1,721,405 research outputs found
Theory of magnetically powered jets
No full textThe magnetic theory for the production of jets by accreting objects is reviewed with emphasis on outstanding problem areas. An effort is made to show the connections behind the occasionally diverging nomenclature in the literature, to contrast the different points of view about basic mechanisms and to highlight concepts for interpreting the results of numerical simulations. The role of dissipation of magnetic energy in accelerating the flow is discussed and its importance for explaining high Lorentz factors. The collimation of jets to the observed narrow angles is discussed, including a critical discussion of the role of "hoop stress." The transition between disk and outflow is one of the least understood parts of the magnetic theory; its role in setting the mass flux in the wind, in possible modulations of the mass flux, and the uncertainties in treating it realistically are discussed. Current views on most of these problems are still strongly influenced by the restriction to two dimensions (axisymmetry) in previous analytical and numerical work; 3-D effects likely to be important are suggested. An interesting problem area is the nature and origin of the strong, preferably highly ordered magnetic fields known to work best for jet production. The observational evidence for such fields and their behavior in numerical simulations is discussed. I argue that the presence or absence of such fields may well be the "second parameter" governing not only the presence of jets but also the X-ray spectra and timing behavior of X-ray binaries
From multiwavelength to mass scaling: accretion and ejection in microquasars and AGN
No full textA solid theoretical understanding of how inflowing, accreting plasma around black holes and other compact objects gives rise to outflowing winds and jets is still lacking, despite decades of observations. The fact that similar processes and morphologies are observed in both X-ray binaries and active galactic nuclei has led to suggestions that the underlying physics could scale with black hole mass, which could provide a new handle on the problem. In the last decade, simultaneous broadband campaigns of the fast-varying X-ray binaries particularly in their microquasar state have driven the development of, and in some cases altered, our ideas about the inflow/outflow connection in accreting black holes. Specifically, the discovery of correlations between the radio, infrared, and X-ray bands has revealed a remarkable connectivity between the various emission regions and argued for a more holistic approach to tackling questions about accretion. This chapter reviews the recent major observational and theoretical advances that focus specifically on the relation between the two "sides" of the accretion process in black holes, with an emphasis on how new tools can be derived forcomparisons across the mass scale
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
BeppoSAX observation of the superluminal galactic source GRS1915+105
No full textWe report on TOO BeppoSAX observations af the superluminal galactic source GRS1915+105 in its high flux state. The TOO mas triggered by the Wide Field Cameras, and the source mas then followed up for about a month with a long (100 ksec) observation and a set of nine 10 ksec snapshots. The source exhibited strong flux and spectral variability over the whole energy range (0.1-300 keV). We present here the spectral analysis carried out on a portion of the longer observation. The low energy (0.1-10 keV) spectrum taken in a phase of lack of spectral variability can be reasonably well fitted with an absorbed power law plus a multicolor blackbody. Clear evidence for a Fe K alpha emission line at 6.7 keV is also found, which an intensity that may be not directly correlated with continuum flux variation
Kilohertz quasi-periodic oscillations and broad iron emission lines as a probe of strong-field gravity.
No full textKilohertz quasi-periodic oscillations (kHz QPOs) and broad iron emission lines are thought to be produced at the inner edge of an accretion disk around neutron stars in low mass X-ray binary systems (LMXBs). The frequency of kHz QPOs and the width of iron lines strongly suggest that these phenomena take place just few kilometers above the surface of the neutron star and hence they should probe the strong gravitational field around these stars. Both QPO variability and iron line hold the promise of searching for signatures of strong-field general relativity, e.g. the ISCO or Lense-Thirring precession. Here I present new results of the kHz QPOs in the transient neutron star LMXB XTE J1701-462, and the kHz QPOs and the broad iron line in the persistent neutron star LMXB 4U 1636-53. In XTE J1701 we find that, contrary to previous claims, properties of the kHz QPO are probably not uniquely determined by the geometry of the space-time. In 4U 1636 we find that there is a complex relation between kHz QPOs and properties of the iron line, which is difficult to explain if both phenomena reflect properties of the same region in the accretion disk
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