1,720,984 research outputs found
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
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
Ensemble X-ray variability of active galactic nuclei. II. Excess variance and updated structure function
Full text linkContext. Most investigations of the X-ray variability of active galactic nuclei (AGN) have been concentrated on the detailed analyses of individual, nearby sources. A relatively small number of studies have treated the ensemble behaviour of the more general AGN population in wider regions of the luminosity-redshift plane.
Aims: We want to determine the ensemble variability properties of a rich AGN sample, called Multi-Epoch XMM Serendipitous AGN Sample (MEXSAS), extracted from the fifth release of the XMM-Newton Serendipitous Source Catalogue (XMMSSC-DR5), with redshift between ~0.1 and ~5, and X-ray luminosities in the 0.5-4.5 keV band between ~10^42 erg/s and ~10^47 erg/s.
Methods: We urge caution on the use of the normalised excess variance (NXS), noting that it may lead to underestimate variability if used improperly. We use the structure function (SF), updating our previous analysis for a smaller sample. We propose a correction to the NXS variability estimator, taking account of the light curve duration in the rest frame on the basis of the knowledge of the variability behaviour gained by SF studies.
Results: We find an ensemble increase of the X-ray variability with the rest-frame time lag τ, given by SF ∝ τ^0.12. We confirm an inverse dependence on the X-ray luminosity, approximately as SF ∝ LX^-0.19. We analyse the SF in different X-ray bands, finding a dependence of the variability on the frequency as SF ∝ ν^-0.15, corresponding to a so-called softer when brighter trend. In turn, this dependence allows us to parametrically correct the variability estimated in observer-frame bands to that in the rest frame, resulting in a moderate (≲15%) shift upwards (V-correction).
Conclusions: Ensemble X-ray variability of AGNs is best described by the structure function. An improper use of the normalised excess variance may lead to an underestimate of the intrinsic variability, so that appropriate corrections to the data or the models must be applied to prevent these effects
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
Exploring the multiphase medium in MKW 08: from the central active galaxy up to cluster scales
Full text linkContext. The study of the brightest cluster galaxy (BCG) coronae embedded in noncool core (NCC) galaxy clusters is crucial to understand the BCG’s role in galaxy cluster evolution as well as the activation of the self-regulated cooling and heating mechanism in the central regions of galaxy clusters.
Aims. We explore the X-ray properties of the intracluster medium (ICM) of the NCC galaxy cluster MKW 08 and the BCG corona, along with their interface region. With recent and deep archival Chandra observations, we study the BCG corona in detail, and with archival XMM-Newton observations, we investigate the implications of the central active galactic nuclei (AGN) on the BCG.
Methods. We carry out imaging and spectral analyses of MKW 08 with archival XMM-Newton and Chandra X-ray observations.
Results. Our spectral analysis suggests the presence of a central AGN by a power-law with a photon index of Γ ≃ 1.8 at the core of its BCG. Although the ICM does not exhibit a cluster scale cool core, the BCG manifests itself as a mini cool core characterized by a cooling time as short as 64 Myr at r = 3 kpc centered at the galaxy. The isothermality of the BCG corona seems to favor mechanical feedback from the central AGN as the major source of gas heating. The gas pressure profile of this mini cool core suggests that the BCG coronal gas reaches pressure equilibrium with the hotter and less dense ICM inside an interface of nearly constant pressure, delimited by radii 4 ≤ r ≤ 10 kpc at the galactic center. As revealed by the presence of a metal enriched tail (Z ≃ 0.5–0.9 Z⊙) extending up to 40 kpc, the BCG corona seems to be experiencing ram-pressure stripping by the surrounding ICM and/or interacting with a nearby galaxy, IC 1042
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
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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