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 $\zphot \gtrsim 3$, \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
\textit{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
($\zspec=5.08$) optically bright ($i^\prime\lesssim 21$) quasars ever detected
in X-ray surveys has been discovered.
