1,720,978 research outputs found
Multi-wavelength observations of Galactic black hole X-ray transients
No full textGalactic black hole X-ray transients (BHXRTs) represent the ideal opportunity to study accretion physics in extreme environments. Questions relating to accretion geometry, the mass of the compact object, how transient outbursts start and how jets form and evolve can only be answered by observing BHXRTs across multiple wavelengths. In this thesis I take a multi-wavelength approach to studying BHXRTs, and attempt to reconcile many aspects of accretion physics using a number of techniques.I have used timing analysis techniques to discover a super-orbital periodicity of 420 days in the X-ray and optical light curves of the BHXRT Swift J1753.5-0127, whilst also uncovering the likely orbital period of the candidate BHXRT MAXI J1305-704. X-ray spectral observations of Swift J1753.5-0127 revealed its first ever transition to a soft accretion state. This was found to be one of the lowest luminosity soft states ever recorded in such a system, at < 1% the Eddington luminosity, which proved crucial in the subsequent radio observations. Collaborators and I found that the compact jet had been quenched by a factor > 25, indicating that jet quenching was not dependent on accretion rate.In the optical regime, spectroscopy can reveal details about the mass of the compact object, which is important for population studies of black holes (BHs). I place a lower limit on the mass of Swift J1753.5-0127at M1 > 7:41:2M, confirming it as a BH and removing it from the so-called 'mass gap.' However, optical observations can also reveal details about the accretion geometry of a source, as in the case of the BHXRT V404 Cyg, in which collaborators and I combined optical spectroscopy and photometry and found evidence for the 2015 outburst initiating 1 week before the X-ray outburst was detected. Our results were found to be consistent with the current disc instability model for transient outbursts. I summarise these findings and suggest avenues for future work in the closing pages of this thesis. Overall, this work shows that we can only truly understand BHXRTs once we have studied them across multiple wavelengths, and only in doing so can we discuss the underlying physics behind some of the most extreme regions of the Universe
Soft excess in the quiescent Be/X-ray pulsar RX J0812.4-3114
No full textWe report a 72 ks XMM-Newton observation of the Be/X-ray pulsar (BeXRP) RX J0812.4-3114 in quiescence (Lx=1.6x10^33 erg/s). Intriguingly, we find a two-component spectrum, with a hard power-law (Gamma=1.5) and a soft blackbody-like excess below =1 keV. The blackbody component is consistent in kT with a prior quiescent Chandra observation reported by Tsygankov et al. and has an inferred blackbody radius of =10 km, consistent with emission from the entire neutron star (NS) surface. There is also mild evidence for an absorption line at =1 keV and/or =1.4 keV. The hard component shows pulsations at P=31.908 s (pulsed fraction 0.84+/-0.10), agreeing with the pulse period seen previously in outbursts, but no pulsations were found in the soft excess (pulsed fraction <~ 31%). We conclude that the pulsed hard component suggests low-level accretion onto the neutron star poles, while the soft excess seems to originate from the entire NS surface. We speculate that, in quiescence, the source switches between a soft thermal-dominated state (when the propeller effect is at work) and a relatively hard state with low-level accretion, and use the propeller cutoff to estimate the magnetic field of the system to be <~8.4x10^11 G. We compare the quiescent thermal Lx predicted by the standard deep crustal heating model to our observations and find that RX J0812.4-3114 has a high thermal Lx, at or above the prediction for minimum cooling mechanisms. This suggests that RX J0812.4-3114 either contains a relatively low-mass NS with minimum cooling, or that the system may be young enough that the NS has not fully cooled from the supernova explosion
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
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
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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