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 _ { 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 _ { ox } - L _ { UV } 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 _ { ox } - L _ { UV } 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 _ { ox } indices .
We use ensemble structure functions to analyse multi-epoch data .

Results : We confirm the anticorrelation of \alpha _ { ox } with L _ { UV } , and do not find any evidence of a dependence of \alpha _ { ox } on z .
The dispersion in our simultaneous data ( \sigma \sim 0.12 ) is not significantly smaller than in previous non-simultaneous studies , suggesting that “ artificial \alpha _ { ox } variability ” introduced by non-simultaneity is not the main cause of dispersion .
“ Intrinsic \alpha _ { ox } variability ” , i.e. , the true variability of the X-ray to optical ratio , is instead important , and accounts for \sim 30 \% of the total variance , or more .
“ Inter-source dispersion ” , due to intrinsic differences in the average \alpha _ { 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 .

Conclusions :