1,721,052 research outputs found
On X-Ray Optical Depth in the Coronae of Active Stars
No full textWe have investigated the optical thickness of the coronal plasma through the analysis of high-resolution X-ray spectra of a large sample of active stars observed with the High Energy Transmission Grating Spectrometer on Chandra. In particular, we probed for the presence of significant resonant scattering in the strong Lyman series lines arising from hydrogen-like oxygen and neon ions. The active RS CVn-type binaries II Peg and IM Peg and the single M dwarf EV Lac show significant optical depth. For these active coronae, the Lyα/Lyβ ratios are significantly depleted as compared with theoretical predictions and with the same ratios observed in similar active stars. Interpreting these decrements in terms of resonance scattering of line photons out of the line of sight, we are able to derive an estimate for the typical size of coronal structures, and from these we also derive estimates of coronal filling factors. For all three sources we find that both the photon path length as a fraction of the stellar radius and the implied surface filling factors are very small and amount to a few percent at most. The measured Lyα/Lyβ ratios are in good agreement with APED theoretical predictions, thus indicating negligible optical depth, for the other sources in our sample. We discuss the implications for coronal structuring and heating flux requirements. For the stellar sample as a whole, the data suggest increasing quenching of Lyα relative to Lyβ as a function of both LX/Lbol and the density-sensitive Mg XI forbidden-to-intercombination line ratio, as might generally be expected
Eclipse mapping of EXO 0748-676: evidence for a massive neutron star
No full textDetermining the maximum possible neutron star (NS) mass places limits on the equation of state (EoS) of ultra-dense matter. The mass of NSs in low-mass X-ray binaries can be determined from the binary mass function, providing independent constraints are placed on both the binary inclination and mass ratio. In eclipsing systems, they relate via the totality duration. EXO 0748–676 is an eclipsing NS low-mass X-ray binary with a binary mass function estimated using stellar emission lines from the irradiated face of the companion. The NS mass is thus known as a function of mass ratio. Here, we model the X-ray eclipses in several energy bands, utilizing archival XMM–Newton data. We find a narrow region of absorbing material surrounding the companion star is required to explain the energy-dependent eclipses. Therefore, we suggest the companion may be experiencing ablation of its outer layers and that the system could transition into a redback millisecond pulsar. Our fit returns a mass ratio of q=0.222+0.07−0.08 and an inclination i=76.5±1.41.1. Combining these with the previously measured radial velocity of 410 ± 5 km s−1, derived from Doppler mapping analysis of H α emission during quiescence, returns an NS mass of ∼2 M⊙ even if the line originates as far from the NS as physically possible, favouring hard EoS. The inferred mass increases for a more realistic emission point. However, a ∼1.4 M⊙ canonical NS mass is possible when considering radial velocity values derived from other emission lines observed both during outburst and quiescence
Hard diffuse X-ray emission around the PSR J2032+4127: A pulsar wind nebula in the Cygnus OB2 association
Full text linkFil: Albacete Colombo, Juan F. Universidad Nacional de Río Negro. Río Negro, Argentina.Fil: Drake, Jeremy J. Smithsonian Astrophysical Observator. Cambridge, USA.Fil: Filocomo, Agostina. Universidad Nacional de Río Negro. Río Negro, Argentina.Fil: Wright, Nick J. Keele University, Astrophysics Group. Keele, UK.The Cygnus\,OB2 region, ∼3.5−5 Myr old, contains one of the most significant populations of massive stars of the Milky Way. Such stars substantially contribute to producing large scale soft (3 keV) diffuse X-ray emission in the direction of the pulsar PSR\,J2032+4127. The torus-shaped emission spans a ≈3′×2′ jet-like structure. It is spatially coincident with the Fermi γ-ray source 4FGL\,J2032.3+4127. We suggest that the hard diffuse X-ray emission is the pulsar wind nebula bearing the pulsar PSR\,J2032+4127, a consequence of a past core-collapse SN explosion in the region.la región de Cygnus OB2, de∼3.5−5×106 años de edad, contiene una de las poblaciones estelaresde alta masa más grandes de la Vía Láctea. Estas estrellas contribuyen a la emisión difusa observada a gran escala en rayos X blandos (3 keV) en direccion del pulsar PSR J2032+4127. Esta emisión elongada de forma toroidal abarca una estructura tipo jetde tamaño ≈3′×2′, y es coincidente con la posición de la fuente de rayos-γ4FGL J2032.3+4127. Confirmamos que la emisión difusa de rayos X duros es la nebulosa pulsante originada por el pulsar PSR J2032+4127, como consecuancia deuna explosion de SN tipocore-collapseen la regi ́on
Hard diffuse X-ray emission around the PSR J2032+4127: A pulsar wind nebula in the Cygnus OB2 association
No full textFil: Albacete Colombo, Juan F. Universidad Nacional de Río Negro. Río Negro, Argentina.Fil: Drake, Jeremy J. Smithsonian Astrophysical Observator. Cambridge, USA.Fil: Filocomo, Agostina. Universidad Nacional de Río Negro. Río Negro, Argentina.Fil: Wright, Nick J. Keele University, Astrophysics Group. Keele, UK.The Cygnus\,OB2 region, ∼3.5−5 Myr old, contains one of the most significant populations of massive stars of the Milky Way. Such stars substantially contribute to producing large scale soft (3 keV) diffuse X-ray emission in the direction of the pulsar PSR\,J2032+4127. The torus-shaped emission spans a ≈3′×2′ jet-like structure. It is spatially coincident with the Fermi γ-ray source 4FGL\,J2032.3+4127. We suggest that the hard diffuse X-ray emission is the pulsar wind nebula bearing the pulsar PSR\,J2032+4127, a consequence of a past core-collapse SN explosion in the region.truela región de Cygnus OB2, de∼3.5−5×106 años de edad, contiene una de las poblaciones estelaresde alta masa más grandes de la Vía Láctea. Estas estrellas contribuyen a la emisión difusa observada a gran escala en rayos X blandos (3 keV) en direccion del pulsar PSR J2032+4127. Esta emisión elongada de forma toroidal abarca una estructura tipo jetde tamaño ≈3′×2′, y es coincidente con la posición de la fuente de rayos-γ4FGL J2032.3+4127. Confirmamos que la emisión difusa de rayos X duros es la nebulosa pulsante originada por el pulsar PSR J2032+4127, como consecuancia deuna explosion de SN tipocore-collapseen la regi ́on
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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