2025/12/05 by Prokhorenko, S. A., Sazonov, S. Yu., Gilfanov, M. R. +5
#Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · doi:10.48550/arxiv.2512.05807
Motivated by the idea of using quasars as standardizable candles for cosmology, we examine the relation between X-ray (at 2 keV, L\rm 2keV) and ultraviolet (at 2500 Angstrom, L\rm 2500) 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 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 L\rm 2keV-L\rm 2500 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 l\rm X≡log(L\rm 2keV/[\rm erg s-1 Hz-1]) and l\rm UV≡log(L\rm 2500/[\rm erg s-1 Hz-1]), we find the slope, γ=0.69±0.02 (hereafter all uncertainties are quoted at the 68% confidence level), and normalization, l\rm X=26.45±0.02 at l\rm UV=30.5, of the L\rm 2keV (L\rm 2500) 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 L\rm 2keV-L\rm 2500 relation in both variables, i.e. in X-ray and UV luminosity. The intrinsic X-ray scatter (σ2\rm intX=0.066±0.005) strongly dominates over the UV one (σ2\rm intUV=0.001+0.003-0.001). 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.