1,720,957 research outputs found
X-RAY-VERSUS OPTICALLY SELECTED ACTIVE GALACTIC NUCLEI - A COMPARATIVE-STUDY OF THE LUMINOSITY FUNCTIONS AND EVOLUTION RATES
No full textWe compare statistical and evolutionary properties of active galactic nuclei (AGNs) and quasars selected at optical and soft X-ray wavelengths. We show that two related questions - the relationship between X-ray and optical emission and the evolution rate of X-ray-selected AGNs . probably require major revisions. New data from ROSAT deep surveys and a reanalysis of the Einstein Medium Sensitivity Survey (EMSS) indicate that the evolution rate of X-ray AGNs could have been underestimated and could actually be comparable to that of optical quasars. We also find that optical and X-ray emissions of X-ray-selected AGNs scale linearly. Adoption of our new best solutions for the optical and X-ray evolution models, and for the L(O)/L(X) dependence, leads to an impressive agreement of the luminosity functions (LFs) over wide luminosity and redshift intervals: the two wavebands appear to select essentially the same population of AGNs. On this basis, we argue that the small fraction of spurious contaminants, which allows unbiased sampling all over the luminosity-redshift plane, provides X-ray observations with considerable advantages over the usual optical selection techniques
ANGULAR-CORRELATIONS OF THE X-RAY-BACKGROUND AND CLUSTERING OF EXTRAGALACTIC X-RAY SOURCES
No full textThe information content of the autocorrelation function (ACF) of intensity fluctuations of the X-ray background (XRB) is analyzed. The tight upper limits set by ROSAT deep survey data on the ACF at arcminute scales imply strong constraints on clustering properties of X-ray sources at cosmological distances and on their contribution to the soft XRB. If quasars have a clustering radius r0 = 12-20 Mpc (H-0 = 50), and their two-point correlation function, xi(r), is constant in comoving coordinates (epsilon = - 1.2), as indicated by optical data, they cannot make up more than 40%-50% of the soft XRB (higher contributions corresponding to lower r0); the maximum contribution may reach 80% in the case of stable clustering (epsilon = 0). If r0 greater-than-or-equal-to 12 Mpc, a slow decrease of the xi(r) of the AGNs with increasing redshift (epsilon congruent-to - 3) is ruled out since it would imply an implausibly low contribution to the XRB. Active star-forming (ASF) galaxies clustered like normal galaxies (r0 congruent-to 12 Mpc) can yield up to 20% or up to 40% of the soft XRB for epsilon = - 1.2 or epsilon = 0, respectively. The ACF on degree scales, typical of existing hard X-ray surveys, essentially reflects the clustering properties of local sources and is proportional to their volume emissivity. The upper limits on scales of a few degrees imply that hard X-ray-selected AGNs have r0 less-than-or-equal-to 25 Mpc if epsilon = 0 or r0 less-than-or-equal-to 20 Mpc if epsilon = - 1.2. No significant constraints are set on clustering of ASF galaxies, due to their low local volume emissivity. The possible signal on scales greater-than-or-equal-to 6-degrees, if real, may be due to AGNs with r0 congruent-to 20 Mpc; the contribution from clusters of galaxies with r0 congruent-to 50 Mpc is a factor congruent-to 2 lower. Other classes of sources clustered like normal galaxies could only account for such signal if their local volume emissivity is almost-equal-to 6 x 10(38) ergs s-1 Mpc-3. This limit is lower than the values indicated by recent, indirect, estimates and implies that the bulk of the XRB is not local. We have also computed the expected ACF in the 2-10 keV energy band on arcminute scales, that will be useful for comparison to the upcoming data from the Astro-D satellite. Although the nature of sources producing the bulk of the soft and of the hard XRB is likely to be different, their clustering properties appear to be not much different from those of normal galaxies
HARD AND SOFT-X-RAY SELECTED ACTIVE GALACTIC NUCLEI - 2 DISTINCT POPULATIONS
No full textWe report the results of a thorough analysis of the evolutionary properties of active galactic nuclei, exploiting data from both hard and soft X-ray surveys [counts, luminosity and redshift distributions, average X-ray spectral properties and P(D) distributions]. Strong indications are found that soft (< 3 keV) and hard X-ray surveys emphasize two distinct classes of AGN that have different X-ray spectra, luminosity functions and evolution. 'Soft X-ray' AGN have higher luminosities, steep source-count slopes and high evolution rates, similar to those of optically selected QSOs; their contribution to the 2-10 keV X-ray background (XRB) is probably < 20 per cent. 'Hard X-ray' AGN have relatively low luminosities and strong photoelectric absorption. The average spectra of AGN detected in deep soft X-ray surveys imply that hard X-ray AGN either undergo a very weak luminosity/density evolution or that such evolution is accompanied by a substantial increase with redshift of the absorbing column densities; in the latter case they can easily contribute the bulk of the XRB above 3 keV. The existence of a new AGN population, essentially invisible below 10 keV, and accounting for a large fraction of the hard XRB, is also discussed: constraints from the HEAO-1 A2 (and to some extent from A4) fluctuation data are pointed out. AGN can hardly account for the soft XRB intensity, especially if it is significantly higher than implied by a simple extrapolation of higher energy measurements; contributions from other classes of source (e.g. clusters of galaxies and active star-forming galaxies) are required
SMALL-SCALE FLUCTUATIONS AND ANGULAR-CORRELATIONS OF THE X-RAY-BACKGROUND IN THE HEAO-1 A-2 ENERGY-BAND - CONSTRAINTS ON CLUSTERING OF X-RAY SOURCES
No full textUsing HEAO 1 A-2 all-sky survey data we have determined the amplitude of intensity fluctuations of the extragalactic 2-10 keV X-ray background (XRB) over an effective solid angle of 1.84 deg2 and their angular correlation function on angular scales of less than 3-degrees (i.e., smaller than the detector's field of view). The observed fluctuations are congruent-to 3 sigma above those expected if the counts keep a Euclidean slope well below the limit of the Piccinotti survey, as indicated by the Einstein Observatory medium sensitivity and deep surveys. A good empirical fit to the data is obtained assuming that the integral counts in the A-2 band have a slope of 1.65(-0.07)+0.06, fully consistent with the best-fit slope of 1.72(-0.10)+0.15 obtained by Piccinotti et al. directly from source counts with the same experiment. Alternatively, the data may imply a significant clustering of extragalactic X-ray sources. However, the non-Poisson constribution of rich clusters of galaxies with clustering radii consistent with optical data (r0 less-than-or-equal-to 50-60 Mpc, H0 = 50) do not fully account for the observed signal. Clustering of AGNs could do the job; the required amplitude of the two-point correlation function depends primarily on the evolution of their luminosity and/or space density and of their clustering properties. The resulting values are consistent with optical data on quasar clustering
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Get PDFWe 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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