1,721,073 research outputs found
A model for the optical high state light curve of AM Herculis
No full textWe present a simple quantitative model that can describe
the photometric B and V band light curves of AM Herculis obtained
during a high state. The double-humped shape of the V band light
curve is dominated by cyclotron emission from a region at the main
accreting pole with an area of ~ and
sustaining an inflow of ~0.06
. The almost unmodulated B band is dominated by emission from the accretion stream. The
contribution of the heated white dwarf to the optical emission is
small in the B band, but comparable to that of the accretion stream
in the V band. The emission of the secondary star is negligible
both in B and in V
RX J1313.2-3259, a missing link in CV evolution?
No full textWe present low-state IUE spectroscopy of the ROSAT-discovered polar RXJ1313.2-3259. The SWP spectrum displays a broad Ly alpha absorption profile, which can be fitted with a two-temperature model of a white dwarf of T(wd) = 15 000 K with a hot spot of T(spot) = 34 000 K which covers f similar to 0.01 of the white dwarf surface. The white dwarf temperature is atypically low for the long orbital period (4.18 h) of RXJ1313.2-3259. This low temperature implies either that the system is a young CV in the process of switching on mass transfer or that it is an older CV found in a prolonged state of low accretion rate, much below that predicted by standard evolution theory. In the first case, we can put a lower limit on the life time as pre-CV of 10(8) yrs. In the second case, the good agreement of the white dwarf temperature with that expected from compressional heating suggests that the system has experienced the current low accretion rate for an extended period > 10(4) yrs. A possible explanation for the low accretion rate is that RXJ1313.2-3259 is a hibernating post nova and observational tests are suggested
BeppoSAX observations of AM Herculis in intermediate and high states
No full textTemporal and spectral analyses from BeppoSAX observations of AM Her performed during both an intermediate and a high stare are presented and discussed. Optical observations taken a few days after the X-ray ones are also presented. During the intermediate state observation, the source was in its "normal", one-pole accretion mode. In the high state it switched to an hitherto unobserved atypical "two-pole" accretion mode, with significant soft and hard X-ray emission from both poles. The emission from the second pole is much softer than that from the primary pole, while the soft X-ray excess of the primary pole is fairly modest in this accretion mode. These facts suggest that accretion onto the secondary is mainly due to blobs penetrating deeply in the photosphere, while that on the primary pole is mostly via a more homogeneous column, giving rise to the classical standing shock. A strong X-ray flaring activity is also observed in the soft X-ray band, but not in the hard X-ray and optical emissions indicating that flares are due to inhomogeneous blobby-accretion events
Far-ultraviolet spectroscopy of magnetic cataclysmic variables
No full textWe have obtained Hubble Space Telescope (HST) STIS data for a total of 11 polars as part of a program aimed at compiling a homogeneous database of high-quality far-ultraviolet (FUV) spectra for a large number of cataclysmic variables (CVs). Of the 11 polars, eight were found in a state of low accretion activity (V347 Pav, VV Pup, V834 Cen, BL Hyi, MR Ser, ST LMi, RX J1554.2+2721, and V895 Cen) and three in a state of high activity ( CD Ind, AN UMa, and UW Pic). The STIS spectra of the low-state polars unambiguously reveal the photospheric emission of their white dwarf (WD) primaries. We have used pure hydrogen WD models to fit the FUV spectra of the low-state systems (except RX J1554.2+2721, which is a high-field polar) in order to measure the WD effective temperatures. In all cases, the fits could be improved by adding a second component, which is presumably due to residual accretion onto the magnetic pole of the WD. The WD temperatures obtained range from 10,800 to 14,300 K for log g = 8.0. Our analysis more than doubles the number of polars with accurate WD effective temperatures. Comparing the WD temperatures of polars with those of nonmagnetic CVs, we find that at any given orbital period the WDs in polars are colder than those in nonmagnetic CVs. The temperatures of polars below the period gap are consistent with gravitational radiation as the only active angular momentum loss mechanism. The differences in WD effective temperatures between polars and nonmagnetic CVs are significantly larger above the period gap, suggesting that magnetic braking in polars might be reduced by the strong field of the primary. We derive distance estimates to the low-state systems from the flux scaling factors of our WD model fits. Combining these distance measurements with those from the literature, we establish a lower limit on the space density of polars of 1.3 x 10(-6) pc(-3)
A 150 MG magnetic white dwarf in the cataclysmic variable RX J1554.2+2721
No full textWe report the detection of Zeeman-split Lyalpha absorption pi and sigma(+) lines in the far-ultraviolet Hubble Space Telescope/Space Telescope Imaging Spectrograph (STIS) spectrum of the magnetic cataclysmic variable RX J1554.2 + 2721. Fitting the STIS data with magnetic white dwarf model spectra, we derive a field strength of B similar or equal to 144 MG and an effective temperature of 17,000 K less than or similar to T-eff less than or similar to 23,000 K. This measurement makes RX J1554.2 + 2721 only the third cataclysmic variable containing a white dwarf with a field exceeding 100 MG. Similar to the other high-field polar AR UMa, RX J1554.2 + 2721 is often found in a state of feeble mass transfer, which suggests that a considerable number of high-field polars may still remain undiscovered
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
Unveiling Accreting White Dwarf Binaries in Hard X-Ray Surveys
No full textAmong hard X-ray sources detected in the INTEGRAL/IBIS and Swift/BAT surveys, those identified as accreting white dwarf binaries recently boosted in number, representing ∼ 20% of the Galactic sample. The majority are identified as Cataclysmic Variables with magnetic white dwarf primaries suggesting that this subclass could be an important costituent of the galactic population of X-ray sources. In this paper I present the results of an on-going follow-up progamme in the X-rays aiming at identifying the true nature of newly discovered sources
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