IDENTIFICATION OF A COMPLETE SAMPLE OF NORTHERN ROSAT ALL-SKY SURVEY X-RAY SOURCES .5. DISCOVERY OF A Z=4.28 QSO NEAR THE RASS SOURCE RX J1028.6-0844

We report on the discovery of the first z > 4 QSO in the ROSAT All Sky Survey (RASS) near the X-ray source RXJ1028.6-0844. From the Ly alpha emission line found in an optical spectrum we measured z = 4.276 +/- 0.005. This is so far the hightest redshift for a QSO in the RASS. The RASS X-ray count rate of 0.035 cts s(-1) leads to an X-ray flux of f( x)=8.33 10(-13) erg s(-1) cm(-2) and a luminosity of L-x approximate t o 6 10(46) erg s(-1) in the 2 - 10 keV rest frame energy band of the Q SO if galactic column density of neutral hydrogen and a photon index G amma=-2 is assumed. The spectral energy distribution as inferred from the observed hardness ratios indicates the presence of considerable in trinsic absorption in the QSO around 2 - 2.5 keV.