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

TL;DR

This study analyzes the correlation between X-ray and UV luminosities in quasars using a large, complete sample, introducing a new method that accounts for variability, selection effects, and intrinsic scatter.

Contribution

It presents a novel approach for determining the X-ray/UV luminosity relation in quasars, explicitly modeling intrinsic scatter in both variables.

Findings

The slope of the $L_{2keV}$-$L_{2500}$ relation is 0.69±0.02.

Intrinsic X-ray scatter dominates over UV scatter in the relation.

The results are consistent with previous studies, with new insights into intrinsic variability.

Abstract

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 $l_{\rm X}\equiv\log(L_{\rm 2keV}/[{\rm erg\,s^{-1}\,Hz^{-1}}])$ and $l_{\rm UV}\equiv\log(L_{\rm 2500}/[{\rm erg\,s^{-1}\,Hz^{-1}}])$, we find the slope, $\gamma=0.69\pm0.02$ (hereafter all uncertainties are quoted at the 68% confidence level), and normalization, $l_{\rm X}=26.47\pm0.02$ 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 ($\sigma^2_{\rm intX}=0.063\pm0.005$) strongly dominates over the UV one ($\sigma^2_{\rm intUV}=0.002^{+0.003}_{-0.002}$). 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.