1,720,990 research outputs found
Quasar spectral slope variability in the optical band
No full textWe 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 alpha-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 variability of quasars from the MACHO project database
No full textV- and R-passband light curves of a sample of 59 QSOs, most of which were detected on the basis of variability, have been recently made public as part of the MACHO project database. The high sampling frequency is unique for a statistical QSO sample. Preliminary results of the color variability as a function of redshift are presented
Medium-deep observations of QSOS/AGNS in the field of SA 57
No full textThe Selected Area 57 is one of the best studied fields of the sky at all wavelengths and AGN samples have been selected with different techniques, including optical variability. We present preliminary analysis of a medium-deep XMM-Newton observation of the field, yielding 140 X-ray sources, 90 of which are identified in the optical band
Variability-Selected Low Luminosity AGNs in the SA57 and in the CDFS
No full textLow Luminosity Active Galactic Nuclei (LLAGNs) are contaminated by the light of their host galaxies, thus they cannot be detected by the usual colour techniques. For this reason their evolution in cosmic time is poorly known. Variability is a property shared by virtually all active galactic nuclei, and it was adopted as a criterion to select them using multi epoch surveys. Here we report on two variability surveys in different sky areas, the Selected Area 57 and the Chandra Deep Field South
Optical variability of quasars: Statistics and cosmological properties
No full textThe 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
Variability and the X-ray/UV ratio of active galactic nuclei. II. Analysis of a low-redshift Swift sample
No full textVariability, both in X-ray and optical/UV, affects the well-known anti-correlation between the 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 values from source to source ("inter-source dispersion"). We want to evaluate the intrinsic variations in individual objects, and their effect on the dispersion of the anti-correlation. We use simultaneous UV/X-ray data from Swift observations of a low-redshift sample, to derive the epoch-dependent indices. We correct for the host galaxy contribution by a spectral fit of the optical/UV data. We compute ensemble structure functions to analyse variability of multi-epoch data. We find a strong "intrinsic variability", which makes an important contribution ( of the total variance) to the dispersion of the 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 AGNs, and are neither due to the high fraction of strongly variable NLS1s, nor to dilution of the optical variability by the host galaxies. Dilution affects instead the slope of the anti-correlation, which steepens, once corrected, becoming similar to higher luminosity sources. The structure function of increases with the time lag up to 1 month. This indicates the important contribution of the intermediate-long timescale variations, possibly generated in the outer parts of the accretion disk
Redshift dependence of QSO spectral variability
No full textWe present a combined analysis of the optical spectral variability for two samples of QSOs, 42 objects at z < 0.4 monitored at the Wise Observatory, plus 59 objects up to z approximate to 3 in the field of the Magellanic Clouds, detected and/or monitored within the MACHO Project database. Our analysis shows some increase of the observed spectral variability as a function of redshift, with a large scatter. These data are compared with a model based on the addition of flares of different, temperatures to a stationary quasar SED, also taking into account the intrinsic scatter of the SEDs
Spectroscopical confirmation of variability-selected active galactic nuclei
No full textThe cosmological evolution of active galactic nuclei (AGN) luminosity function is poorly known at the faint end, since active nuclei fainter than their host galaxies cannot be selected by color techniques. A sample of low luminosity AGN candidates has been selected on the basis of their variability. We carried out spectroscopic observations with the WYFFOS multi-fiber facility at the 4.2 m William Herschel Telescope. Preliminary results are presented, indicating the validity of the selection technique
XMM-Newton observation of the cluster Zw 1305.4+2941 in the field SA 57
No full textWe report the details of an XMM-Newton observation of the cluster of galaxies Zw 1305.4+2941 at the intermediate redshift z = 0.241, increasing the small number of interesting X-ray constraints on properties of similar to 3 keV systems above z = 0.1. Based on the similar to 45 ks XMM-Newton observation, we find that within a radius of 228 kpc the cluster has an unabsorbed X-ray flux of f(X) = (2.07 +/- 0.06) x 10(-13) ergs cm(-2) s(-1), a temperature of kT = 3.17 +/- 0.19 keV, in good agreement with the previous ROSAT determination, and an abundance of 0.93(-0.29)(+0.24) Z(circle dot). Within r(500) = 723 +/- 16 kpc the rest-frame bolometric X-ray luminosity is L-X(r(500)) = (1.25 +/- 0.16) x 10(44) h(70)(-2) ergs s(-1). The cluster obeys the scaling relations for L-X, T, and the velocity dispersion sigma(v) derived at intermediate redshift for kT less than or similar to 4 keV, for which we provide new fits for all objects in the literature. The mass derived from an isothermal NFW model fit is M-vir = (2.77 +/- 0.21) x 10(14) M-circle dot, with the concentration parameter c = 7.9 +/- 0.5
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