1,720,962 research outputs found
PSR J1012+5307: younger than it looks?
No full textLorimer et al. have recently reported that the spin-down age (similar to 7 x 10(9) yr) of the low-mass binary pulsar PSR J1012+5307 is much higher than the cooling age (3 x 10(8) yr) of its white dwarf companion. The proposed solutions for this discrepancy are outlined and discussed. In particular, the revised cooling age estimate proposed by Alberts ct al. agrees with data from other low-mass binary pulsar systems if a transition to the 'classical' cooling regime occurs between similar to 0.14 and similar to 0.28 M.. If this transition is excluded, PSR J1012+5307 seems to have finished its accretion phase far from the spin-up line
Spin and orbital evolution in low-mass binary pulsars
No full textWe consider the spin and orbital evolution of low-mass binary radio pulsars which are not members of globular clusters. We show that their current spin periods can be understood in terms of their likely mass-transfer history. Systems with orbital periods greater than or similar to 50 days are the endpoints of low-mass X-ray binaries whose orbital evolution is driven by the nuclear expansion of the secondary. For shorter periods, orbital angular momentum losses are significant, and the mass-transferring progenitor undergoes a phase of Eddington or super-Eddington mass transfer which spins up the neutron star to an Eddington-limited equilibrium period. These systems subsequently enter a propeller phase and are probably undetectable as low-mass X-ray binaries, largely resolving the inferred discrepancy between the current number of binary pulsars and their likely progenitors. Spin-down of the neutron stars in this propeller phase is evidently ineffective. The observed relation between spin period, magnetic field, and orbital period is in apparent agreement with simple ideas of the equilibrium spin period but requires much lower fastness parameters than predicted by current theory
The age of PSR J1012+5307
No full textLorimer et al. have recently reported that the spin-down age (similar to 7 x 10(9) yr) of the millisecond pulsar PSR J1012 + 5307 is much longer than the cooling age (3 x 10(8) yr) of its white dwarf companion. A scenario where the neutron star magnetic field decays spontaneously, and conservative accretion of all the mass lost by the secondary then 'recycles' the neutron star as a millisecond pulsar, cannot explain this discrepancy: the pulsar spins too rapidly at the end of the accretion phase to reach the current value within the white dwarf cooling time. However, if the decay of the neutron star magnetic field is induced by the accretion process itself, spin-up and field decay occur on similar time-scales, and the neutron star spin period cannot in general reach the usual short equilibrium value. The pulsar is thus able to accrete all the mass lost by the companion, reaching its current period and period derivative within the white dwarf cooling time
A Ginga observation of the X-ray pulsar 4U 0352+30
No full text4U 0352+30 (X Persei) is a low-luminosity binary X-ray pulsar with a pulse period of 835 s. We present timing and spectral analysis of a Ginga observation of X Persei from 1990 January 26 to January 29. The observation shows the peculiar spectral behavior of X Persei: the pulse-averaged hardness ratio exhibits a sharp hardening episode at phase minimum of the light curve. In order to explain the shape of the observed hardness ratio, we discuss the possible geometry of the emitting region. Simple models of fan emission from a hollow accretion column can reproduce the qualitative features of the observed light curve and hardness ratio
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
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
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