1,721,068 research outputs found
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
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
The evolution of the QSO luminosity function up to z = 4: The role of K-correction uncertainties
No full textUncertainties in the spectral index of the optical spectrum of QSOs induce systematic biases in the estimate of the cosmological evolution of the luminosity function. In a framework of pure luminosity evolution, we obtain a slower intrinsic evolution of the luminosity function for z < 2.2 after correction for uncertainties in the adopted K-correction. The slower evolution is consistent with the apparent density decline of high-redshift QSOs (z > 2.5) observed in the red band. Indeed, we find that a scenario of uniform luminosity evolution up to z congruent-to 4 is suggested. The apparent decline in the redshift distribution is explained in terms of the sliding down along the steep side of the luminosity function for increasing redshift
Quasar spectral variability from the XMM-Newton serendipitous source catalogue
Full text linkContext. X-ray spectral variability analyses of active galactic nuclei (AGN)
with moderate luminosities and redshifts typically show a softer when brighter
behaviour. Such a trend has rarely been investigated for high-luminosity AGNs
( erg/s), nor for a wider redshift range (e.g.
). Aims. We present an analysis of spectral variability
based on a large sample of 2,700 quasars, measured at several different epochs,
extracted from the fifth release of the XMM-Newton Serendipitous Source
Catalogue. Methods. We quantified the spectral variability through the
parameter defined as the ratio between the change in the photon index
and the corresponding logarithmic flux variation,
. Results. Our analysis confirms a softer
when brighter behaviour for our sample, extending the previously found general
trend to high luminosity and redshift. We estimate an ensemble value of the
spectral variability parameter . We do not find dependence
of on redshift, X-ray luminosity, black hole mass or Eddington ratio. A
subsample of radio-loud sources shows a smaller spectral variability parameter.
There is also some change with the X-ray flux, with smaller (in
absolute value) for brighter sources. We also find significant correlations for
a small number of individual sources, indicating more negative values for some
sources
Ensemble spectral variability study of Active Galactic Nuclei from the XMM-Newton serendipitous source catalogue
No full textThe variability of the X-Ray spectra of active galactic nuclei (AGN) usually includes a change of the spectral slope. This has been investigated for a small sample of local AGNs by Sobolewska and Papadakis [1], who found that slope variations are well correlated with flux variations, and that the spectra are typically steeper in the bright phase (softer when brighter behaviour). Not much information is available for the spectral variability of high-luminosity AGNs and quasars. In order to investigate this phenomenon, we use data from the XMM-Newton Serendipitous Source Catalogue, Data Release 5, which contains X- Ray observations for a large number of active galactic nuclei in a wide luminosity and redshift range, for several different epochs. This allows to perform an ensemble analysis of the spectral variability for a large sample of quasars. We quantify the spectral variability through the spectral variability parameter β, defined by Trevese and Vagnetti [2] as the ratio between the change in spectral slope and the corresponding logarithmic flux variation. We find that the spectral variability of quasars has a softer when brighter behaviour, similarly to local AGNs
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
A new approach to the variability characterization of active galactic nuclei
No full textThe normalized excess variance is a popular method used by many authors to estimate the variability of active galactic nuclei (AGNs), especially in the X-ray band. We show that this estimator is affected by the cosmological time dilation, so that it should be appropriately corrected when applied to AGN samples distributed in wide redshift intervals. We propose a formula to modify this estimator, based on the use of the structure function. To verify the presence of the cosmological effect and the reliability of the proposed correction, we use data extracted from the XMM-Newton Serendipitous Source Catalogue, data release 5 (XMMSSC-DR5), and cross-matched with the Sloan Digital Sky Survey quasar catalogue, of data release 7 and 12
The evolutionary unified scheme. I. Quasars and radio galaxies in the viewing angle-redshift plane
No full textWe present a study of the distribution of quasars and radio galaxies in the viewing-angle-redshift plane in the framework of the evolutionary unified scheme (Vagnetti, Giallongo, and Cavaliere 1991). Results are presented for some illustrative cases, including a distribution of the Lorentz factors and appropriate luminosity functions for quasars and their host galaxies. A cosmologically increasing [Gamma] is found, in agreement with the previous paper. It is argued that the appearance of sources as quasars or radio galaxies can depend on the viewing angle and on the redshift, due to the balance between the beamed component and the luminosity of the host galaxy. Within the assumptions of our evolutionary unified scheme, we find that low-Gamma objects can be observed as quasars mainly at z greater than or similar to 0.3, while a substantial fraction of the low-z radio galaxies could consist of quasar remnants
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