A QSO discovered at the redshift of the extended X-ray cluster RX J0152.7-1357

The galaxy cluster RX J0152.7 - 1357 at z = 0.8325 is one of the most luminous X-ray clusters known. It is elongated toward the bright, nearby, X-ray active galaxy NGC 720 about 14' away. At the same distance on the other side of that galaxy is an X-ray blue stellar object with a flux of 5.8 counts ks(-1) in the ROSAT PSPC detector. An intermediate-resolution optical spectrum of this blue stellar object taken with the Keck I telescope has resulted in the unambiguous identification of this source with a QSO at redshift z = 0.8312, the same redshift as the galaxy cluster. We discuss the implications of this discovery in the framework of the standard model for large-scale structure formation.