The variability of 30 PG quasars has been observed in the red band during three years. A rest-frame structure function analysis shows an increase of variability with the time interval up to 0.2 mag r.m.s. after two years. A comparison with the IUE data available for PG quasars shows that the variability increases with the rest-frame frequency at each time interval. Evidence for this effect is supported by the analysis of the average variability of other published quasar samples. The effect can completely account for the increase of variability with redshift found in other studies.Comment: 23 pages, uuencoded, compressed postscript file including figures, also available at http://itovf2.roma2.infn.it/vagnetti/cimp.ps, submitted to Ap

Light curves of eight BL Lac objects in the BVRI bands have been analyzed. All of the objects tend to be bluer when brighter. However spectral slope changes differ quantitatively from those of a sample of QSOs analyzed in a previous paper (Trevese & Vagnetti 2002) and appear consistent with a different nature of the optical continuum. A simple model representing the variability of a synchrotron component can explain the spectral changes. Constraints on a possible thermal accretion disk component contributing to the optical luminosity are discussed.

We performed a new analysis of B and R light curves of a sample of PG quasars. We confirm the variability-redshift correlation and its explanation in terms of spectral variability, coupled with the increase of rest-frame observing frequency for quasars at high redshift. The analysis of the instantaneous spectral slope for the whole quasar samples indicates both an inter-QSO and intra-QSO variability-luminosity correlation. Numerical simulations show that the latter correlation cannot be entirely due to the addition of the host galaxy emission to a nuclear spectrum of variable luminosity but constant shape, implying a spectral variability of the nuclear component. Changes of accretion rate are also insufficient to explain the amount of spectral variation, while hot spots possibly caused by local disk instabilities can explain the observations.Comment: 20 pages, 6 figures, to appear in Ap.J., January 200

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.