1,721,052 research outputs found
Radiative Transfer of Polarized X-rays: Magnetized Thomson Scattering in Neutron Stars
No full textThis thesis is a focused study of the polarization characteristics of radiative transfer in a strong magnetic field. The main process examined here is magnetized Compton scattering in a non-relativistic regime (i.e.\ magnetized Thomson scattering), and we focus on applying this study to predict polarization properties of the X-ray emission from magnetars.
Magnetars are a highly magnetic sub-class of neutron stars, characterized by their extremely high surface magnetic fields, comparable to or exceeding the quantum critical field ( Gauss) at which an electron's cyclotron energy and rest mass energy are equal.
There are 29 known/candidate magnetars at this time, and they commonly exhibit persistent quasi-thermal surface emission in soft X-rays with flat tails extending into the hard X-rays up to around 150 keV, as well as transient bursting activity in hard X-rays attributed to magnetospheric flares.
Magnetized Thomson scattering refers to electron-photon scattering in a background magnetic field.
The field introduces anisotropy to the problem, giving it a more complicated angular dependence.
It also produces a strong frequency dependence to the cross section: it is resonant at the cyclotron frequency .
Additionally, electron motion perpendicular to the field becomes increasingly suppressed at higher field strengths, leading to a reduction in the cross section for certain incoming photon angles when .
There are complicated polarization characteristics for the process as well, with the differential cross section depending on the initial and final polarization state of the photon.
An important distinction occurs between photons that have a component of linear polarization parallel to the field and those that are fully orthogonal to it.
We explore this process in detail using a Monte Carlo simulation model, treating the transfer primarily in slab geometries, a common simplification. This allows a direct comparison with previous work and is an important step towards achieving more complicated geometries and scattering regimes. We fully map the frequency and angular dependence of this process in the optically thick regime, capturing both resonant and non-resonant properties of the scattering.
We present results for a model of magnetar persistent surface emission, as well as a simple magnetar flare model. In both cases the transfer is purely due to magnetic Thomson scattering, and we superimpose the emission from regions of different temperature, density, and magnetic field.
For magnetar surface emission, we see a phase-dependent linear polarization, forming either a single- or double-peaked pattern with a maximum level of roughly , depending on the angle between the observer and the spin axis.
This could have important implications for polarimetric determination of the effect of vacuum birefringence as polarized X-rays transfer through the magnetosphere to infinity. For the flare model we see much stronger polarization signals as the emission is coming from more localized regions, and it is highly dependent on viewing geometry and frequency.
A secondary process is also examined due to its importance in magnetized plasmas: the so-called generalized Faraday effect. This is analogous to the ordinary Faraday effect, where the phase lag caused by birefringence of circular eigenmodes of electromagnetic waves produces a constant rotation of the plane of linear polarization for a propagating wave. The generalized effect occurs when the eigenmodes are no longer circular, and it produces a very similar rotation when viewed in terms of the Poincar\'e sphere. When this effect is assumed to be prolific, the transfer can be reformulated in terms of the normal modes in what is called the normal mode description. We explore the parameter space in which this description is valid, and develop a method to handle transfer in certain regimes where it is invalid. This prepares the way for incorporating such nuances in future developments of the magnetized radiative transfer problem
Magnetic pair creation transparency in gamma-ray pulsars
No full textMagnetic pair creation, γ [arrow right] e + e - , is a key component in polar cap models of gamma-ray pulsars, and has informed assumptions about the still poorly understood radio emission. The Fermi Gamma-Ray Space Telescope has now detected more than 100 γ-ray pulsars, providing rich information for the interpretation of young energetic pulsars and old millisecond pulsars. Fermi observations have established that the high-energy spectra of most of these pulsars have exponential turnovers in the 1-10 GeV range. These turnovers are too gradual to arise from magnetic pair creation in the strong magnetic fields of pulsar inner magnetospheres, so their energy can be used to provide a physically motivated lower bound to the typical altitude of GeV band emission. This work computes pair creation opacities for photon propagation in neutron star magnetospheres. It explores the constraints that can be placed on the emission location of Fermi γ-rays due to single-photon pair creation transparency below the turnover energy, as well as the limitations of this technique. These altitude bounds are typically in the range of 2-6 neutron star radii for the Fermi pulsar sample, and provide one of the few possible constraints on the emission altitude in radio quiet pulsars that do not possess double-peaked pulse profiles
Polarized Radiative Transfer in the Magnetospheres and Atmospheres of Neutron Stars
No full textNeutron stars are sources of strongly polarized emission in X-rays or soft gamma-rays due to the presence of strong magnetic fields. Radiation transport of soft X-rays in neutron star surface layers is critical to the determination of the emergent anisotropy of light intensity and polarization signatures. Additionally high-energy photons propagating in neutron star magnetospheres can be attenuated by QED processes like photon splitting and magnetic pair creation. In this thesis, I explore the scattering transport in the classical magnetic Thomson domain using Monte Carlo technique. Representative results for emergent polarization signals from surface layers are presented for both localized and extended surface regions with magnetic field strengths that are of broad applicability to different neutron star classes. These results provide an important background for observations acquired by polarimetry missions like IXPE. I also explore polarization-dependent opacities for the two QED processes in static dipolar or twisted magnetospheres of highly magnetized neutron stars like magnetars, calculating attenuation lengths and determining escape energies, which are the maximum photon energies for transparency out to infinity. These opacity calculations put constraints on the possible emission locales and the strengths of the magnetospheric twists, and apply not only to magnetar flares but also to their quiescent hard X-ray tail emission. An exploration of photon splitting attenuation in the context of a resonant inverse Compton scattering model for the hard X-ray tails derives distinctive phase-resolved spectroscopic and polarimetric signatures, of significant interest for future MeV-band missions such as AMEGO and e-ASTROGAM
Resonant Compton Scattering in Highly-Magnetized Pulsars
No full textSoft gamma repeaters and anomalous X-ray pulsars are subset of slow-rotating neutron stars, known as magnetars, that have extremely
high inferred surface magnetic fields, of the order 100-1000 TeraGauss. Hard, non-thermal and pulsed persistent X-ray emission extending between 10 keV and 230 keV has been seen in a number of magnetars by RXTE, INTEGRAL, and Suzaku. In this thesis, the author considers inner magnetospheric models of such persistent hard X-ray emission where resonant Compton upscattering of soft thermal photons is anticipated to be the most efficient radiative process. This high efficiency is due to the relative proximity of the surface thermal photons, and also because the scattering becomes resonant at the cyclotron frequency. At the cyclotron resonance, the effective cross section exceeds the classical Thomson one by over two orders of magnitude, thereby enhancing the efficiency of continuum production and cooling of relativistic electrons. In this thesis, a new Sokolov and Ternov formulation of the QED Compton scattering cross section for strong magnetic fields is employed in electron cooling and emission spectra calculations. This formalism is formally correct for treating spin-dependent effects and decay rates that are important at the cyclotron resonance. The author presents electron cooling rates at arbitrary interaction points in a magnetosphere using the QED cross sections. The QED effects reduce the rates below high-field extrapolations of older magnetic Thomson results. The author also computes angle-dependent upscattering model spectra, formed using collisional integrals, for uncooled monoenergetic relativistic electrons injected in inner regions of pulsar magnetospheres. These spectra are integrated over closed field lines and obtained for different observing perspectives. The spectral cut-off energies are critically dependent on the observer viewing angles and electron Lorentz factor. It is found that electrons with energies less than around 15 MeV will emit most of their radiation below 250 keV, consistent with the observed turnovers in magnetar hard X-ray tails. Moreover, electrons of higher energy still emit most of the radiation below 1 MeV, except for very select viewing perspectives that sample tangents to field lines. This small parameter space makes it difficult to observe signals extending into the Fermi-LAT band. Polarization dependence in spectra is illustrated, offering potential constraints for models of magnetar emission in anticipation of a future hard X-ray polarimetry missions
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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