Optical Identification of New Candidates for Quasars at 3 < z < 5.5 from the XMM-Newton X-ray Survey

We present the results of optical spectroscopy for 39 quasar candidates at photometric redshifts z _phot ≳ 3 from the catalog by Khorunzhev et al. (2016) (K16). This is a catalog of quasar candidates and known type 1 quasars selected among the X-ray sources of the 3XMM-DR4 catalog of the XMM-Newton serendipitous survey. We have performed spectroscopy for new candidates at the 1.6-m AZT-33IK telescope of the Sayan Solar Observatory and the 6-m BTA telescope of the Special Astrophysical Observatory. Predominantly X-ray bright quasar candidates that could be used to construct the X-ray luminosity function at luminosities L _X,2-10 > 10⁴⁵ erg s^-1 have been selected. The purity of the candidates from K16 with a high X-ray flux has turned out to be lower than the purity of the quasi-random sample investigated by us previously. We have estimated the observing efficiency of quasar candidates as a function of their photometric redshift.