| Abstract
| - Abstract. We report the discovery of two radio-loud quasars with redshifts greater than 4: GB1428+4217, with z=4.72, and GB1713+2148 with z=4.01. This doubles the number of published radio-selected quasars with z>4, bringing the total to 4. GB1428+4217 is the third most distant quasar known and the highest redshift radio and X-ray source currently known. It has a radio flux density at 5 GHz of 259±31 mJy and an optical magnitude of R~20.9. The rest frame absolute UV magnitude, Mv(1450 Å), is −26.7, similar to that of the archetypal radio-selected quasar 3C273 [z=0.158; Mv(1450 Å)= −26.4]. GB1428+4217 is tentatively detected in ROSAT PSPC observations, which has been confirmed by more recent ROSAT observations described in a companion paper by Fabian et al. Both quasars were discovered during the CCD imaging phase of an investigation into the evolution of the space density of radio-loud quasars at high redshift. Combined with our earlier survey results, these objects give a lower limit on the space density of quasars with radio power P5 GHz>5.8×1026 W Hz−1 sr−1 between z=4 and z=5 of 1.4±0.9×10−10 Mpc−3. This can be compared to 2.9±0.2×10−10 Mpc−3 at z=2 from Dunlop & Peacock for flat-spectrum sources of the same luminosity.
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