1,720,985 research outputs found
X-ray spectral variability of Active Galactic Nuclei from XMM-Newton data
Full text linkThe X-ray spectral variability of Active Galactic Nuclei typically follows a “softer when brighter” trend, which is believed to be originated mainly from the superposition of different spectral components, varying independently from each other, although some intrinsic variations of the continuum are also possible. We analyzed the MEXSAS sample, made up by more than 7800 observations from 2700 quasars from the fifth release of the XMM-Newton Serendipitous Source Catalogue (3XMM-SSC), cross-matched with two quasar catalogues, SDSS-DR7Q and SDSS-DR12Q. We developed a technique that is able to compute estimates of the photon index from approximate spectral fits, using the fluxes in the catalogue. Following Trevese & Vagnetti (2002), we quantify the spectral variability using β = −∆Γ/∆ log F. We find an ensemble softer when brighter trend, extending therefore this result to quasars, and the same result is found for eight single sources extracted from the catalogue, plus one (PG 1114+445) discarded because of the presence of a prominent warm absorber, although with different extent. To investigate the reason for this range of different values of beta, we investigated a sample of X-ray
bright sources taken from the samples of Sobolewska & Papadakis (2009), plus M81, for which it is possible to obtain accurate photon indices. We compute both the accurate and approximate photon indices and we confront β obtained with both methods. Finally, we studied the spectra of 13 observations of PG 1114+445, finding multiple absorbers, one possibly being a highly ionized, ultra-fast outflow, with velocity of about 15% of the speed of light, which is observable in four
observations
Ensemble quasar spectral variability from the XMM-Newton Serendipitous Source Catalogue
No full textVariations of the X-ray spectral slope have been found in many Active Galactic Nuclei (AGN) at moderate luminosities and redshifts, typically showing a "softer when brighter" behaviour. However, similar studies are not usually performed for high-luminosity AGNs. We present an analysis of the spectral variability based on a large sample of quasars in wide intervals of luminosity and redshift, measured at several different epochs, extracted from the fifth release of the XMM Newton Serendipitous Source Catalogue. Our analysis confirms
a "softer when brighter" trend also for our sample, extending to high luminosity and redshift the general behaviour previously found. These results can be understood in light of current spectral models, such as intrinsic variations of the X-ray primary radiation, or superposition with a constant reflection component
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
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
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
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
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
- …
