VLA observations reveal the source to have a flat-spectrum radio core plus an unresolved radio lobe 2.5" away with a spectral index of α = -1.45. The lobe is connected to the core by a weak jet. The integrated flux density of the source at 20 cm is 0.66 Jy. The V magnitude of the quasar is 19.5, and the tentative x-ray source has a very soft spectrum. We discuss the properties of this object in the context of known high-redshift radio sources and existing x-ray selected quasar samples, and comment on the potential of our discovery technique for exploring the properties of very distant active galaxies.