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Study of external fields around magnetars and analysis of interstellar absorption
Full text linkThis thesis describes mainly two topics: the analysis of the external field of hyper-magnetized neutron stars (magnetars), and the distribution of galactic interstellar absorption.
Magnetars have been introduced as a theoretical model to account for the peculiar characteristics of a limited group (~15 objects out of more than two thousands known) of isolated neutron stars. Though being so few, these objects are extensively studied due to their extreme properties. In the magnetar model, the emission of such sources is explained as energy released from the magnetic field (hence the name) rather than from rotational, gravitational or thermal energy as for the majority of the neutron stars. From measures of spin
periods and their variations, the magnetic fields involved are estimate to be of order 10^14-10^15 G (10^10-10^11 T), making these stars the ’strongest magnets’ of the Universe. In this frame the study of magnetars opens the possibility to analyze the effects of the magnetic fields on plasma under really extreme conditions. The magnetic fields we deal with exceed the quantum threshold B_QED~ 4·10^13 G (4·10^9 T) and cause strong alterations of the properties of matter (for a review see Harding & Lai [60]). Not only such an intense field strongly interacts with the plasma and modify the atomic structure of matter, but it also modifies the properties of vacuum too. One of the more noticeable effectsis the vacuum polarization that alters the propagation of photons introducing ordinary and extraordinary modes.
The only tool to study these objects is the analysis of their emitted radiation. This is the reason why a great effort has been devoted to synthetically model the spectra of magnetars.
The aim of these studies is to take into account the known effects of the field and indirectly derive, from the collected spectra, some constraint on the physical conditions in the emitting zones, both on the star surface and in the magnetosphere.
The magnetar model has been developed in different works, starting in 1992 with the one of Thompson and Duncan. In 2002, in a paper by Thompson, Lyutikov and Kulkarni, the external field of magnetars was described as a sheared dipole. In this model the torsion comes from the interior and gradually twist the crust, shearing, as a consequence, the external field too. The bursts are fueled by a sudden release of great amount of energy due to fractures of the crust occurring as the stress imparted from the field become too strong.
Assuming this configuration of the field, the magnetar model is able to reproduce the spectral behaviour of the majority of the Anomalous X-Ray Pulsars and Soft Gamma Repeaters.
Surprisingly, the magnetar model also suggested a link between these two classes that had become clearer only when a number of sources with intermediate properties between the two has been discovered. Nevertheless, as more studies are conducted on magnetar spectra,
the underlying model need to be refined. Some recent works suggest, for example, small emitting zones that do not cover the entire star, or the presence of different temperatures
on the surface of the magnetars. In some cases there are indications that the twist may be confined only to a portion of the magnetosphere, and/or that the large scale field is more
complex than a simple dipole.
The studies conducted in this work follow this researchline, and are intended to extend the preexisting model. The followed idea is to include field variations on small scales, achieved considering multipolar fields of various orders.
The other subject of this work deals with interstellar absorption in our Galaxy. Interstellar extinction is the subject of many studies, as it modifies the radiation emitted by all the sources. As the dust and the interstellar particles are not emitting (with the exception of the infrared emission from silicates and polycyclic aromatic hydrocarbon (PAH) molecules), they are particularly difficult to analyze. Generally the interstellar absorbing medium is visible only when illuminated from starlight; therefore, when possible, it is directly measured
by the collected radiation, both analyzing low-ionization lines in spectra, and from photometric analysis. These methods, though, give a measure of extinction only for a number of objects and cannot cover extensively the observed sky. To obtain a value for the extinction
towards directions for which there are no available measures, it is common to use a model of the distribution of dust in the Galaxy.
While the advantages of an extinction models are evident, as they allow to infer absorption quantities even where there is a lack of measures, the disadvantage is the need to make assumptions on the stellar and/or dust components of the Galaxy. This introduces again other uncertainties on the final result. A different approach is based on the analysis of the
dust infrared emission (Schlegel, Finkbeiner & Davis [107]) in order to map the interstellar medium. Unfortunately even this method has limitations, as it gives informations only on the total column density in each direction, without the possibility to resolve the three dimensional structure of the distribution of interstellar medium.
In this context the work performed in this thesis aims at constructing an algorithm to interpolate the extinction measures available in literature, without recurring to a model of the Milky Way components. The idea followed is to collect the greater possible set of absorption
data, in order to create a coarse-grained three dimensional map of the galactic extinction, and interpolate these data to infer the values between the known points. As extinction is a cumulative measure of the absorption occurring between the observer and the star, before
interpolating the data, a deconvolution of these measured quantities into local values of absorption, is needed. Some preliminary results obtained with the developed code and conclusions are drawn.Gli argomenti trattati nella presente tesi sono due: l’ analisi del campo attorno a stelle di neutroni iper-magnetizzate (Magnetar) e la distribuzione dell’ assorbimento interstellare nella nostra Galassia.
Le Magnetar sono state introdotte come modello teorico per spiegare le caratteristiche peculiari di un ristretto sottoinsieme di stelle di neutroni isolate, circa 15 oggetti su più di 2000 noti. Nonostante siano così pochi, questi oggetti sono argomento di numerosi studi
a causa delle loro proprietà estreme. All’interno del modello di magnetar, l’emissione di queste sorgenti viene spiegata come energia rilasciata dal campo magnetico (da cui il nome) piuttosto che da riserve di energia rotazionale, gravitazionale o termica come avviene per la maggior parte delle stelle di neutroni. Dalle misure dei periodi orbitali e dalla loro variazione nel tempo, i campi magnetici coinvolti sono stimati essere dell’ordine di 10^14-10^15 G (10^10-10^11 T), portando queste stelle ad essere “i magneti più intensi” dell’Universo. In questo quadro, lo studio delle magnetars apre la possibilità di analizzare gli effetti di campi magnetici su plasmi in condizioni davvero estreme. I campi magnetici che vengono trattati superano la soglia B_QED~ 4·10^13 G (4·10^9 T) e quindi provocano grandi alterazioni delle proprietà intrinseche della materia (per una review si veda Harding & Lai [60]). Campi così intensi non solo interagiscono pesantemente col plasma modificando la struttura atomica della materia, ma alterano anche le proprietà del vuoto. Uno degli aspetti più significativi è la polarizzazione del vuoto che introduce i modi ordinario e straordinario di propagazione
dei fotoni. L’unico strumento per studiare questi oggetti è l’analisi della radiazione emessa; questo è il motivo per cui sono stati compiuti grandi sforzi per creare modelli sintetici di spettri delle magnetars. Lo scopo di queste ricerche è di derivare, indirettamente dagli spettri raccolti,
alcuni vincoli sulle condizioni fisiche delle zone emittenti, sia sulla superficie della stella che nella magnetosfera.
Il modello è stato sviluppato in vari articoli a partire dal 1992 con il lavoro di Thompson e Duncan. Nel 2002, in un articolo di Thompson, Lyutikov e Kulkarni, il campo esterno delle magnetars è stato descritto come un dipolo twistato attorno al proprio asse magnetico.
In questo modello una torsione si propaga dalle zone interne e gradualmente deforma la crosta, provocando una distorsione anche nel campo esterno. I burst osservati sarebbero quindi alimentati dall’improvviso rilascio di una grande quantità di energia dovuta a fratture della crosta, che avvengono quando lo stress sulla superficie diventa troppo forte. Assumendo questa con?gurazione del campo, il modello è in grado di riprodurre le caratteristiche spettroscopiche della maggior parte delle Anomalous X-Ray Pulsars e dei Soft Gamma
Repeaters. Sorprendenetemente il modello delle magnetar suggeriva anche un legame tra queste due classi di oggetti, che è stato confermato in seguito, dalla scoperta di oggetti con proprietà intermedie tra le due classi. Ciò nonostante, a mano a mano che nuovi studi vengono condotti sull’emissione delle magnetar, il modello necessità di essere affinato. Alcuni lavori recenti, ad esempio, suggeriscono o che le zone emittenti siano piccole e non coprano l’intera superficie della stella, oppure che siano presenti zone a diversa temperatura sulla superficie. In alcuni casi, il twist sembra essere confinato solo a delle porzioni della magnetosfera e/o il campo, su larga scala, sembra essere più complesso di un semplice dipolo.
Gli studi condotti si inseriscono in questo quadro e sono rivolti a generalizzare il modello esistente. L’idea che è stata seguita è quella di includere delle variazioni del campo su piccola scala considerando multipoli di ordini diversi. A differenza di quanto accade per campi
generici, in questo caso il procedimento è complicato della non linearità delle equazioni che descrivono i campi twistati.
Il secondo argomento di questa tesi riguarda l’assorbimento interstellare nella nostra Galassia. L’estinzione interstellare è oggetto di molti studi in quanto modifica la radiazione osservata proveniente dalle sorgenti celesti. Le polveri e le particelle interstellari non emettono radiazione, ad eccezione dell’emissione infrarossa, dovuta a silicati e idrocarburi policiclici aromatici, e dunque sono molto difficili da analizzare. Generalmente, il mezzo interstellare è visibile solo quando viene illuminato da altre sorgenti, quindi, ove possibile, viene
misurato direttamente dalla radiazione emessa da oggetti lontani ed osservata sia analizzando righe spettroscopiche di bassa ionizzazione che da analisi fotometriche.
Questi metodi tuttavia, forniscono una misura dell’estinzione solo tra
l’ osservatore e alcuni oggetti, ma non coprono estensivamente tutto il cielo. Per ottenere misure di estinzione laddove le misure non sono presenti, generalmente viene impiegato un modello di distribuzione delle polveri nella Galassia. Mentre i vantaggi riguardanti l’uso dei modelli di estinzione sono evidenti, dato che permettono di ricavare la quantità di estinzione anche in zone dove le misure sono assenti, lo svantaggio è dovuto alla necessità di adottare delle assunzioni sulle proprietà delle stelle e/o sulle polveri della Galassia, che introducono ulteriori incertezze nel risultato finale. Un approccio differente è basato sull’analisi dell’emissione infrarossa delle polveri (Schlegel, Finkbeiner & Davis [107]) per mappare il mezzo interstellare. Purtroppo anche questo
metodo ha delle limitazioni, in quanto fornisce informazioni solo sull’intera column density in una determinata direzione, senza la possibilità di risolvere la struttura tridimensionale
della distribuzione del mezzo interstellare.
In questo contesto il presente lavoro è rivolto alla realizzazione di un algoritmo utile ad interpolare le misure di estinzione disponibili in letteratura, senza dover ricorrere ad un
modello per le componenti della Via Lattea. L’idea che è stata seguita è quella di raccogliere la maggior quantità possibile di dati di assorbimento, al fine di ottenere una mappa tridimensionale (seppur grossolana) dell’estinzione nella nostra Galassia, e interpolare questi dati per ricavare il valore di assorbimento in posizioni diverse da quelle già note.
Poiché l’ estinzione è una misura cumulativa dell’assorbimento distribuito tra l’osservatore e la stella, prima di interpolare i dati è necessario effettuare fare una scomposizione degli assorbimenti misurati in valori locali di assorbimento.
Le conclusioni e alcuni risultati preliminari del codice che è stato sviluppato sono presentati
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
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
Author Under Sail The Imagination of Jack London, 1893-1902
No full textIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Intro -- Title Page -- Copyright Page -- Dedication -- Contents -- Acknowledgments -- Introduction -- 1. Spirit Truth -- 2. From Absorption to Theatricality and Back Again -- 3. "I Will Build a New Present" -- 4. Sons as Authors -- 5. Fathers as Publishers -- 6. The Daughter as Author -- 7. Lovers as Authors -- 8. At Sea with the Family -- 9. Yellow News, Yellow Stories -- 10. The Return Home -- Notes -- Bibliography -- Index -- About Jay WilliamsIn Author Under Sail, Jay Williams offers the first complete literary biography of Jack London as a professional writer engaged in the labor of writing. It examines the authorial imagination in London's work, the use of imagination in both his fiction and nonfiction, and the ways he defined imagination in the creative process in his business dealings with his publishers, editors, and agents. In this first volume of a two-volume biography, Williams traverses the years 1893 to 1902, from London's "Story of a Typhoon" to The People of the Abyss. The Jack London who emerges in the pages of Author Under Sail is a writer whose partnership with publishers, most notably his productive alliance with George Brett of Macmillan, was one of the most formative in American literary history. London pioneered many author models during the heyday of realism and naturalism, blurring the boundaries of these popular genres by focusing on absorption and theatricality and the representation of the seen and unseen. London created an impassioned, sincere, and extremely personal realism unlike that of other American writers of the time. Author Under Sail is a literary tour de force that reveals the full range of London as writer, creative citizen, and entrepreneur at the same time it sheds light on the maverick side of machine-age literature.Description based on publisher supplied metadata and other sources.Electronic reproduction. Ann Arbor, Michigan : ProQuest Ebook Central, YYYY. Available via World Wide Web. Access may be limited to ProQuest Ebook Central affiliated libraries
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