The optical identification of large and complete subsamples of X-ray sources from the ROSAT All Sky Survey (RASS) showed about 40% of the sources detected at high galactic latitude to have AGN as optical counterparts (e.g. Zickgraf et al. 1997 (Paper II), Thiering et al. 1997 (Paper III), Bade et al. 1996 ). The bulk of these AGN is at low to intermediate redshifts. Zickgraf et al. (1997 ), e.g., found median redshifts of typically 0.2 to 0.3 for AGN in three study areas with nearly complete identification and X-ray flux limits on the order of a few times 10-13 erg s-1 cm-2. At high redshifts the frequency distribution has a tail up to (Krautter et al. 1997 , Paper IV).
At very high redshifts, i.e. , only a few quasars have been detected so far at X-ray energies. One of these, RX J1759.4+6638, is the first X-ray-selected quasar () (Henry et al. 1994 ). It was found in a deep pointed ROSAT PSPC observation with a flux of erg s-1 cm-1 in the 0.5-2.0 keV energy range, well below the flux limits of the above mentioned RASS samples. The radio-selected quasar GB 1508+5714 () was detected with the EINSTEIN IPC (Mathur & Elvis 1995 ), and the optically selected quasar 0000-263 ()(Bechtold et al. 1994 ) was detected in deep pointed ROSAT PSPC exposures. However, no quasar at was found until now in the RASS.
In this paper we report on the first discovery of a QSO at in the RASS. The quasar was found among the RASS sources studied by us within a project for optical identification of a complete sample of RASS X-ray sources at high galactic latitude in the northern hemisphere (). The project has been described in detail by Zickgraf et al. (1997 ). The full catalog of identifications and a statistical evaluation of the results can be found in Paper III and Paper IV, respectively.
In Sect. 2 the observations are presented. The results are discussed in Sect 3. Throughout this paper we use the cosmological parameters km s-1 and , an energy index as and likewise the photon index with photon flux density in photons cm-2 s-1 keV-1.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998