X-ray QSO evolution from a very deep ROSAT survey

Some 32 quasi-stellar objects (QSOs) were identified to a flux limit of 2 x 10(exp -15) erg/sq cm in the deepest optically identified X-ray survey performed to date. This Rosat position sensitive proportional counter survey (PSPC) is spectroscopically 86 percent complete. The surface density of quasars at this flux limit is 210 +/- 37 per sq deg. The QSO X-ray luminosity function was measured at low luminosities and high redshifts, the highest of which was 3.4.