1,721,811 research outputs found
Internal shock model for microquasars
No full textWe present a model for the radio outbursts of microquasars based on the assumption of quasi-continuous jet ejection. The jets are 'lit up' by shock fronts traveling along the jets during outbursts. The shocks accelerate relativistic particles which emit the observed synchrotron radiation. The observed comparatively flat decay light curves combined with gradually steepening spectral slopes are explained by a superposition of the radiation of the aging relativistic particle population left behind by the shocks. This scenario is the low energy, time-resolved equivalent to the internal shock model for GRBs. We show that this model predicts energy contents of the radiating plasma similar to the plasmon model. At the same time, the jet model relaxes the severe requirements on the central source in terms of the rate at which this energy must be supplied to the jet. Observations of 'mini-bursts' with flat spectral slopes and of infrared emission far from the source centre suggest two different states of jet ejections: (i) A 'mini-burst' mode with relatively stable jet production and weak radio emission with flat spectra and (ii) an outburst mode with strong variations in the jet bulk velocities coupled with strong radio emission with steeper spectra. We also show that the continuous jets in microquasars should terminate in strong shocks and possibly inflate radio lobes similar to extragalactic jet sources. We investigate the possibility of testing the predictions of this model with resolved radio observations. Finally, we suggest that Doppler-shifted X-ray iron lines, and possibly H-alpha lines, may be emitted by the jet flow of microquasars if thermal instabilities analogous to those in SS433 exist in their jets
Comptonization of low-frequency radiation in accretion disks Angular distribution and polarization of hard radiation
No full textIn accretion disks the angular distribution and polarization of hard radiation forming via Comptonization, i.e., multiple scatterings in the disk, depend only on the optical thickness of the disk and are independent of either the photon frequency or the geometric distribution of low-frequency photons. This paper presents calculations of the angular distribution and polarization made for several values of the optical thickness of disks, for the case of the Thomson scattering. The method of successive approximations over the number of scatterings was employed in the calculations, without taking into account the frequency dependence of the scattering cross-section and indicatrix. Specific astrophysical applications of the results obtained are discussed. A simplified theory of electron temperature dependence on the vertical coordinate in the disk is given. The density distribution over the vertical coordinate in the radiation-dominated disks is derived
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
MODELING X-RAY DATA OF BLACK HOLE BINARIES
Full text linkWe test the truncated disc models using multiwavelength (optical/UV/X-ray) data from the 2005 hard state outburst of the black hole SWIFT J1753.5-0127. This system is both fairly bright and has fairly low interstellar absorption, so gives one of the best datasets to study the weak, cool disc emission in this state. We fit these data using models of an X-ray illuminated disc to constrain the inner
disc radius throughout the outburst. Close to the peak, the observed soft X-ray component is consistent with being produced by the inner disc, with its intrinsic emission enhanced in temperature and luminosity by reprocessing of hard X-ray illumination in an overlap region between the disc and corona. This disc emission provides the seed photons for Compton scattering to produce the hard X-ray
spectrum, and these hard X-rays also illuminate the outer disc, producing the optical emission by reprocessing.
However, towards the end of the outburst, all these conclusions may change. The optical points clearly lie on an extrapolation of the hard X-ray flux, which may indicate that the seed photons for Compton scattering are now self-generated in the flow by Cyclo-Synchrotron radiation rather than being from the disc. The weak soft X-ray emission implies a small disc radius, unchanged from the outburst peak, in conflict with the expectations of the truncated disc model. However, this also requires that the energy to power the corona is advected vertically and radially in a dissipationless fashion from the disc. Thus it seems more likely that the soft X-ray component is not direct emission from the disc itself. We show that a similarly dim low/hard state spectrum from XTE J1118+480 puts similar constraints on the soft X-ray emission region, but here the very low interstellar absorption (an order of magnitude smaller than in SWIFT J1753.5-0127) allows detection of a much larger, cooler, UV component which is well fit by a truncated disc. Thus whatever the origin of the additional weak soft X-ray emission (irradiation of the inner face as opposed to the top surface of the inner edge of the disc, residual inner disc left from evaporation, ionised reflection, jet etc), its existence as a clearly separate component from the truncated disc in XTE J1118+480 shows that it does not trace the inner disc radius, so cannot be used to constrain the truncated disc models
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
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