Dario Trevese

The ensemble variability properties of faint QSOs

A refined sample of 64 variable objects with stellar image structure has been identified in SA 57 to B approximately = 22.5, over a time baseline of 15 yr, sampled at 11 distinct epochs. The photometric data typically have a root-mean-square (rms) error at B = 22 of only 0.05 mag. Thirty-five quasars in this field have already been spectroscopically confirmed, 34 of which are among the sample of variables. Of the other variables, six are known spectroscopically to be stars, 10 additional objects are stars based on reliable detection of proper motion, and one is spectroscopically a narrow emission line galaxy. Of the 13 remaining variables, it is argued that they are a mixture of distant halo subdwarfs and quasars with starlike colors. We compute the ensemble average structure function and autocorrelation function from the light curves in the respective quasar rest frames, which are used to investigate the general dependences on apparent magnitude, absolute magnitude, and redshift.

Quasar spectral slope variability in the optical band

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 an intra-QSO α-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.

Spectral Slope Variability of BL Lacertae Objects in the Optical Band

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 by Trevese & Vagnetti and appear to be 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.

An X-Ray and Optical Study of Matter Distribution in Clusters of Galaxies

A sample of 12 Abell clusters of galaxies has been studied in the optical and X-ray bands. The optical data are derived from automatic photometry based on microdensitometric scans of Palomar Schmidt F-band plates. ROSAT PSPC data were obtained from the public archive. Galaxy density and X-ray luminosity profiles have been constructed and deprojected by parametric and nonparametric techniques to obtain volume density profiles. We find a relation between the gas density ρgas and the galaxy density ρgal consistent with the prediction of the hydrostatic isothermal model. We confirm that more than 70% of the cluster mass is dark within 1.5 Mpc h -->−150 from the cluster center. Outside 250 kpc we find a nearly constant M/LV ratio, which on average is 137 M☉/L☉, implying that the galaxy mass traces the total matter distribution. The dark mass-to-light ratio has a similar behavior. We also find a baryonic fraction fb ≈ 0.2, which, assumed as representative of the cosmic value and compared with nucleosynthesis calculations, constrains the cosmological parameter Ω0 to be smaller than 0.25.

Continuum Variability of Active Galactic Nuclei in the Optical-Ultraviolet Range

The variability of the continuum spectral energy distribution has been analyzed for a complete magnitude-limited sample of quasars in Selected Area 57, observed at two epochs in the photographic U, BJ, F, and N bands with the Mayall 4 m telescope at Kitt Peak National Observatory. Changes δα of the spectral slope α appear correlated with brightness variations δ log fν, indicating an average hardening of the spectrum in the bright phases. This confirms that the correlation of variability with redshift, found in a single observing band, is due to intrinsic spectral changes. The average observed δα-δ log fν relation is consistent with the spectral change due to temperature variation of a blackbody of about 2.5 × 104 K.

The orientation of galaxies in clusters

We present a statistical analysis of directional data for the 10-20 brightest galaxies in a sample of 55 clusters. The data are derived from the digitization of Palomar 48 in. Schmidt telescope plates. We consider the orientation of clusters, of each galaxy, the direction of the line joining each galaxy with the cluster center or with the brightest galaxy, and all possible relative angles between these directions. We apply various statistical tests and we find no significant alignment for galaxies fainter than the two first ranked cluster members. We confirm a strong alignment of the brightest galaxy major axis with the long axis of the parent cluster

A PHOTOMETRICALLY DETECTED FORMING CLUSTER OF GALAXIES AT REDSHIFT 1.6 IN THE GOODS FIELD

We report the discovery of a localized overdensity at z∼1.6 in the GOODS-South Field, presumably a poor cluster in the process of formation. The three-dimensional galaxy density has been estimated on the basis of well calibrated photometric redshifts from the multiband photometric GOODS-MUSIC catalog using the (2+1)D technique. The density peak is embedded in the larger scale overdensity of galaxies known to exist at z=1.61 in the area. The properties of the member galaxies are compared to those of the surrounding field and we found that the two populations are significantly different supporting the reality of the structure. The reddest galaxies, once evolved according to their best fit models, have colors consistent with the red sequence of lower redshift clusters. The estimated M200 total mass of the cluster is in the range 1.3 × 10 14 − 5.7 × 10 14 M⊙, depending on the assumed bias factor b. An upper limit for the 2-10 keV X-ray luminosity, based on the 1Ms Chandra observations, is LX = 0.5 � 10 43 erg s −1 , suggesting that the cluster has not yet reached the virial equilibrium. Subject headings: Galaxies: clusters: general – Galaxies: evolution – Galaxies: formation – Galaxies: distances and redshifts

Variable Extended Objects in Selected Area 57

We have isolated a sample of 14 candidate variable objects with extended image structure to BJ = 22.5 in 0.284 deg2 of Selected Area 57. The majority of candidates are blue (U-B -23 active galactic nuclei (AGNs) to BJ = 22, including stellar sources, is ~40 deg-2, in good agreement with other surveys at this depth. The confirmed AGNs in extended sources make up 36% of this population. Thus, the application of a variability criterion to images with extended structure enhances the completeness of the census of active nuclei. If the majority of our candidates are bona fide AGNs, the surface density could be as high as 82 deg-2 for MB > -23 and 162 deg-2 for all luminosities to BJ = 22, with extended sources contributing up to 33% of the total.

Variability and the X-ray/UV ratio of active galactic nuclei

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 αox 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 αox − 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 αox − 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 αox indices. We use ensemble structure functions to analyse multi-epoch data. Results. We confirm the anticorrelation of αox with LUV, and do not find any evidence of a dependence of αox on z. The dispersion in our simultaneous data (σ ∼ 0.12) is not significantly smaller than in previous non-simultaneous studies, suggesting that “artificial αox variability” introduced by non-simultaneity is not the main cause of dispersion. “Intrinsic αox 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 αox 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.

Optical variability of quasars: Statistics and cosmological properties

The long-term optical variability of a composite, complete sample of quasars is investigated. While no correlation with luminosity is detected, a positive correlation with redshift is found (rho = 0.25). These results are reconciled with previous opposite findings of other authors, arguing that the analysis of variability can be biased by the combined effect of the structure function of light curves and of cosmological time dilation. Such bias becomes negligible if variability is measured by a magnitude difference at a fixed and large time lag (4 yr). Interpretation of this cosmological trend is given in terms of spectral variability. Statistical biases on the estimate of the evolution rate for the quasar population are discussed.