1,721,007 research outputs found
Monitoring the closest supermassive black hole: X-ray and infrared variability of Sgr A*
No full textEmission from Sgr A* is highly variable at both X-ray and infrared (IR) wavelengths. Observations over the last ~20 years have detected X-ray flares that rise above a quiescent thermal background about once per day, while faint X-ray flares from Sgr A* are undetectable below the constant thermal emission. In contrast, the IR emission of Sgr A* can be seen to be continuously variable. Recently, simultaneous observations have observed a rise in IR activity around the same time as every distinct X-ray flare, while the opposite is not always true (peaks in the IR emission are not always coincident with an X-ray flare). Characterizing the behaviour of these simultaneous X-ray/IR events and measuring any time lag between them can constrain models of the accretion flow and the emission mechanism. Using 100+ hours of overlapping data from a coordinated campaign between the Spitzer Space Telescope and the Chandra X-ray Observatory, we present results of the longest simultaneous IR and X-ray observations of Sgr A* taken to date. The cross-correlation between the IR and X-ray light curves in this unprecedented dataset, which include four modest X-ray/IR flares, show evidence that flaring in the X-ray may lead the IR by 10-20 minutes with 68% confidence, but remain consistent with simultaneity at 99.7% confidence. Simultaneous multi-wavelength observations of more bright flares will improve our ability to constrain the flare timing characteristics and emission mechanisms, and must be a priority for Galactic Center observing campaigns.L'émission du trou noir supermassif Sagittarius A* (Sgr A*) est variable à la fois aux longueurs d'onde des rayons X et infrarouges (IR). Les observations faites au cours des ~20 dernières années ont détecté des éruptions de rayons X comme des événements distincts au-dessus d'un arrière-plan calme, se produisant en moyenne une fois par jour. En revanche, l'émission IR semble varier continuellement. Récemment, des observations simultanées ont observé que les éruptions de rayons X s'accompagnent toujours d'une augmentation de l'activité IR, alors que l'inverse n'est pas toujours vrai (les pics de l'IR ne sont pas tous accompagnés de pics dans les rayons X). Caractériser le comportement de ces événements simultanés et mesurer tout décalage entre eux peut contraindre les modèles du flux d'accrétion et du mécanisme d'émission. Utilisant plus de 100 heures de données de chevauchement provenant d'une campagne coordonnée entre le Spitzer Space Telescope et le Chandra X-ray Observatory, nous présentons les résultats des plus longues observations IR et X simultanées de Sgr A * à ce jour. La corrélation croisée entre les courbes IR et rayons X montre que les pics des rayons X peuvent devancer ceux de l'IR de 10 - 20 minutes avec une confiance de 68%, mais rester cohérente avec la simultanéité à un niveau de confiance de 99,7%
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
Change points in sgr A*’s X-ray flaring rate: fact or artifact?
No full textAn unusual object, G2, had its pericenter passage around Sgr A*, the 4 million solar masses supermassive black hole in the Galactic Centre, in Summer 2014. Several research teams have reported evidence that following G2's pericenter encounter the rate of Sgr A*'s bright X-ray flares increased significantly. Our analysis carefully treats varying flux contamination from a nearby magnetic neutron star and is free from complications induced by using data from multiple X-ray observatories with different sensitivities and spatial resolutions. We test this scenario using a massive dataset from the Chandra X-ray Observatory, the only X-ray instrument that can spatially distinguish between Sgr A* and the nearby Galactic Centre magnetar throughout the full extended period encompassing G2's encounter with Sgr A*. We use X-ray data from the 3 Ms observations of the Chandra X-ray Visionary Program (XVP) in 2012 as well as an additional 1.5 Ms of observations up to 2018. We use detected flares to make distributions of flare properties. Using a simulation model of X-ray flares accounting for important factors such as the different Chandra instrument modes, we test the null hypothesis on Sgr A*'s bright (or any flare category) X-ray flaring rate around different potential change points. In contrast to previous studies, our results are consistent with the null hypothesis; the same model parameters produce distributions consistent with the observed ones around any plausible change point.Un objet inhabituel, G2, a atteint son péricentre autour de Sgr A*, le trou noir supermassif de 4 millions de masses solaires au centre de la galaxie, durant l'été 2014. Certains groupes de recherche ont affirmé que, suite à cela, le taux d'éruptions brillantes a augmenté significativement. Notre analyse traite avec soin la contamination de flux variable provenant du magnétar à proximité et n'a pas de complications induites par l'utilisation de données provenant de plusieurs observatoires à rayons X avec différentes sensibilités et résolutions spatiales. Nous testons ce scénario en utilisant un ensemble de données volumineux du Chandra X-ray Observatory, le seul instrument à rayons X capable de distinguer spatialement Sgr A* et le magnétar du Centre Galactique pendant toute la durée de la rencontre entre G2 et Sgr A*. Nous utilisons les données de rayons X issues des observations de 3 Ms du X-ray Visionary Program (XVP) de Chandra en 2012, ainsi que 1,5 Ms supplémentaires d’observations jusqu’en 2018. Nous utilisons les éruptions détectées afin de créer des distributions de leurs propriétés. À l'aide d'un modèle de simulations d'éruptions rayons X prenant en compte des facteurs importants tels que les différents modes d'instrument de Chandra, nous testons l'hypothèse nulle sur le taux d'éruptions rayons X lumineuses (ou de n'importe quelle autre catégorie) de Sgr A* autour de différents points de changement potentiels. Contrairement aux études précédentes, nos résultats sont en accord avec l'hypothèse nulle; les mêmes paramètres du modèle produisent des distributions cohérentes avec les distributions observées autour de tout point de changement plausible
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
- …
