| Abstract
| - Context. The observed relation between the X-ray radiation from active galactic nuclei, originating in the corona, and the optical/UV radiation from the disk is usually described by the anticorrelation between the UV to X-ray slope $\alpha_{\rm ox}$ and the UV luminosity. Many factors can affect this relation, including: i) enhanced X-ray emission associated with the jets of radio-loud AGNs, ii) X-ray absorption associated with the UV broad absorption line (BAL) outflows, iii) other X-ray absorption not associated with BALs, iv) intrinsic X-ray weakness, v) UV and X-ray variability, and non-simultaneity of UV and X-ray observations. The separation of these effects provides information about the intrinsic $\alpha_{\rm ox}$ - LUV relation and its dispersion, constraining models of disk-corona coupling. Aims. We use simultaneous UV/X-ray observations to remove the influence of non-simultaneous measurements from the $\alpha_{\rm ox}$ - LUV relation. Methods. We extract simultaneous data from the second XMM-Newton serendipitous source catalogue (XMMSSC) and the XMM-Newton Optical Monitor Serendipitous UV Source Survey catalogue (XMMOMSUSS), and derive the single-epoch $\alpha_{\rm ox}$ indices. We use ensemble structure functions to analyse multi-epoch data. Results. We confirm the anticorrelation of $\alpha_{\rm ox}$ with LUV, and do not find any evidence of a dependence of $\alpha_{\rm ox}$ on z. The dispersion in our simultaneous data ( σ~0.12) is not significantly smaller than in previous non-simultaneous studies, suggesting that “artificial $\alpha_{\rm ox}$ variability” introduced by non-simultaneity is not the main cause of dispersion. “Intrinsic $\alpha_{\rm ox}$ variability”, i.e., the true variability of the X-ray to optical ratio, is instead important, and accounts for ~30% of the total variance, or more. “Inter-source dispersion”, due to intrinsic differences in the average $\alpha_{\rm ox}$ values from source to source, is also important. The dispersion introduced by variability is mostly caused by the long timescale variations, which are expected to be driven by the optical variations.
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