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We have compiled a catalog of 903 candidates for type 1 quasars at redshifts3<z<5.5 selected among the X-ray sources of the serendipitous XMM-Newton surveypresented in the 3XMM-DR4 catalog (the median X-ray flux is 5x10^{-15}erg/s/cm^2 the 0.5-2 keV energy band) and located at high Galactic latitudes>20 deg in Sloan Digital Sky Survey (SDSS) fields with a total area of about300 deg^2. Photometric SDSS data as well infrared 2MASS and WISE data were usedto select the objects. We selected the point sources from the photometric SDSScatalog with a magnitude error Delta z<0.2 and a color i-z<0.6 (to firsteliminate the M-type stars). For the selected sources, we have calculated thedependences chi^2(z) for various spectral templates from the library that wecompiled for these purposes using the EAZY software. Based on these data, wehave rejected the objects whose spectral energy distributions are betterdescribed by the templates of stars at z=0 and obtained a sample of quasarswith photometric redshift estimates 2.75<zphot<5.5. The selection completenessof known quasars at z>3 in the investigated fields is shown to be about 80%.The normalized median absolute deviation is 0.07, while the outlier fraction iseta= 9. The number of objects per unit area in our sample exceeds the number ofquasars in the spectroscopic SDSS sample at the same redshifts approximately bya factor of 1.5. The subsequent spectroscopic testing of the redshifts of ourselected candidates for quasars at 3<z<5.5 will allow the purity of this sampleto be estimated more accurately.

Catalog of Candidates for Quasars at 3 < z < 5.5 Selected among X-Ray Sources from the 3XMM-DR4 Survey of the XMM-Newton Observatory