1,720,996 research outputs found
The High-Energy Spectra of Accreting Black Holes: Observational Evidence for Bulk-Motion Infall
No full textWe discuss the emergent spectra from accreting black holes, considering in particular the case where the accretion is characterized by relativistic bulk motion. We suggest that such accretion is likely to occur in a wide variety of black hole environments, where the strong gravitational field is expected to dominate the pressure forces, and that this is likely to lead to a characteristic high-energy spectroscopic signature: an extended power-law tail. It is in the high (soft) state that matter impinging upon the event horizon can be viewed directly, and the intrinsic power law is seen. Certain types of active galactic nuclei (AGNs) may represent the extragalactic analog of the high-soft state accretion, which would further support our ideas, demonstrating the stability of the ( alpha ~1.8 ) power law. This stability is a result of the asymptotic independence of the spectral index on the mass accretion rate and its weak dependence on plasma temperatures. We have computed the expected spectral energy distribution for an accreting black hole binary in terms of our three model parameters: the disk color temperature, a geometric factor related to the illumination of the black hole site by the disk, and a spectral index related to the efficiency of the bulk-motion up-scattering. We emphasize that this is a fully self-consistent approach, and it is not to be confused with the more common phenomenological methods employing additive power law and blackbody or multicolor disk. A test of the model is presented using observational data from the Compton Gamma Ray Observatory (CGRO) and the Rossi X-Ray Timing Explorer (RXTE), covering ~=2-200 keV for two recent galactic black hole X-ray nova outbursts. The resulting model fits are encouraging and, along with some observational trends cited from the literature, they support our bulk-motion hypothesis
A Method for Black Hole Mass Determination in Accretion-powered X-Ray Sources
No full textWe describe a method for the determination of black hole masses based on information inferred from high-energy spectra. It is required that the spectral energy distribution consist of thermal and Comptonized components. One can then, in principle, infer the depth of the gravitational potential well for sources of known distance. The thermal component is inferred by the integration of a blackbody spectral form over the disk. We assume that the color temperature distribution in the disk has a specific shape given by the Shakura-Sunyaev disk model that goes to zero at the inner disk radius and at infinity and has a maximum at 4.2 R_S. In this formulation there is only one parameter, the so-called color correction factor, relating the apparent temperature to effective temperature, which characterizes the thermal emission component. We have made use of improved Galactic black hole binary dynamical mass determinations to derive, in effect, an empirical calibration of this factor. We then present our analysis of observational data for representative objects of several classes: Galactic black hole X-ray binaries, narrow-line Seyfert galaxies (NLS1s), and ``ultraluminous'' extragalactic X-ray sources (ULXs). We then apply our mass determination calculation and present our results. We argue that this approach can potentially fill a void in the current knowledge of NLS1 and ULX properties and discuss how a deeper understanding of both classes has relevance to the broader issues of how cosmic black holes, beyond the stellar-mass realm, are formed and what is their overall mass distribution
Mass Determination of Black Holes in LMC X-1 and Nova MUSCAE 1991 from Their High-Energy Spectra
No full textWe offer a brief description of the bulk motion Comptonization (BMC) model for accretion onto black holes, illustrated by its application to observational data for LMC X-1 and Nova Muscae 1991. We then extract some physical parameters of these systems from observables within the context of the BMC model, drawing from results on GRO J1655-40, for which we presented extensive analysis previously. We derive estimates of the mass, (16+/-1)M_solarx[0.5/cos(i)]^1/2, and mass accretion rate in the disk in Eddington units (m_d~=2) for LMC X-1 and (24+/-1)M_solarx[0.5/cos(i)]^1/2d_5.5 and (m_d~=3) for Nova Muscae 1991, where cos(i) and d_5.5 are the inclination angle cosine and distance in 5.5 kpc units, respectively. Differences between these estimates and previous estimates based on dynamical studies are discussed. It is further shown that the disk inner radius increases with the high-to-low-state transition in Nova Muscae 1991. Specifically, our analysis suggests that the inner disk radius increases to r_in~=17 Scwarzschild radii as the transition to the low-hard state occurs
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
New Evidence for a Black Hole in the Compact Binary Cygnus X-3
Full text linkThe bright and highly variable X-ray and radio source known as Cygnus X-3 was among the first X-ray sources discovered, yet it remains in many ways an enigma. Its known to consist of a massive. Wolf-Rayet primary in an extremely tight orbit with a compact object. Yet one of the most basic of pa.ranietern the mass of the compact object - is not known. Nor is it even clear whether its is a neutron star or a black hole. In this Paper we present our analysis of the broad-band high-energy continua covering a substantial range in luminosity and spectral morphology. We apply these results to a recently identified scaling relationship which has been demonstrated to provide reliable estimates of the compact object mass in a number of accretion powered binaries. This analysis leads us to conclude that the compact object in Cygnus X-3 has a mass greater than 4.2 solar mass thus clearly indicative of a black hole and as such resolving a longstanding issue. The full range of uncertainty in our analysis and from using a. range of recently published distance estimates constrains the compact object mass to lie between 4.2 solar mass and 14.4 solar mass. Our favored estimate, based on a 9.0 kpc distance estimate is approx. l0 solar mass, with the. error margin of 3.2 solar masses. This result may thus pose challenges to shared-envelope evolutionary models of compact binaries. as well as establishing Cygnus X-3 as the first confirmed accretion-powered galactic gamma: ray source
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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