1,721,061 research outputs found
X-ray variability in the Chandra Deep Field South
No full textWe present the preliminary results of an X-ray variability study of sources detected in the Chandra Deep Field South (Giacconi et al. 2002). We both investigate the long-term variability, binning the lightcurves in order to achieve the maximum S/N ratio for faint sources, and analyze the short-term variability of stronger sources.
We found that the fraction of variable objects increases with total counts, consistent with the better statistics. To validate our quantitative analysis we perform extensive simulations mimicking the observing conditions for each source. We use our simulations to predict the fraction of sources which are variable.
We will quantify the variable flux fraction, and the dependence of variability on several physical parameters
X-ray variability in the Chandra Deep Field South
No full textWe present the preliminary results of an X-ray variability study of sources detected in the Chandra Deep Field South (Giacconi et al. 2002). We both investigate the long-term variability, binning the lightcurves in order to achieve the maximum S/N ratio for faint sources, and analyze the short-term variability of stronger sources.
We found that the fraction of variable objects increases with total counts, consistent with the better statistics. To validate our quantitative analysis we perform extensive simulations mimicking the observing conditions for each source. We use our simulations to predict the fraction of sources which are variable.
We will quantify the variable flux fraction, and the dependence of variability on several physical parameters
GOODS Optically Variable Galaxy Nuclei and Their Multiwavelength Properties
No full textIn both GOODS fields, the survey's HST/ACS observations were taken in five epochs with uniform ̃ 45-day spacing, for the primary purpose of high-z supernova detection and follow-up. However, this combination of sensitivity, high resolution, and time spacing is also well-suited to detecting optical variables (OVs) among ̃ 50000 moderate-z galaxy nuclei on rest-frame timescales of weeks to months. The overwhelming majority of these variable galaxy nuclei will be AGN; the small fraction arising from SNe have already been meticulously culled by the high-z SNe search effort. Previous OV studies in the HDF-N have shown relatively poor overlap with X-ray-identified AGN from CXO 2-Msec observations. Thus OV galaxy nuclei potentially represent a significant addition to the census of distant AGN subject to multiwavelength scrutiny with GOODS. We present the results of our z-band variability analysis, including the discovery of ̃ 170 ``significantly variable'' galaxy nuclei at z850 < 24.5AB whose light-curve excess variance is statistically inconsistent with non-variability at the 10-4 level or better. Although the GOODS sensitivity is less than previous two-epoch OV searches in the HDF-N, the much larger GOODS area compared to HDF-N results in a several-fold increase to the number of known OV AGN in the Chandra Deep Fields. Moreover, the GOODS OVs are broadly distributed in redshift out to z ̃ 1.3, comparable to the redshift distribution of the Chandra Deep Field sources
Gravitational lensing by cosmic string: what we learn from the CSL-1 case
No full textCosmic strings were postulated by Kibble in 1976 and, from a theoretical point of view, their existence finds support in modern superstring theories, both in compactification models and in theories with extended additional dimensions. Their eventual discovery would lead to significant advances in both cosmology and fundamental physics. One of the most effective ways to detect cosmic strings is through their lensing signatures which appear to be significantly different from those introduced by standard lenses (i.e. compact clumps of matter). In 2003, the discovery of the peculiar object CSL-1 raised the interest of the physics community since its morphology and spectral features strongly argued in favour of it being the first case of gravitational lensing by a cosmic string. In this paper we provide a detailed description of the expected observational effects of a cosmic string and show, by means of simulations, the lensing signatures produced on background galaxies. While high angular resolution images obtained with Hubble Space Telescope, revealed that CSL-1 is a pair of interacting ellipticals at redshift 0.46, it represents a useful lesson to plan future surveys
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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