1,720,987 research outputs found
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
Signal Processing and Instrument Characterizationon a Large-N Radio Interferometer
Get PDFThe CHIME radio interferometer telescope was designed with the purpose of studying dark energy during an era of accelerated expansion. It will conduct a survey of large scale structure to sample the matter density field in HI emission and measure the time evolution of the dark energy equation of state, w. CHIME is a large-N telescope and features the largest GPU correlator in radio astronomy. Here I present my contributions to the CHIME project, which took place during my graduate career, between 2015-2022.
The GPU array cluster is supported by an enterprise scale software backend that I helped design to be portable and extensible by an object-oriented code infrastructure. The CHIME backend, kotekan, is built for multi-tiered task allocation across the GPU cluster that includes support tasks such as RFI excision, in addition to several operational modes including beamforming. Extensions to the architecture are designed for new science objectives and compliment the cosmology functionality to include new capabilities for FRB surveys and pulsar timing. The GPU management infrastructure I wrote and beamforming code I optimized are described here and directly impacted these objectives.
Calibration of the system is performed by precisely measuring the response of the instrument. This is accomplished by either setting configurations in advance of observation cycles or by manipulating the correlated visibilities offline. Statistical analysis of the data using conventional methods such as principal component analysis are applied to test the quality of the data by estimating the redundancy of baselines that are averaged to improve signal-to-noise. The work toward optimal calibration is an evolving process for greater fundamental understanding of the system.
The achieved performance of the existing calibration of the telescope can be evaluated through application in a scientific study. The study presented here is of the Cygnus Loop and a spatial spectral index variation mapping across the surface of the feature. The study incorporated every calibration developed by the collaboration and addressed every systematic present in the system. The calibrator 3C286 was selected to calibrate the spectral flux density and several beam models were applied to calibrate the spectral response.Ph.D
Instrument Design and Analysis Techniques for Low-Redshift 21 cm Cosmology and Transient Detection with CHORD and CHIME
No full textThe Canadian Hydrogen Observatory and Radio-transient Detector (CHORD) is a compact, innovative “large- , small-” radio interferometric array set for construction in British Columbianext to its older sibling, the Canadian Hydrogen Intensity Mapping Experiment (CHIME). With its 0.3–1.5 GHz band, sub-30 K system temperature, sub-millimeter precision dish surfaces, nearly 15,000 m collecting area, and very broad field of view, it is designed to map the Universe’s matter distribution up to redshifts of = 3.7 and boost fast radio burst (FRB) detection rate.
This work emphasizes that CHORD’s array elements be located as close as possible to enhance sensitivity to lower k-modes of the matter power spectrum and minimize transient search cost. It also introduces arguments for using very deep dishes () based on coupling and ground illumination considerations. Preliminary calibration results from the Deep Dish Development Array (D3A6) suggest that the signal chain’s system temperature matches specifications at lower frequencies ( < 0.8 GHz), while further testing is required for higher frequencies.
The CHORD feed, designed for the array, is inexpensive and quick to assemble, while maintaining a 30 K system temperature using ambient-temperature analog receiver hardware across the band. The design’s major novelty is an oversized backshort on an exponentially tapered (“Vivaldi”) design, adaptable to an observatory’s impedance and beam shape requirements via a simple optimization algorithm, particularly suitable for very deep dishes (). In those dishes, the aperture efficiency remains above 50% over the band.
Cosmological measurements are complicated by instrumental coupling and foregrounds. We show that coupling can be estimated from electromagnetic (EM) simulations of the array elements using a combination of their S-parameters, time-domain response, and beam shapes. Lowering the focal ratio of the dishes emerged as a powerful method to reduce coupling power. Foregrounds are typically removed from data using their spectral smoothness via a Fourier decomposition along the frequency axis; this can also be done with a singular value decomposition, and we successfully demonstrate this approach on CHIME data.Ph.D
Digital Signal Processing for the Canadian Hydrogen Intensity Mapping Experiment
No full textThis thesis presents the design, development, operation, and performance of digital signal processing systems for the Canadian Hydrogen Intensity Mapping Experiment (CHIME) and its Pathfinder. CHIME is designed to investigate the behaviour of Dark Energy in order to constrain its physical mechanism; it will do so by examining the geometry of the recent universe using as a `standard ruler' the imprint of the Baryon Acoustic Oscillation (BAO) feature in the 21 cm emission of neutral Hydrogen.
In order to construct the complete set of visibilities for CHIME's 2048 inputs and 400\,MHz of bandwidth, the FX correlator's X-Engine must be capable of ingesting 6 Tib/s of data and completing 8.4×1014 complex multiply-and-accumulate operations per second; these requirements make CHIME's X-Engine by far the largest currently in operation. Using mass-produced Graphics Processing Units (GPUs), commercial off-the-shelf computer hardware, and a sealed-loop liquid-cooling system, the CHIME correlator X-Engine achieves high performance and exceptional efficiency. This thesis presents a detailed description of the X-Engine hardware design and a thorough evaluation of its performance, with additional commentary on aspects of the design and deployment which may warrant additional consideration by those planning similar systems.
The substantial computational power available in the correlator system provided an opportunity for pre-integration excision of transient Radio-Frequency Interference (RFI). A number of RFI mitigation techniques were examined, including Median Absolute Deviation and Spectral Kurtosis measures; the latter was implemented on CHIME and the Pathfinder, and results from tests of the RFI detection system are presented.
The CHIME Pathfinder, in addition to acting as a test-bed for hardware and analysis techniques, produced an archive of visibility data spanning three years; from this, maps were produced covering the entire northern sky for 609 sidereal days. This thesis details the data processing pipeline which was employed to generate these maps as well as analysis of the resulting data products with regards to their overall variability and the properties of known point-like sources.Ph.D
Fast Radio Burst Localization with very Long Baseline Interferometry
No full textThis thesis covers the development of Very Long Baseline Interferometry (VLBI) localization techniques for Fast Radio Bursts (FRBs). FRBs were discovered only in the 2000s and have revolutionized the radio transient field due to their unknown and complex emission mechanisms, their brightness, and the potential use as probes of the Universe's cosmology. The aim of this thesis project is to enhance the localization potential of Canadian Hydrogen Intensity Mapping Experiment (CHIME) in order to identify galactic hosts, which are necessary to precisely measure redshifts of FRBs. The VLBI prototype station chosen, was the Algonquin Radio Observatory (ARO) 10-m telescope a decommissioned radio dish from the 1960s. During the first two years of PhD studies, a complete refurbishment of the system was done, which included an update on the analog signal chain and a new acquisition system. While upgrades were on their way, CHIME Fast Radio Burst Project (CHIME/FRB) detected its first FRB in 2018, and today more than thousand have been observed. The ARO 10-m telescope has proved to be robust and stable over the years, with scientific results since its functioning on 2019 until the completion of this thesis (February 15th, 2022). Observations of at least three bright radio bursts have been recorded and analyzed at the ARO 10-m telescope, which have paved the way for the upcoming CHIME/FRB Outriggers project. A VLBI study has been carried out with the ARO testbed, including a new fringe fitting correlator model and a new method to phase calibrate using pulsars, particularity developed for the 400--800 MHz low frequency bandpass. The upcoming CHIME/FRB Outriggers project will have multiple baselines across North America in order to localized thousands of FRBs in the next years, to 50 mas precision, which no other experiment has been able to accomplish. Therefore, these results will open a new era in transient astronomy and unlock the unknown matter content of the Universe.Ph.D
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
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