1,721,172 research outputs found
Suzaku Observations of 4U 1957+11: Potentially the Most Rapidly Spinning Black Hole in (the Halo of) the Galaxy
Get PDFWe present three Suzaku observations of the black hole candidate 4U 1957+11 (V 1408 Aql) - a source that exhibits some of. the simplest and cleanest examples of soft, disk-dominated spectra. 4U 1957+ II also presents among the. highest peak temperatures found from disk-dominated spectra. Such temperatures may be associated with rapid black hole spin. The 4U 1957+11 spectra also require a very low normalization, which can be explained by a combination of small inner disk radius and a large distance (> 10 kpc) which places 4U 1957+ 11 well into the Galactic halo. We perform Joint fits to the Suzaku spectra with both relativistic and Comptonized disk models. Assuming a low mass black hole and the nearest distance (3 Stellar Mass, 10 kpc), the dimensionless spin parameter a* = Jc/GM(sup 2)> or approx. 0.9. Higher masses and farther distances yield a* approx. = 1. Similar conclusions are reached with Comptonization models; they imply a combination of small inner disk radii (or, equivalently, rapid spin) and large distance. Low spin cannot be recovered unless 4U 1957+11 is a low mass black hole that is at the unusually large distance of > or approx.40 kpc. We speculate whether the suggested maximal spin is related to how the system came to reside in the halo
Si K EDGE STRUCTURE AND VARIABILITY IN GALACTIC X-RAY BINARIES
No full textWe survey the Si K edge structure in various absorbed Galactic low-mass X-ray binaries (LMXBs) to study states of silicon in the inter- and circum-stellar medium. The bulk of these LMXBs lie toward the Galactic bulge region and all have column densities above 10 22 cm ⁻². The observations were performed using the Chandra High Energy Transmission Grating Spectrometer. The Si K edge in all sources appears at an energy value of 1844 ± 0.001 eV. The edge exhibits significant substructure that can be described by a near edge absorption feature at 1849 ± 0.002 eV and a far edge absorption feature at 1865 ± 0.002 eV. Both of these absorption features appear variable with equivalent widths up to several mÅ. We can describe the edge structure using several components: multiple edge functions, near edge absorption excesses from silicates in dust form, signatures from X-ray scattering optical depths, and a variable warm absorber from ionized atomic silicon. The measured optical depths of the edges indicate much higher values than expected from atomic silicon cross sections and interstellar medium abundances, and they appear consistent with predictions from silicate X-ray absorption and scattering. A comparison with models also indicates a preference for larger dust grain sizes. In many cases, we identify Si xiii resonance absorption and determine ionization parameters between log ξ = 1.8 and 2.8 and turbulent velocities between 300 and 1000 km s ⁻¹. This places the warm absorber in close vicinity of the X-ray binaries. In some data, we observe a weak edge at 1.840 keV, potentially from a lesser contribution of neutral atomic silicon. Keywords: dust, extinction; ISM: abundances; techniques: spectroscopic; X-rays: binaries; X-rays: IS
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
XIPE: the x-ray imaging polarimetry explorer
Get PDFXIPE, the X-ray Imaging Polarimetry Explorer, is a mission dedicated to X-ray Astronomy. At the time of writing XIPE is in a competitive phase A as fourth medium size mission of ESA (M4). It promises to reopen the polarimetry window in high energy Astrophysics after more than 4 decades thanks to a detector that efficiently exploits the photoelectric effect and to X-ray optics with large effective area. XIPE uniqueness is time-spectrally-spatially-resolved X-ray polarimetry as a breakthrough in high energy astrophysics and fundamental physics. Indeed the payload consists of three Gas Pixel Detectors at the focus of three X-ray optics with a total effective area larger than one XMM mirror but with a low weight. The payload is compatible with the fairing of the Vega launcher. XIPE is designed as an observatory for X-ray astronomers with 75 % of the time dedicated to a Guest Observer competitive program and it is organized as a consortium across Europe with main contributions from Italy, Germany, Spain, United Kingdom, Poland, Sweden
The Ionized Stellar Wind in Vela X-1 during Eclipse
No full textWe present a first analysis of a high-resolution X-ray spectrum of the ionized stellar wind of Vela X-1 during eclipse. The data were obtained with the High-Energy Transmission Grating Spectrometer on board the Chandra X-Ray Observatory. The spectrum is resolved into emission lines with fluxes between 0.02 and 1.04 × 10[superscript -4] photons cm[superscript -2] s[superscript -1]. We identify lines from a variety of charge states, including fluorescent lines from cold material and a warm photoionized wind. We can exclude signatures from collisionally ionized plasmas. For the first time, we identify fluorescent lines from L-shell ions from lower Z elements. We also detect radiative recombination continua from a kT = 10 eV (1.2 × 10[superscript 5] K) photoionized optically thin gas. The fluorescent line fluxes imly the existence of optically thick and clumped matter within or outside the warm photoionized plasma.Smithsonian Astrophysical Observatory (SV1-61010)United States. National Aeronautics and Space Administration (Contract NAS 8-39073
The High-Resolution X-Ray Spectrum of SS 433 Using the Chandra HETGS
No full textWe present observations of SS 433 using the Chandra High Energy Transmission Grating Spectrometer. Many emission lines of highly ionized elements are detected with relativistic blue and red Doppler shifts. The lines are measurably broadened to 1700 km s[superscript -1] (FWHM), and the widths do not depend significantly on the characteristic emission temperature, suggesting that the emission occurs in a freely expanding region of constant collimation with an opening angle of 1[° over .]23 ± 0[° over .]06. The blueshifts of lines from low-temperature gas are the same as those of high-temperature gas within the uncertainties, again indicating that the hottest gas we observe to emit emission lines is already at terminal velocity. This velocity is 0.2699 ± 0.0007c, which is larger than the velocity inferred from optical emission lines by 2920 ± 440 km s[superscript -1]. Fits to the emission-line fluxes give a range of temperatures in the jet from 5 × 10[superscript 6] to 1 × 10[superscript 8] K. We derive the emission measure as a function of temperature for a four-component model that fits the line flux data. Using the density-sensitive Si XIII triplet, the characteristic electron density is 10[superscript 14] cm[superscript -3], where the gas temperature is about 1.3 × 10[superscript 7] K. Based on an adiabatic expansion model of the jet and a distance of 4.85 kpc, the electron densities drop from ~2 × 10[superscript 15] to 4 × 10[superscript 13] cm[superscript -3] at distances of (2-20) × 10[superscript 10] cm from the apex of the cone that bounds the flow. The radius of the base of the visible jet is estimated to be ~10[superscript 8] cm, and the mass outflow rate is 1.5 × 10[superscript -7] M[subscript ☉] yr[superscript -1]. The kinetic power is 3.2 × 1038 ergs s[superscript -1], which is 1000 times larger than the unabsorbed 2-10 keV X-ray luminosity. The bremsstrahlung emission associated with the lines can account for the entire continuum; we see no direct evidence for an accretion disk. The image from zeroth order shows extended emission at a scale of ~2'', aligned in the general direction of the radio jets.United States. National Aeronautics and Space Administration (Contract NAS 8-38249)United States. National Aeronautics and Space Administration (Contract SVI-61010
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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