Journal of High Energy Astrophysics
07/2026
JHEAp
SRG/eROSITA-SDSS view on the relation between X-ray and UV emission for quasars
Прохоренко, С.
Сазонов, С.
Гильфанов, М.
Balashev S.
Мещеряков, А.
Motivated by the idea of using quasars as standardizable candles for cosmology, we examine the relation between X-ray (at 2 keV, L2keV) and ultraviolet (at 2500 Å, L2500) monochromatic luminosities of quasars using a sample of 2414 X-ray sources from the SRG/eROSITA all-sky survey cross-matched with the Sloan Digital Sky Survey data release 16 quasar catalogue (SDSS DR16Q), at spectroscopic redshifts between 0.5 and 2.5. These objects are bright both in X-rays and in the optical, so that the sample is characterized by nearly 100% statistical completeness. We have developed a new method for determining the L2keV─L2500 relation, which consistently takes into account (i) X-ray and UV flux limited object selection, (ii) X-ray and UV variability of quasars, and (iii) the decreasing space density of quasars with increasing luminosity. Assuming a linear relation between lX≡logL2keVergs−1Hz−1 and lUV≡logL2500ergs−1Hz−1, we find the slope, γ=0.69±0.02 (hereafter all uncertainties are quoted at the 68% confidence level), and normalization, lX=26.47±0.02 at lUV=30.5, of the L2keV (L2500) dependence. These values are not substantially different from the results of previous studies. A key novel aspect of our work is allowance for intrinsic scatter (which adds to the dispersion induced by quasar variability and flux measurement uncertainties) of the L2keV─L2500 relation in both variables, i.e. in X-ray and UV luminosity. The intrinsic X-ray scatter (σintX2=0.063±0.005) strongly dominates over the UV one (σintUV2=0.002−0.002+0.003). Further studies should seek to explain this behaviour in terms of accretion onto supermassive black holes and orientation of quasars with respect to the observer.
