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 $\text{zphot}\gtrsim 3$, \Nobs \ 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 $\mathit{\text{3XMM-DR4}}$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 \ telescope of the Sayan Solar Observatory and the 6-m BTA telescope of the Special Astrophysical Observatory. The spectra at \Azt \ were taken with the new low- and medium-resolution ADAM spectrograph that was produced and installed on the telescope in 2015. Fourteen of the \Nobs \ candidates actually have turned out to be quasars; 10 of them are at spectroscopic redshifts z > 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 ($\text{zspec}=5.08$) optically bright ($i^{\mathrm{\prime }}\lesssim 21$) quasars ever detected in X-ray surveys has been discovered.