Optical spectroscopy of candidates for quasars at 3 < z < 5.5 from the XMM-newton X-ray survey. A distant X-ray quasar at z = 5.08

We present the results of optical spectroscopy for 19 quasar candidates at photometric redshifts z _phot ≳ 3, 18 of which enter into the Khorunzhev et al. (2016) catalog (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 a quasi-random sample of 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. The spectra at AZT-33IK were taken with the new low- and medium-resolution ADAM spectrograph that was produced and installed on the telescope in 2015. Fourteen of the 18 candidates actually have turned out to be quasars; 10 of them are at spectroscopic redshifts z _spec > 3. The high purity of the sample of new candidates suggests that the purity of the entire K16 catalog of quasars is probably 70-80%. One of the most distant ( z _spec = 5.08) optically bright ( i' ≲ 21) quasars ever detected in X-ray surveys has been discovered.