1,721,441 research outputs found
The inside-out view on neutron-star magnetospheres
No full textWe construct hydromagnetic neutron star equilibria which allow for a non-zero electric current distribution in the exterior. The novelty of our models is that the neutron star's interior field is in equilibrium with its magnetosphere, thus bridging the gap between previous work in this area, which either solves for the interior assuming a vacuum exterior or solves for the magnetosphere without modelling the star itself. We consider only non-rotating stars in this work, so our solutions are most immediately applicable to slowly rotating systems such as magnetars. Nonetheless, we demonstrate that magnetospheres qualitatively resembling those expected for both magnetars and pulsars are possible within our framework. The ‘inside-out’ approach taken in this paper should be more generally applicable to rotating neutron stars, where the interior and exterior regions are again not independent but evolve together
Resistive relativistic magnetohydrodynamics from a charged multifluids perspective
No full textWe consider general relativistic magnetohydrodynamics from a charged multifluids point of view, taking a variational approach as our starting point. We develop the case of two charged components in detail, accounting for a phenomenological resistivity, providing specific examples for pair plasmas and proton-electron systems. We discuss both cold, low-velocity, plasmas and hot systems where we account for a dynamical entropy component. The results for the cold case (which accord with recent work in the literature) provide a complete model for resistive relativistic magnetohydrodynamics, clarifying the assumptions that lead to various models that have been used in astrophysical applications. The analysis of the hot case is (as far as we are aware) novel, accounting for the relaxation times that are required to ensure causality and demonstrating the explicit coupling between fluxes of heat and charg
Variational multi-fluid dynamics and causal heat conductivity
No full textWe discuss heat conductivity from the point of view of a variational multi-fluid model, treating entropy as a dynamical entity. We demonstrate that a two-fluid model with a massive fluid component and a massless entropy can reproduce a number of key results from extended irreversible thermodynamics. In particular, we show that the entropy entrainment is intimately linked to the thermal-relaxation time that is required to make heat propagation in solids causal. We also discuss non-local terms that arise naturally in a dissipative multi-fluid model, and relate these terms to those of phonon hydrodynamics. Finally, we formulate a complete heat-conducting two-component model and discuss briefly the new dissipative terms that arise. <br/
The g-mode spectrum of reactive neutron star cores
No full textWe discuss the impact of nuclear reactions on the spectrum of gravity g-modes of a mature neutron star, demonstrating the anticipated disappearance of these modes when the timescale associated with the oscillations is longer than that of nuclear reactions. This is the expected result, but different aspects of the demonstration may be relevant for related problems in neutron star astrophysics. In particular, we develop the framework required for an explicit implementation of finite-time nuclear reactions in neutron star oscillation problems and demonstrate how this formulation connects with the usual bulk viscosity prescription. We also discuss implications of the absence of very high order g-modes for problems of astrophysical relevance
The asymptotic quasinormal mode spectrum of non-rotating black holes
No full textA conjectured connection to quantum gravity has led to a renewed interest in highly damped black-hole quasinormal modes (QNMs). In this paper we present simple derivations (based on the WKB approximation) of conditions that determine the asymptotic QNMs for both Schwarzschild and Reissner–Nordström black holes. This confirms recent results obtained by Motl and Neitzke, but our analysis fills several gaps left by their discussion. We study the Reissner–Nordström results in some detail, and show that, in contrast to the asymptotic QNMs of a Schwarzschild black hole, the Reissner–Nordström QNMs are typically not periodic in the imaginary part of the frequency. This leads to the charged black hole having peculiar properties which complicate an interpretation of the results in the context of quantum gravity
Probing neutron-star superfluidity with gravitational-wave data
No full textWe discuss the possibility that future gravitational-wave detectors may be able to detect various modes of oscillation of old, cold neutron stars. We argue that such detections would provide unique insights into the superfluid nature of neutron-star cores, and could also lead to a much improved understanding of pulsar glitches. Our estimates are based on a detector configuration with several narrow-band (cryogenic) interferometers operating as a "xylophone" which could lead to high sensitivity at high frequencies. We also draw on recent advances in our understanding of the dynamics of pulsating superfluid neutron-star cores
Merger-inspired rotation laws and the low-T/W instability in neutron stars
No full textImplementing a family of differential rotation laws inspired by binary neutron-star merger remnants, we consider the impact of the rotation profile on the low-T/W instability. We use time evolutions of the linearised dynamical equations, in Newtonian gravity, to study non-axisymmetric oscillations and identify the unstable modes. The presence and evolution of the low-T/W instability is monitored with the canonical energy and angular momentum, while the growth time is extracted from the evolved kinetic energy. The results for the new rotation laws highlight similarities with the commonly considered j-constant law. The instability sets in when an oscillation mode co-rotates with the star (i.e. whenever there is a point where the mode's pattern speed matches the bulk angular velocity) and grows faster deep inside the co-rotation region. However, the new profiles add features, like an additional co-rotation point to the problem, which affect the onset of instability. The rotation laws influence more drastically the oscillation frequencies of the l=m=2 f-mode in fast rotating models, but affect the instability growth time at any rotation rate. We also identify models where the low-T/W instability appears to be triggered by inertial modes. We discuss to what extent the inferred qualitative behaviour is likely to be of observational relevance
Gravitational waves from freely precessing neutron stars
No full textIn this paper we model the gravitational wave emission of a freely precessing neutron star. The aim is to estimate likely source strengths, as a guide for gravitational wave astronomers searching for such signals. We model the star as a partly elastic, partly fluid body with quadrupolar deformations of its moment of inertia tensor. The angular amplitude of the free precession is limited by the finite breaking strain of the star's crust. The effect of internal dissipation on the star is important, with the precession angle being rapidly damped in the case of a star with an oblate deformation. We then go on to study detailed scenarios where free precession is created and/or maintained by some astrophysical mechanism. We consider the effects of accretion torques, electromagnetic torques, glitches and stellar encounters. We find that the mechanisms considered are either too weak to lead to a signal detectable by an Advanced LIGO interferometer, or occur too infrequently to give a reasonable event rate. We therefore conclude that, using our stellar model at least, free precession is not a good candidate for detection by the forthcoming laser interferometers
A covariant action principle for dissipative fluid dynamics: from formalism to fundamental physics
No full textWe present a new variational framework for dissipative general relativistic fluid dynamics. The model extends the convective variational principle for multi-fluid systems to account for a range of dissipation channels. The key ingredients in the construction are (i) the use of a lower dimensional matter space for each fluid component, and (ii) an extended functional dependence for the associated volume forms. In an effort to make the concepts clear, the formalism is developed step-by-step with model examples considered at each level. Thus we consider a model for heat flow, derive the relativistic Navier-Stokes equations and discuss why the individual dissipative stress tensors need not be spacetime symmetric. We argue that the new formalism, which notably does not involve an expansion away from an assumed equilibrium state, provides a conceptual breakthrough in this area of research. We also provide an ambitious list of directions in which one may want to extend it in the future. This involves an exciting set of problems, relating to both applications and foundational issues
Quasinormal modes of nearly extreme Reissner-Nordstrom black holes
No full textWe present detailed calculations of the quasinormal modes of Reissner-Nordström black holes. While the first few, slowly damped, modes depend on the charge of the black hole in a relatively simple way, we find that the rapidly damped modes show several peculiar features. The higher modes generally spiral into the value for the extreme black hole as the charge increases. We also discuss the possible existence of a purely imaginary mode for the Schwarzschild black hole: Our data suggest that there is a quadrupole quasinormal mode that limits to ?M=-2i as Q?0
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