Identification of a complete sample of northern ROSAT All-Sky Survey X-ray sources. V. 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 RX J1028.6-0844. From the Lyα emission line found in an optical spectrum we measured z=4.276+/-0.005. This is so far the highest redshift for a QSO in the RASS. The RASS X-ray count rate of 0.035ct/s leads to an X-ray flux of f_x_=8.3310^-13^erg/s/cm and a luminosity of L_x_=~6x10^46^erg/s in the 2-10keV rest frame energy band of the QSO if galactic column density of neutral hydrogen and a photon index {GAMMA}=-2 is assumed. The spectral energy distribution as inferred from the observed hardness ratios indicates the presence of considerable intrinsic absorption in the QSO around 2-2.5keV.