1,721,036 research outputs found
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
Fluctuations in Astronomical Masers
Full text linkThe radiation of astronomical masers fluctuates on the time scale 1/Gamma, where Gamma is the levels\u27 loss rate. These intensity fluctuations reflect fluctuations of the level poulations around their mean, steady state values over the length scale λc = c/Γ. In saturated masers, the intensity fluctuations are dominated by passage through the unsaturated core. The effects of the saturated zones and of the seed radiation that the masers amplify can be neglected.
These fluctuations may have been detected in recent observations by Clegg and Cordes (1991) of Galactic H II/OH masers, providing a possible direct determination of the saturation intensity Js (= Γ/2B), an important input parameter for theoretical modeling. Long time-scale variations in the fluctuation amplitudes can provide information on phenomena such as passage of hydrodynamic waves in the maser region
Polarization of Astronomical Maser Radiation. IV. Circular Polarization Profiles
Full text linkProfile comparison of the Stokes parameters V and I is a powerful tool for maser data analysis, which provides the first direct methods for unambiguous determination of (1) the maser saturation stage, (2) the amplification optical depth and intrinsic Doppler width of unsaturated masers, and (3) the comparative magnitudes of Zeeman splitting and Doppler line width. Circular polarization recently detected in OH 1720 MHz emission from the Galactic center appears to provide the first direct evidence for maser saturation
Polarization of Astronomical Maser Radiation
Full text linkThe polarization of maser radiation when the source is permeated by an aligned magnetic field is derived for arbitrary angular momenta of the transition states. This generalization is made possible by an analysis of the structure of the propagating waves in a frame aligned with the magnetic axis. The key elements in determining the polarization properties are the assumption of independent and incoherent pump and loss processes for all magnetic sublevels, and the beaming of maser radiation. The radiation propagating in the direction of maximal intensity growth is polarized according to the solutions derived by Goldreich, Keeley, and Kwan for J = 1 → 0 masers, using various assumptions about the pump rates and the relation between line width and Zeeman splitting. These solutions are shown to be the most general ones for the dominant rays (those propagating along the longest chords through the source) of steady state beamed masers with arbitrary spins in the relevant domains of parameter space. The beamed nature, and polarization properties, of astronomical maser radiation are established during the unsaturated growth phase, when an increase in overall source dimension results in proportionate tightening of the beaming factor Ω/4π
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
On the Current Status of Maser Polarization Theory
Full text linkDespite recent criticism, past conflicts among theoretical studies of maser polarization are by now resolved and clarified. All analytical methods proposed thus far produce the same polarization solutions, solutions that agree with observations. Current numerical studies do not reproduce these solutions. Instead, for any pumping employed, their results are in direct conflict with both analytical theory and observations. These problems reflect the fact that the basic formulation of numerical modeling has yet to incorporate the statistical nature of the radiation field
Radiative Transfer in Astronomical Masers. I. The Linear Maser
Full text linkThis is the first in a series of three papers presenting a comprehensive, unified approach for radiative transfer in astronomical maser sources. This paper provides the general formalism and presents the detailed analytic solution of the linear maser, including the case of a source illuminated by background radiation
On the Theory of Astronomical Masers in Three Dimensions
Full text linkIn the standard theory of three-dimensional astronomical masers, the radiation field is described as if the source were comprised of a collection of linear masers. To leading order, the standard theory is shown to provide the correct description of three-dimensional masers and its results remain intact, but only within a frequency core whose half-width is χsΔνD, where ΔνD is the Doppler width and χs is a dimensionless parameter. For any given geometry, χs is ~1θsat, where θsat is the beaming angle of a maser with that geometry that has just saturated. For typical pumping schemes, χs is ~2 in spherical masers, ~2.5-3 in disk masers, and ~3-5 in cylindrical masers. For frequencies outside this core region, maser operation corresponds to a mode that will be called suppressed, and the standard theory breaks down. In this frequency domain, interaction with core rays that are slightly slanted to the direction of propagation suppresses photon production. In contrast with the core region, in the suppressed regime the rate of maser photon generation never reaches the maximum allowed by the pump processes; this regime effectively corresponds to a maser whose inherent strength is weaker than that of a linear maser whose properties are otherwise identical. Observed maser radiation is effectively confined to the core region since frequencies in a suppressed domain are practically unobservable. In essence, χs provides an effective cutoff, defining a width at zero intensity that depends on the geometry but is unaffected by growth at line center. In practice, suppression affects only extreme maser outbursts. Their profiles change in such a way that when fitted with a Gaussian, the linewidth decreases when the line center intensity increases, even for masers that are saturated at the line core -- in marked contrast with the predictions of standard analysis of maser linewidths. This behavior could perhaps be related to the inverse relationship between intensity and line width displayed in some intense H2O maser flares in star forming regions
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