INT+Prime Focus, WHT+ISIS

During an imaging survey of the evolution of the space density of radio-loud quasars, a quasar at z=4.72 was discovered. This quasar, GB 1428+4217, is the third most distant quasar 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 is M (1450 Å) = –26.7. Another radio-loud quasar was discovered during the same CCD imaging survey, GB 1713+2148, with z = 4.01. Combined with earlier survey results, these objects give a lower limit on the space density of quasars with radio power P_{5 GHz} > 5.8 × 10²⁶ W Hz^–1 sr^–1 between z = 4 and z = 5 of 1.4 ± 0.9 × 10^–10 Mpc^–3.
 

B and R image of GB1428+4217 (the very red object in the center). GB1428+4217 is the third most distant quasar and the highest redshift radio and X-ray source currently known. This picture was taken using the Prime Focus camera at the INT. [ GIF ]

References
 

• A C Fabian et al, 1997, "The extreme X-ray luminosity of the z=4.72 radio-loud quasar GB 1428+4217", MNRAS, 291, L5.
• I M Hook and R G McMahon, 1998, "Discovery of radio-loud quasars with z=4.72 and z=4.01", MNRAS, 294, L7.