Speaker
Description
The statistical analysis of the relationship between ultraviolet (UV) and X-ray emission from active galactic nuclei (AGN) allows for a better understanding of the processes occurring during accretion of matter onto a supermassive black hole in the center of AGN, and also tests the possibility of using quasars as "standard candles" for measuring distances on cosmological scales in the Universe. We have developed a new method for finding the parameters of this 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. We examine the relation between X-ray (at 2 keV, $L_{\rm X}$) and UV (at 2500Å, $L_{\rm UV}$) monochromatic luminosities of quasars using a sample of 2414 sources from the SRG/eROSITA all-sky survey and the Sloan Digital Sky Survey data release 16 quasar catalogue (SDSS DR16Q), which are sufficiently bright in X-ray and the optical to have well defined X-ray fluxes and nearly 100% statistical completeness. Assuming the relation of the form $l_{\rm X}=\gamma l_{\rm UV}+\beta$, where $l_{\rm X}≡\log(L_{\rm X}/[\rm erg\,s^{-1}\,Hz^{-1}])$ and $l_{\rm UV}\equiv \log(L_{\rm UV}/[\rm erg\,s^{-1}\,Hz^{-1}])$, we find $\gamma=0.69\pm0.02$, and normalization, $l_{\rm X}=26.47\pm0.02$ at $l_{\rm UV}=30.5$. A 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 $L_{\rm X} - L_{\rm UV}$ 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 ($\sigma^2_{\rm intUV}=0.002^{+0.003}_{-0.002}$).