1,721,295 research outputs found
THE IRON LINE AND HIGH-ENERGY BUMP AS X-RAY SIGNATURES OF COLD MATTER IN SEYFERT-1 GALAXIES
No full textGinga observations of several bright Seyfert 1 galaxies have recently revealed the presence in their X-ray spectra of both the iron K fluorescence line and a high energy bump, which together are currently interpreted as due to the reprocessing of the primary radiation by cold matter located near the putative black hole. A number of quantitative model predictions on the reprocessed radiation, when the matter is assumed to be distributed in an accretion disk, are presented in this paper; they were obtained, on the basis of Monte Carlo simulations, for different geometries of the disk surface and of the primary X-ray source. The case of a flat disk and a point source placed above the disk on its symmetry axis is treated in greater detail, taking into account the general relativistic and kinematic effects, which have dramatic consequences on the shape of the K line and edge. These effects are less relevant for the equivalent width (W-alpha) of the line and for the strength of the high energy bump, and were therefore ignored for simplicity in computing these two quantities for other source geometries (an optically thin corona, which turns out to be well approximated by the point source, and an optically thick sphere), and for other configurations of the disk surface (conical and concave). Concerning W-alpha, its maximum value is about 150 eV for a flat, 180 eV for a conical and 230 eV for a concave disk seen face-on. Concerning the strength of the bump, it turns out to be an excellent indicator of strong deviations from the planar configuration, and it is shown that, being practically independent of the iron abundance, it can be used to constrain the abundance when used in conjunction with W-alpha. Several observational implications are discussed, in particular it is noted that the relativistic and kinematic effects on the shape of the edge might lead to a misinterpretation of low resolution measurements in terms of absorption by highly ionized iron
The ROSAT PSPC spectrum of the Seyfert 2 galaxy NGC 4388
No full textThe ROSAT PSPC spectrum of the Seyfert 2 galaxy NGC 4388 is analyzed, with the aim to better understand the nature of the extended (a few kpc) soft X-ray emission discovered by the ROSAT HRI. Fitting the spectrum with an optically thin thermal model we find evidence for a metal abundance significantly lower than solar. We therefore argue that the emission is due at least in part to another component, maybe unresolved emission from compact sources
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
Spectral Properties of the Prompt X-ray emission and Afterglow from the Gamma-Ray Burst of 28 February 1997
No full textASTROPHYSICS JOURNA
Collective olfactory search in a turbulent environment
No full textFinding the source of an odor dispersed by a turbulent flow is a vital task for many organisms. When many individuals concurrently perform the same olfactory search task, sharing information about other members' decisions can potentially boost the performance. But how much of this information is actually exploitable for the collective task? Here we show, in a model of a swarm of agents inspired by moth behavior, that there is an optimal way to blend the private information about odor and wind detections with the public information about other agents' heading direction. Our results suggest an efficient multiagent olfactory search algorithm that could prove useful in robotics, e.g., in the identification of sources of harmful volatile compounds
AG Draconis
No full textL. Piro, Istituto TESRE, Bologna; A. Cassatella, IUE Observatory, Villafranca; L. Spinoglio and R. Viotti, Istituto Astrofisica Spaziale, Frascati; and A. Altamore, Istituto Astronomico, Rome, telex: "We detected x-ray emission from the high-velocity symbiotic star AG Dra at minimum phase. The star was observed with EXOSAT on June 5. Using a bremsstrahlung model with log nH = 20.2, we find kT = 24 eV and a 0.2-1.0-keV flux of 3.4 x 10**-16 J m**-2 s**-1. Coordinated IUE observations on June 9 showed an ultraviolet spectrum typical of minimum phase. There was no ultraviolet contamination of EXOSAT fluxes. The fine-error-sensor magnitude was 9.8. Infrared photometry on May 2 and 23 gave: J = 7.12, H = 6.36, K = 6.27, confirming that the K star is substantially stable.
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