X-ray Luminosity Function of Quasars at 3 < z < 5 from XMM-Newton Serendipitous Survey Data.

The X-ray luminosity function of distant (3 < z < 5.1) type 1 quasars has been measured. A sample of distant high-luminosity (1045 erg s−1 ≤ LX,2−10 < 7.5×1045 erg s−1 in the 2-10 keV energy band) quasars from the catalog by Khorunzhev et al. (2016) compiled from the data of the 3XMM-DR4 catalog of the XMM-Newton serendipitous survey and the Sloan Digital Sky Survey (SDSS) has been used. This sample consists of 101 sources. Most of them (90) have spectroscopic redshifts zspec ⩾ 3; the remaining ones are quasar candidates with photometric redshift estimates zphot ⩾ 3. The spectroscopic redshifts of eight sources have been measured with the BTA and AZT-33IK telescopes. Owing to the record sky coverage area (≃250 sq. deg at X-ray fluxes ~10−14 erg s−1 cm−2 in the 0.5-2 keVband) from which the sample was drawn, we have managed to obtain reliable estimates of the space density of distant X-ray quasars with luminosities LX,2−10 > 2×1045 erg s−1 for the first time. Their comoving space density remains constant as the redshift increases from z = 3 to 5 to within a factor of 2. The power-law slope of the X-ray luminosity function of distant quasars at its bright end (above the break) has been reliably constrained for the first time. The range of possible slopes for the quasar luminosity and density evolution model is γ2 = 2.72−0.12+0.19 ± 0.21, where initially the lower and upper boundaries of γ2 with the remaining uncertainty in the detection completeness of X-ray sources in SDSS and subsequently the statistical error of the slope are specified. [ABSTRACT FROM AUTHOR]