1,721,110 research outputs found
IRON K-ALPHA LINES FROM IONIZED DISKS IN Z-TYPE X-RAY BINARIES
No full textThe properties of the iron Kalpha line emitted by a photoionized accretion disc in a Z-type X-ray binary system are determined for different values of the mass accretion rate and disc inclination angle. We consider a geometry where the accretion disc is illuminated by X-rays emitted from a spherical, optically thick Comptonized cloud, representing the magnetosphere on the horizontal branch, and the magnetosphere plus puffed accretion disc on the normal and flaring branches. We find that line profiles can be complex due to Doppler, relativistic, shadowing, line-blending, resonant trapping and other effects. The iron Kalpha line equivalent width, intrinsic width and centroid energy vary in a tortuous manner as a function of the accretion rate, cloud radius and inclination angle. We examine the reasons for this behaviour and discuss how it will affect inferences about the physical states of Z-sources based on their iron line properties. We compare our results with available data, in particular on Cygnus X-2, and indicate how tests of Z-source models could be made using high-energy-resolution measurements like those expected from ASCA
The iron K alpha line complex in Compton-thick Seyfert 2 galaxies
No full textWe calculate the intensities of the fluorescent and resonantly scattered iron K alpha lines in those active galactic nuclei (AGN) in which the primary radiation at the line energy is completely blocked by intervening matter along the line of sight (the so-called 'Compton-thick' Seyfert 2 galaxies). In the framework of unification models for Seyfert galaxies, both a 'cold' fluorescent line from the obscuring torus and highly ionized lines from the warm material responsible for the scattering and polarization of optical photons are expected. The neutral iron line from the obscuring torus has an equivalent width, with respect to the continuum reflected by the same matter, of order 1-2 keV, provided that the matter is Compton thick. The line emission from the warm material, in the optically thin regime for all relevant processes, is dominated by the resonantly scattered lines. However, for plausible physical conditions of this matter, the resonant lines are likely to be optically thick in their centres. In this case, the resonant line intensity can significantly decrease, and for tau(T) greater than or similar to 0.1 the fluorescent/recombination lines dominate the resonant lines. We present analytical formulae for the equivalent widths of iron lines in the optically thin case, and numerical results (obtained by means of Monte Carlo simulations) for the optically thick case. We apply the theoretical results to the most famous Compton-thick source, the Seyfert 2 galaxy NGC 1068, The large uncertainties in the continuum spectral shape and line equivalent widths prevent any definite conclusions, but the application can be regarded as an example of how the results presented in this paper could be used to estimate physical and geometrical quantities in obscured AGN
A broad-band X-ray view of NGC 4945
No full textWe present the results of a BeppoSAX observation of the nearby spiral galaxy NGC 4945 in the 0.1-200 keV energy band. The nuclear X-ray emission emerges above similar or equal to 7 keV, through an absorber with column density N-H similar to a few 10(24) cm(-2) Its remarkable variability (doubling/halving timescale similar to 10(4) s) is not associated with any appreciable spectral changes, ruling out variations of the interposed absorber or changes in the primary continuum shape. The intense iron K-alpha, fluorescent emission line is likely to be produced by the same nuclear absorbing matte. Our estimate of the intrinsic nuclear power suggests that the active nucleus dominates the bolometric energy output ofNGC 4945, if its nucleus has a typical quasar L-X/L-bol ratio. This supports the idea that X-rays are the best energy band to search for absorbed AGN. The 0.1-5 keV emission is extended along the plane of the galaxy, and most likely due to a population of unresolved binaries, as believed to happen in several early-type galaxies. Alternatively, hot gas associated with a starburst-driven superwind outflow could substantially contribute to the extended emission above 1 keV
Probing the extremes of Seyfert activity: BeppoSAX observations of Narrow-Line Seyfert 1 galaxies
No full textResults are presented for the first year of observations of a selected sample of Narrow-Line Seyfert 1 galaxies (Ton S 180, RE J1034+396, Ark 564) obtained with the imaging instruments onboard BeppoSAX. These are the first simultaneous broad band (0.1-10 keV) spectra so far obtained for this class of objects
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
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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