Context. Most investigations of the X-ray variability of active galactic nuclei (AGN) have been concentrated on the detailed analyses of individual, nearby sources. A relatively small number of studies have treated the ensemble behaviour of the more general AGN population in wider regions of the luminosity-redshift plane. Aims. We want to determine the ensemble variability properties of a rich AGN sample, called Multi-Epoch XMM Serendipitous AGN Sample (MEXSAS), extracted from the fifth release of the XMM-Newton Serendipitous Source Catalogue (XMMSSC-DR5), with redshift between ∼0.1 and ∼5, and X-ray luminosities in the 0.5-4.5 keV band between ∼10 42 erg/s and ∼10 47 erg/s. Methods. We urge caution on the use of the normalised excess variance (NXS), noting that it may lead to underestimate variability if used improperly. We use the structure function (SF), updating our previous analysis for a smaller sample. We propose a correction to the NXS variability estimator, taking account of the light curve duration in the rest frame on the basis of the knowledge of the variability behaviour gained by SF studies. Results. We find an ensemble increase of the X-ray variability with the rest-frame time lag τ, given by SF ∝ τ 0.12. We confirm an inverse dependence on the X-ray luminosity, approximately as SF ∝ L −0.19 X. We analyse the SF in different X-ray bands, finding a dependence of the variability on the frequency as SF ∝ ν −0.15 , corresponding to a so-called softer when brighter trend. In turn, this dependence allows us to parametrically correct the variability estimated in observer-frame bands to that in the rest frame, resulting in a moderate (15%) shift upwards (V-correction). Conclusions. Ensemble X-ray variability of AGNs is best described by the structure function. An improper use of the normalised excess variance may lead to an underestimate of the intrinsic variability, so that appropriate corrections to the data or the models must be applied to prevent these effects.

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 α 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 α 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 α 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 α ox indices. We use ensemble structure functions to analyse multi-epoch data.Results. We confirm the anticorrelation of α ox with L UV , and do not find any evidence of a dependence of α 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 α ox variability" introduced by non-simultaneity is not the main cause of dispersion. "Intrinsic α 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 α 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.

Context. Variability, both in X-ray and optical/UV, affects the well-known anti-correlation between the α ox spectral index and the UV luminosity of active galactic nuclei, contributing part of the dispersion around the average correlation (intra-source dispersion) in addition to the differences among the time-average α ox values from source to source (inter-source dispersion). Aims. We aim to evaluate the intrinsic α ox variations in individual objects and their effect on the dispersion of the α ox − L UV anticorrelation. Methods. We used simultaneous UV/X-ray data from Swift observations of a low-redshift sample to derive the epoch-dependent α ox (t) indices. We corrected for the host galaxy contribution by a spectral fit of the optical/UV data. We computed ensemble structure functions to analyse the variability of multi-epoch data.Results. We find a strong intrinsic α ox variability, which significantly contributes (∼40% of the total variance) to the dispersion of the α ox − L UV anti-correlation (intra-source dispersion). The strong X-ray variability and weaker UV variability of this sample are comparable to other samples of low-z active galactic nuclei, and are neither caused by the high fraction of strongly variable narrow line Seyfert 1 galaxies, nor by dilution of the optical variability by the host galaxies. Dilution instead affects the slope of the anticorrelation, which steepens, once corrected, and becomes similar to higher luminosity sources. The structure function of α ox increases with the time lag up to about one month. This indicates the important contribution of the intermediate-to-long timescale variations, which are possibly generated in the outer parts of the accretion disk.

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Italian teams have been involved many times in Space Weather observational campaigns from space and from the ground, contributing in the advancing of our knowledge on the properties and evolution of the related phenomena. Numerous Space Weather forecasting and now-casting modeling efforts have resulted in a remarkable add-on to the overall progress in the field, at both national and international level. The Italian Space Agency has participated several times in space missions with science objectives related to Space Weather; indeed, an important field for the Italian scientific and industrial communities interested in Heliophysics and Space Weather, is the development of new instrumentation for future space missions. In this paper, we present a brief state-of-the-art in Space Weather science in Italy and we discuss some ideas on a long-term plan for the support of future scientific research in the related disciplines. In the context of the current roadmap, the Italian Space Agency aims to assess the possibility to develop a national scientific Space Weather data centre to encourage synergies between different science teams with interest in the field and to motivate innovation and new mission concept development. Alongside with the proposed recommendations, we also discuss how the Italian expertise could complement international efforts in a wider international Space Weather context.