1,721,050 research outputs found
Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables - IV. Observations of frequency doubling and tripling in VW Hyi
No full textWe present new observations of the rapid oscillations in the dwarf nova VW Hyi, made late in outburst. These dwarf nova oscillations (DNOs) increase in period until they reach 33 s, when a transition to a strong 1st harmonic and weak fundamental takes place. After further period increase the 2nd harmonic appears; often all three components are present simultaneously. This 1:2:3 frequency suite is similar to what has been seen in some neutron star and black hole X-Ray binaries, but has not previously been seen in a cataclysmic variable. When studied in detail the fundamental and 2nd harmonic vary similarly in phase, but the 1st harmonic behaves independently, though keeping close to twice the frequency of the fundamental. The fundamental period of the DNOs, as directly observed or inferred from the harmonics, increases to ~ 100 s before the oscillation disappears as the star reaches quiescence. Its maximum period is close to that of the ‘longer period’ DNOs observed in VW Hyi. The quasi-periodic oscillations (QPOs), which have fundamental periods 400 – 1000 s, behave in the sameway, showing 1st and 2nd harmonics at approximately the same times as the DNOs. We explore some possible models. One in which the existence of the 1st harmonic is due to transition from viewing a single accretion region to viewing two regions, and the rate of accretion onto the primary is modulated at the frequency of the 1st harmonic, as in the ‘beat frequency model’, can generate the suite of DNO frequencies observed. But the behaviour of the QPOs is not yet understood
Pulsating white dwarfs in cataclysmic variables: the marriage of ZZ Cet and dwarf nova (in: Variable Stars in the Local Group (IAU Colloquium 193))
No full textThere are now four dwarf novae known with white dwarf primaries that show large amplitude non-radial oscillations of the kind seen in ZZ Cet stars. We compare the properties of these stars and point out that by the end of Sloan Digital Sky Survey more than 30 should be known
Dwarf nova oscillations and quasi-periodic oscillations in cataclysmic variables – II. A low-inertia magnetic accretor model
No full textThe dwarf nova oscillations observed in cataclysmic variable (CV) stars are interpreted in the context of a low-inertia accretor model, in which accretion on to an equatorial belt of the white dwarf primary causes the belt to vary its angular velocity. The rapid deceleration phase is attributed to propellering. Evidence that temporary expulsion rather than accretion of gas occurs during this phase is obtained from the large drop in extreme ultraviolet flux.We show that the quasi-periodic oscillations are most probably caused by a vertical thickening of the disc, moving as a travelling wave near the inner edge of the disc. This alternately obscures and ‘reflects’ radiation from the central source, and is visible even in quite low inclination systems. A possible excitation mechanism, caused by winding up and reconnection of magnetic field lines, is proposed.We apply the model, deduced largely from VW Hyi observations, to re-interpret observations of SS Cyg, OY Car, UX UMa, V2051 Oph, V436 Cen and WZ Sge. In the last of these we demonstrate the existence of a 742-s period in the light curve, arising from obscuration by the travelling wave, and hence show that the two principal oscillations are a dwarf nova oscillation and its reprocessed companion
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
The broad spectral energy distributions of accreting white dwarfs
Full text linkCataclysmic Variable stars (CVs) are excellent examples of multi-wavelength objects. They have been detected at all frequencies, from radio to γ-ray. However, while large sets of multi-wavelength data exist, few studies have been performed on the broad spectral energy distributions (SEDs) of these objects. I perform such a study here, and present the largest collection of broad CV SEDs yet constructed. I define a sample of well-studied, nearby cataclysmic variables of several different CV classes, and gather existing multi-wavelength data over a broad frequency domain. I identify the contributions to the spectrum and compare the spectra between classes. I then use the SEDs to constrain the mass accretion rates by modelling the accretion disc as a series of concentric annuli, each of which radiates as a black body with effective temperature appropriate to the radius of that annulus. The resulting bestfit accretion rates are compared to published values of mass accretion rates for the systems. My method of modelling the emission from a CV accretion disc as a sum of blackbodies produces accretion rate estimates that are reliable to within a factor of 5 for those systems that have steady state discs. However, the data are significantly underfitted by these simple models in all cases. While this method is a useful tool for modelling spectra and finding accretion rates, it cannot be applied to a small amount of data quickly and simply. It must be performed on as large a dataset as possible, and other contributions to the spectrum must be accounted for. As a result, an attempt to obtain accretion rate estimates for a larger, volume-limited sample of non-magnetic CVs was unsuccessful. I also investigate the ratios of flux densities in different wavebands. My data reveal two previously reported correlations, namely that shorter period CVs have higher X-ray to optical flux ratios and that magnetic CVs are X-ray bright compared to non-magnetic CVs. In addition, I find that polars are radio bright compared to IPs and non-magnetic CVs, and illustrate this, for the first time
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
A spectroscopic analysis of the eclipsing Nova-like EC21178-54
No full textIncludes bibliographical references.EC21178-54 is a 13.7 magnitude eclipsing nova-like (NL) cataclysmic variable (CV) discovered in the Edinburgh-Cape survey of blue objects. It has an orbital period of 3.708 hours. High-speed photometric analysis of its light curve reveal that this system is a rich source of quasi-periodic oscillations (QPOs), dwarf nova oscillations (DNOs) and the other type of periodic oscillations called longer period dwarf nova oscillations (lpDNOs). Its light curve shows a deep symmetrical V-shaped primary eclipse, and its spectra show strong broad double-peaked emission. In this project we present phase-resolved spectroscopy of the eclipsing nova-like CV EC21178-54. The average spectrum of EC21178-54 shows broad double-peaked emission lines from HeII 4686Å, and the Balmer series. HeII 4686Å dominates the Balmer lines; and in the Balmer lines the blueshifted component is the stronger of the two. The high-excitation feature, CIII/NIII at 4640-4650 Å, is also present and appear broad in emission. A number of other lines, mostly HeI, are clearly present in absorption and/or emission
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
Rapid oscillations in cataclysmic variable stars
No full textIncludes bibliographical references (p. 129-146).Rapid quasi-coherent oscillations were detected in the optical light curves of 24 cataclysmic variable stars (CVS). In 12 of these systems (TU Men, WW Cet, HX Peg, BP CrA, BR Lup, HP Nor, AG Hya, TW Vir, PU CMa, V426 Ooph, V1193 Ori, and CR Boo) for the first time. The results contribute to the observational record of the phenomenology of dwarf nova oscillations (DNOs), quasi-periodic oscillations (QPOs), and longer period dwarf nova oscillations (lpDNOs), strengthen the correlation, valid over nearly six orders of magnitude in frequency, between the ratio of time scales of different classes of oscillations in white dwarf, neutron star, and black hole binaries, and show that the recently recognized lpDNOs occur fairly commonly in high mass transfer rate CVs
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