SRG/eROSITA-SDSS view on the relation between X-ray and UV emission for quasars

Abstract

Motivated by the idea of using quasars as standardizable candles for cosmology, we examine the relation between X-ray (at 2 keV, $L_{2keV}$) and ultraviolet (at 2500 Angstrom, $L_{2500}$) 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 $L_{2keV}-L_{2500}$ 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 $l_{\mathrm{X}}\equiv \log (L_{2keV}/[\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{H}{\mathrm{z}}^{-1}])$ and $l_{\mathrm{UV}}\equiv \log (L_{2500}/[\mathrm{erg}{\mathrm{s}}^{-1}\mathrm{H}{\mathrm{z}}^{-1}])$, we find the slope, $\gamma =0.69\pm 0.02$ (hereafter all uncertainties are quoted at the 68% confidence level), and normalization, $l_{\mathrm{X}}=26.47\pm 0.02$ at $l_{\mathrm{UV}}=30.5$, of the $L_{2keV}$ ($L_{2500}$) 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 $L_{2keV}-L_{2500}$ relation in both variables, i.e. in X-ray and UV luminosity. The intrinsic X-ray scatter (${\sigma }_{\mathrm{intX}}^2=0.063\pm 0.005$) strongly dominates over the UV one (${\sigma }_{\mathrm{intUV}}^2=0.002_{-0.002}^{+0.003}$). 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.

arXiv comment29 pages, 5 tables, 18 figures

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