1,720,967 research outputs found
The dark side of the Universe
Full text linkThe knowledge of the Universe surrounding us, nevertheless the big steps forward of the last decade, presents a lot of obscure or not well understood parts, that are usually indicated with the adjective dark. These parts touch every component of the Universe: we have well known radiation in form of photons, and its dark counterpart in form of neutrinos, dark matter beside the ordinary (baryonic) matter and also an obscure form of energy whose nature is completely unknown.
In this context the big challenge of the next generation cosmology is to determine the intrinsic properties of these exotic components by working on available observables and by determining new ways to look at our Universe.
Astrophysicists and Cosmologists developed during the years different techniques to observe the sky, some of which focus on the static properties of the Universe (distances and topology, actual composition), while others point towards a description of its dynamics (evolution of the components' perturbations, expansion of the background and so on). As we will see in detail in this thesis, the general relativity that rules the Universe evolution works in such a way to link statical and dynamical behaviour. This feature gives the Cosmologists the possibility to infer dynamical properties starting from simple geometrical measurements, and this, in last analysis, leads to development of new techniques to study directly also the evolutionary part of the Cosmo. We already have some measurements of this kind, but the best is to come with recently developed experiments that started in the last couple of years or that should start soon. This kind of observations will concentrate on precise and complete dynamical data that hopefully will lead us to comprehend also the darkest part of our knowledge: the dark energy.
My Ph.D. work has been principally focused on the study of the properties of the dark sector of the Universe using all probes actually available
X-ray spectral variability of Active Galactic Nuclei from XMM-Newton data
Full text linkThe X-ray spectral variability of Active Galactic Nuclei typically follows a “softer when brighter” trend, which is believed to be originated mainly from the superposition of different spectral components, varying independently from each other, although some intrinsic variations of the continuum are also possible. We analyzed the MEXSAS sample, made up by more than 7800 observations from 2700 quasars from the fifth release of the XMM-Newton Serendipitous Source Catalogue (3XMM-SSC), cross-matched with two quasar catalogues, SDSS-DR7Q and SDSS-DR12Q. We developed a technique that is able to compute estimates of the photon index from approximate spectral fits, using the fluxes in the catalogue. Following Trevese & Vagnetti (2002), we quantify the spectral variability using β = −∆Γ/∆ log F. We find an ensemble softer when brighter trend, extending therefore this result to quasars, and the same result is found for eight single sources extracted from the catalogue, plus one (PG 1114+445) discarded because of the presence of a prominent warm absorber, although with different extent. To investigate the reason for this range of different values of beta, we investigated a sample of X-ray
bright sources taken from the samples of Sobolewska & Papadakis (2009), plus M81, for which it is possible to obtain accurate photon indices. We compute both the accurate and approximate photon indices and we confront β obtained with both methods. Finally, we studied the spectra of 13 observations of PG 1114+445, finding multiple absorbers, one possibly being a highly ionized, ultra-fast outflow, with velocity of about 15% of the speed of light, which is observable in four
observations
Ensemble quasar spectral variability from the XMM-Newton Serendipitous Source Catalogue
No full textVariations of the X-ray spectral slope have been found in many Active Galactic Nuclei (AGN) at moderate luminosities and redshifts, typically showing a "softer when brighter" behaviour. However, similar studies are not usually performed for high-luminosity AGNs. We present an analysis of the spectral variability based on a large sample of quasars in wide intervals of luminosity and redshift, measured at several different epochs, extracted from the fifth release of the XMM Newton Serendipitous Source Catalogue. Our analysis confirms
a "softer when brighter" trend also for our sample, extending to high luminosity and redshift the general behaviour previously found. These results can be understood in light of current spectral models, such as intrinsic variations of the X-ray primary radiation, or superposition with a constant reflection component
Spectral slope variability of BL Lacertae objects in the optical band
No full textLight curves of eight BL Lac objects in the BVRI bands have been analyzed. All of the objects tend to be bluer when brighter. However, spectral slope changes differ quantitatively from those of a sample of QSOs analyzed in a previous paper by Trevese & Vagnetti and appear to be consistent with a different nature of the optical continuum. A simple model representing the variability of a synchrotron component can explain the spectral changes. Constraints on a possible thermal accretion disk component contributing to the optical luminosity are discussed
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
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