1,721,131 research outputs found
Diffuse gamma-ray emission observed by the Fermi Large Area Telescope: massive stars, cosmic rays and the census of the interstellar medium in the Galaxy
Full text linkGalactic diffuse gamma-ray emission is produced by interactions of cosmic rays (CRs) with interstellar gas and low-energy radiation fields. This is the brightest component of the high-energy gamma-ray sky, surveyed since 2008 with unprecedented sensitivity and angular resolution by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope. Galactic diffuse emission constitutes not only a bright and structured background which needs to be modeled in order to study individual sources and fainter diffuse components, but it can be used also as a probe of the interstellar environment of the Milky Way.
We present in-depth studies of LAT gamma-ray observations of selected regions in the local and outer Galaxy. LAT data are compared with multiwavelength tracers of the interstellar medium (ISM), including radio/mm-wave lines of gas and infrared emission/extinction from dust. The impact of the H I optical depth, often overlooked in the past, is carefully examined and recognized currently as the dominant source of uncertainty in the interpretation of observations.
On one hand, we discuss the constraints provided by the gamma-ray data on the census of the interstellar gas. We determine the Xco=N(H2) /Wco ratio for several clouds, finding no significant gradients in the Galactic disc over a range of 3.5 kpc in Galactocentric radius, and variations of a factor <2 in nearby local clouds. We also find evidence for an ubiquitous dark phase of interstellar gas which does not shine at radio/mm wavelengths and which provides a mass of about 50% of that traced by CO. For the first time we determine its gamma-ray spectrum which is found to be well correlated with that of H I, thus further confirming that the emission originates from interstellar gas.
On the other hand, we use the emissivity per hydrogen atom to infer the distribution of CRs in distant locations not accessible by direct measurements. While the local H I emissivity is consistent with the CR spectra measured near the Earth, no significant decrease is found toward the outer Galaxy in spite of the steep decline in number density of putative CR sources. The implications for CR origin and propagation are detailed.
Our models successfully reproduce LAT data over most of the regions considered. A remarkably hard and extended excess of gamma-rays is detected in the innermost 100 pc of the massive-star forming region in Cygnus. The gamma-ray excess fills a ionized cavity, which apparently
extends from the Cyg OB2 stellar cluster toward the g Cygni supernova remnant, and it hints to the presence of an excess of high-energy particles with respect to the surrounding clouds. We discuss its origin, the relationship with spatially coincident TeV emission detected by Milagro and the possible link with CR acceleration in the region.L’emissione gamma Galattica diffusa è prodotta dalle interazioni dei raggi cosmici con il gas e I campi di radiazione di bassa energia nello spazio interstellare. Essa costituisce la componente più brillante del cielo gamma ad alte energie, osservato dal 2008 con una sensibilità e una risoluzione angolare senza precedenti dal Large Area Telescope (LAT) a bordo del
telescopio spaziale Fermi . L’emissione Galattica diffusa costituisce non solo un fondo intenso e strutturato che è necessario modellare per studiare le sorgenti gamma individuali e le componenti diffuse più deboli, ma anche un mezzo per sondare l’ambiente interstellare della Via Lattea.
In questa tesi si riportano alcuni studi delle osservazioni gamma del LAT per regioni selezionate della Galassia locale ed esterna. I dati del LAT sono confrontati con traccianti multifrequenza del mezzo interstellare, tra cui righe radio e millimetriche del gas e emissione e estinzione
dovute alle polveri. L’impatto dello spessore ottico dell’idrogeno atomico, spesso trascurato in passato, è esaminato con cura e identificato allo stato attuale come la principale sorgente di incertezza nell’interpretazione delle osservazioni.
Da un lato si discutono i vincoli dati dall’emissione gamma sul censimento
del gas interstellare. Il rapporto Xco= N(H2) /Wco è determinato per varie nubi: non si osserva un gradiente significativo nel disco Galattico su una distanza di 3.5 kpc in raggio Galattocentrico, ma si evidenziano variazioni di un fattore <2 per le nubi locali. L’emissione
gamma mette in evidenza in tutte le regioni studiate una fase oscura del gas interstellare che non brilla alle lunghezze d’onda radio o millimetriche, la cui massa rappresenta circa il 50% di quella tracciata dal CO; l’origine dell’emissione gamma da parte di gas oscuro è dimostrata dalla prima misura mai effettuata del suo spettro, che risulta ben correlato a
quello dell’idrogeno atomico.
Dall’altro lato l’emissività per atomo di idrogeno serve per tracciare la distribuzione dei raggi cosmici in regioni lontane non accessibili con misure dirette. Mentre l’emissività locale risulta consistente con lo spettro dei raggi cosmici misurato in prossimità della Terra, non si osserva nessuna diminuzione significativa nella Galassia esterna malgrado il rapido decremento nella densità delle sorgenti putative di raggi cosmici. Le implicazioni per la loro origine e propagazione sono discusse in dettaglio.
I nostri modelli riproducono i dati del LAT nella maggior parte delle regioni considerate. Si rivela un eccesso esteso di emissione gamma dura nei 100 pc centrali della regione attiva di formazione stellare del Cigno. La “bolla” gamma riempie una cavità ionizzata, che apparentemente si estende dall’ammasso di stelle massicce Cyg OB2 in direzione del resto di supernova
gamma Cygni, e suggerisce la presenza di un eccesso di particelle di alta energia rispetto alle nubi che la circondano. I possibili meccanismi di accelerazione alla sua origine sono esaminati anche in riferimento all’associazione con emissione gamma al TeV rivelata da Milagro
Bruno Benedetto Rossi
No full textIn occasione del centenario della scoperta dei raggi cosmici (1912-2012), il Dipartimento di Fisica e Astronomia dell’Università di Padova ha pensato di rendere nuovamente disponibile al pubblico questo libro, ormai introvabile ma prezioso, che raccoglie una conferenza tenuta da Bruno Benedetto Rossi a Milano nel 1986 e una sua intervista rilasciata allora. Ai testi originali è stata premessa una biografia scientifica che nell’ultima parte ripercorre le fiorenti ricerche condotte nel settore negli anni successivi alla morte di Rossi. L’edizione originale conteneva un album fotografico che qui è stato arricchito. Un modo per avvicinare il pubblico a una delle più affascinanti avventure della scienza moderna. Un omaggio a uno dei grandi protagonisti di queste ricerche, costretto purtroppo ad abbandonare l’Italia a causa delle infami leggi razziali
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
The spatial distribution of the interstellar gamma-ray emissivity in the Milky Way
No full textTalk given by Luigi Tibaldo at CRATER2018, Gran Sasso Science Institute, L'Aquil
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
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