1,721,101 research outputs found
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
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
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
Individual optical variability of active galactic nuclei from the MEXSAS2 sample
No full textAt present, most of the variability studies of active galactic nuclei (AGNs) are based on ensemble analyses. Nevertheless, it is interesting to provide estimates of the individual variability properties of each AGN, in order to relate them with intrinsic physical quantities. A useful data set is provided by the Catalina Surveys Data Release 2 (CSDR2), which encompasses almost a decade of photometric measurements of ∼500 million objects repeatedly observed hundreds of times. We aim to investigate the individual optical variability properties of 795 AGNs originally included in the Multi-Epoch XMM Serendipitous AGN Sample 2 (MEXSAS2). Our goals consist of (i) searching for correlations between variability and AGN physical quantities and (ii) extending our knowledge of the variability features of MEXSAS2 from the X-ray to the optical. We use the structure function (SF) to analyse AGN flux variations. We model the SF as a power law, SF(τ) = A(τ/τ0)γ, and we compute its variability parameters. We introduce the V-correction as a simple tool to correctly quantify the amount of variability in the rest frame of each source. We find a significant decrease of variability amplitude with increasing bolometric, optical and X-ray luminosity. We obtain the indication of an intrinsically weak positive correlation between variability amplitude and redshift, z. Variability amplitude also appears to be positively correlated with αox. The slope of the power-law SF, γ, is weakly correlated with the bolometric luminosity Lbol and/or with the black hole mass MBH. When comparing optical to X-ray variability properties, we find that X-ray variability amplitude is approximately the same for those AGNs with larger or smaller variability amplitude in the optical. On the contrary, AGNs with steeper SF in the optical do present steeper SF in the X-ray, and vice versa
Individual optical variability of active galactic nuclei from the MEXSAS2 sample
Full text linkMost of the variability studies of active galactic nuclei (AGNs) are based on
ensemble analyses. Nevertheless, it is interesting to provide estimates of the
individual variability properties of each AGN, in order to relate them with
intrinsic physical quantities. A useful dataset is provided by the Catalina
Surveys Data Release 2 (CSDR2), which encompasses almost a decade of
photometric measurements of million objects repeatedly observed
hundreds of times. We aim to investigate the individual optical variability
properties of 795 AGNs originally included in the Multi-Epoch XMM Serendipitous
AGN Sample 2 (MEXSAS2). Our goals consist in: (i) searching for correlations
between variability and AGN physical quantities; (ii) extending our knowledge
of the variability features of MEXSAS2 from the X-ray to the optical. We use
the structure function (SF) to analyse AGN flux variations. We model the SF as
a power-law, , and we compute its
variability parameters. We introduce the V-correction as a simple tool to
correctly quantify the amount of variability in the rest frame of each source.
We find a significant decrease of variability amplitude with increasing
bolometric, optical and X-ray luminosity. We obtain the indication of an
intrinsically weak positive correlation between variability amplitude and
redshift, . Variability amplitude is also positively correlated with
. The slope of the SF, , is weakly correlated with
the bolometric luminosity and/or with the black hole mass
. When comparing optical to X-ray variability properties, we find
that X-ray variability amplitude is approximately the same for those AGNs with
larger or smaller variability amplitude in the optical. On the contrary, AGNs
with steeper SF in the optical do present steeper SF in the X-ray, and vice
versa
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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