1,720,995 research outputs found
The high-energy view of young radio sources: X-ray and gamma-ray observations
No full textAccording to theoretical predictions, young extragalactic radio sources may release a significant fraction of their total radiative power in the high-energy domain. X-ray emission is expected from the central active galactic nucleus (AGN), and from the extended radio components, jet, hot spots, and lobes. The X-rays should also trace the interactions of the radio source with its environment, and reveal to us how the young source clears its way out of the host galaxy and finally reaches the giant stage. Observational campaigns with Chandra and XMM-Newton telescopes have confirmed that young radio sources are indeed relatively X-ray luminous. Recently, efforts have been undertaken to obtain a complete X-ray view of the whole population of young sources, focusing on the youngest and faintest objects. At the same time, the study of the X-ray properties of the samples of quasars and galaxies provides us with clues on the nature of the observed high-energy emission. In specific cases, X-ray and multi-wavelength data allow for a direct test of the different models. A detection in the γ-ray band would represent a robust proof of a non-thermal, high-energy component and the launch of the Fermi telescope has opened the hunt to young radio sources in the γ-ray window
High Energy Emission in Relativistic Jets of AGN: Theory predictions and gamma-ray view
Get PDFThe last decade has been a golden epoch for the observations of extra-galactic jets. The extension of the observational window from the radio through infrared and optical up to the high energy band has provided us with a wealth of new data. The study of the jets by modeling their spectral energy distribution has become a very effective approach. Multi-band observations are determinant to constrain the model parameters and probe the radiative environment where the jets form and expand. The broadband fitting allows us to infer the main physical parameters
of the emitting plasma and to consequently estimate the total jet kinetic power.
In this thesis a simple leptonic synchrotron and inverse Compton model is adopted and generalized by taking into account the main local and external radiative fields which can act as seed photons. The model is used to derive constraints on the jet contribution to the total high energy emission in compact radio sources. These are supposed to be the young counterparts of the giant radio sources. Unveiling the origin of their high energy emission is crucial to understand their subsequent evolution and the nature of the feedback mechanism with the intergalactic medium. First, we discuss the modeled broadband emission of jets in low and high power young radio sources observed at increasing viewing angles. The role of the seed photons is investigated for different jet velocities and linear sizes. We then test the model on the spectral energy distribution of the compact quasar 3C 186.
The presence of a velocity structure internal to the jet appears to be a crucial requirement to ensure its radiative relevance at the high energies. The internal structure of the jet is the subject of a dedicated study carried out on the FR I radio galaxy NGC 6251. The two radiative models used to describe its non-thermal broadband nuclear mission
return back two deeply different pictures: a heavy and slow jet for the synchrotron self Compton model while a light but highly dissipative one for the structured jet model (spine-layer)
The high energy view of FR0 radio galaxies
No full textA new class of low-power compact radio sources with limited jet structures, named FR 0, is emerging from recent radio-optical surveys. This abundant population of radio galaxies, five times more numerous than FR Is in the local Universe (z < 0.05), represent a potentially interesting target at high and very-high energies (greater than 100 GeV), as demonstrated by a single case of Fermi detection. Furthermore, these radio galaxies have been recently claimed to contribute non-negligibly to the extra-galactic γ-ray background and to be possible cosmic neutrino emitters. Here, we review the radio through X-ray properties of FR 0s to predict their high-energy emission (from MeV to TeV), in light of the near-future facilities operating in this band.</p
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
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