1,721,059 research outputs found
Axial quasi-normal modes of neutron stars: accounting for the superfluid in the crust
No full textWe present the results of the first study of global oscillations of relativistic stars with both elastic crusts and interpenetrating superfluid components. For simplicity, we focus on the axial quasi-normal modes. Our results demonstrate that the torsional crust modes are essentially unaffected by the coupling to the gravitational field. This is as expected since these oscillations are known to be weak gravitational-wave sources. In contrast, the presence of a loosely coupled superfluid neutron component in the crust can have a significant effect on the oscillation spectrum. We show that the entrainment between the superfluid and the crust nuclei is a key parameter in the problem. Our analysis highlights the need for a more detailed understanding of the coupled crust-superfluid at the microphysical level. Our numerical results have, even though we have not considered magnetized stars, some relevance for efforts to carry out seismology based on quasi-periodic oscillations observed in the tails of magnetar flares. In particular, we argue that the sensitive dependence on the entrainment may have to be accounted for in attempts to match theoretical models to observational data.<br/
Oscillations in the neutron star crust: Neutron star seismology from QPOs after flares
No full textWe investigate the spectrum of torsional modes in the neutron star crust and discuss what conclusions may be drawn about the global properties of the star from observations of such modes
Neutron star asteroseismology: axial crust oscillations in the Cowling approximation
No full textRecent observations of quasi-periodic oscillations in the aftermath of giant flares in soft gamma-ray repeaters suggest a close coupling between the seismic motion of the crust after a major quake and the modes of oscillations in a magnetar. In this paper, we consider the purely elastic modes of oscillation in the crust of a neutron star in the relativistic Cowling approximation (disregarding any magnetic field). We determine the axial crust modes for a large set of stellar models, using a state-of-the-art crust equation of state and a wide range of core masses and radii. We also devise useful approximate formulae for the mode-frequencies. We show that the relative crust thickness is well described by a function of the compactness of the star and a parameter describing the compressibility of the crust only. Considering the observational data for SGR 1900+14 and SGR 1806?20, we demonstrate how our results can be used to constrain the mass and radius of an oscillating neutron star
Magnetohydrodynamics of superfluid and superconducting neutron star cores
No full textMature neutron stars are cold enough to contain a number of superfluid and superconducting components. These systems are distinguished by the presence of additional dynamical degrees of freedom associated with superfluidity. In order to consider models with mixtures of condensates, we need to develop a multifluid description that accounts for the presence of rotational neutron vortices and magnetic proton fluxtubes. We also need to model the forces that impede the motion of vortices and fluxtubes, and understand how these forces act on the condensates. This paper concerns the development of such a model for the outer core of a neutron star, where superfluid neutrons co-exist with a type II proton superconductor and an electron gas. We discuss the hydrodynamics of this system, focusing on the role of the entrainment effect, the magnetic field, the vortex/fluxtube tension and the dissipative mutual friction forces. Our final results can be directly applied to a number of interesting astrophysical scenarios, e.g. associated with neutron star oscillations or the evolution of the large-scale magnetic fiel
A characteristic approach to the quasi-normal mode problem
No full textIn this paper we discuss a new approach to the quasi-normal mode problem in general relativity. By combining a characteristic formulation of the perturbation equations with the integration of a suitable phase-function for a complex-valued radial coordinate, we reformulate the standard outgoing-wave boundary condition as a zero Dirichlet condition. This has a number of important advantages over previous strategies. The characteristic formulation permits coordinate compactification, which means that we can impose the boundary condition at future null infinity. The phase function avoids oscillatory behaviour in the solution, and the use of a complex radial variable allows a clean distinction between out- and ingoing waves. We demonstrate that the method is easy to implement, and that it leads to high precision numerical results. Finally, we argue that the method should generalize to the important problem of rapidly rotating neutron star spacetimes
Monthly notices letters. L74. Elastic or magnetic? A toy model for global magnetar oscillations with implications for QPOs during flares
No full textA toy model for global magnetar oscillation
No full textThe presence of a magnetic field in a neutron star interior results in a dynamical coupling between the fluid core and the elastic crust. We consider a simple toy-model where this coupling is taken into account and compute the system’s mode oscillations. Our results suggest that the notion of pure torsional crust modes is not useful for the coupled system, instead all modes excite Alfvén waves in the core. However, we also show that among a rich spectrum of global MHD modes the ones most likely to be excited by a fractured crust are those for which the crust and the core oscillate in concert. For our simple model, the frequencies of these modes are similar to the “pure crustal” frequencies. We advocate the significant implications of these results for the attempted theoretical interpretation of QPOs during magnetar flares in terms of neutron star oscillations
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
Everyday Life in the Culture of Surveillance
No full textOver the recent decades, the possibilities to surveil people have increased and been refined with the ongoing digital transformation of society. Surveillance can now go in any direction, and various forms of online surveillance saturate most people’s lives, which are increasingly lived in digital environments. To understand this situation and nuance the contemporary discussions about surveillance – not least in the highly digitalised context of the Nordic countries – we must adopt cultural and ethical perspectives in studying people’s attitudes, motives, and behaviours. The “culture of surveillance”, to borrow David Lyon’s term, is a culture where questions about privacy and publicness, and rights and benefits, are once again brought to the fore.
This anthology takes up this challenge, with contributions from a variety of disciplinary and theoretical frameworks that discuss and shed light on the complexity of contemporary surveillance and thus problematise power relations between the many actors involved in the development and performance of surveillance culture. The contributions highlight how more and more actors and practices play a part in our increasingly digitalised society.
The book is an outcome of the research project “iAccept: Soft surveillance – between acceptance and resistance”, financed by the Marcus and Amalia Wallenberg Foundation. The anthology’s editors are project members, all based at Umeå University, Sweden: Lars Samuelsson, associate professor of philosophy; Coppélie Cocq, professor of Sámi studies and digital humanities; Stefan Gelfgren, associate professor of sociology of religion; and Jesper Enbom, associate professor of media studies.PublishedOver the recent decades, the possibilities to surveil people have increased and been refined with the ongoing digital transformation of society. Surveillance can now go in any direction, and various forms of online surveillance saturate most people’s lives, which are increasingly lived in digital environments. To understand this situation and nuance the contemporary discussions about surveillance – not least in the highly digitalised context of the Nordic countries – we must adopt cultural and ethical perspectives in studying people’s attitudes, motives, and behaviours. The “culture of surveillance”, to borrow David Lyon’s term, is a culture where questions about privacy and publicness, and rights and benefits, are once again brought to the fore.
This anthology takes up this challenge, with contributions from a variety of disciplinary and theoretical frameworks that discuss and shed light on the complexity of contemporary surveillance and thus problematise power relations between the many actors involved in the development and performance of surveillance culture. The contributions highlight how more and more actors and practices play a part in our increasingly digitalised society.
The book is an outcome of the research project “iAccept: Soft surveillance – between acceptance and resistance”, financed by the Marcus and Amalia Wallenberg Foundation. The anthology’s editors are project members, all based at Umeå University, Sweden: Lars Samuelsson, associate professor of philosophy; Coppélie Cocq, professor of Sámi studies and digital humanities; Stefan Gelfgren, associate professor of sociology of religion; and Jesper Enbom, associate professor of media studies
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