arXiv subjects

Nils Andersson

Publications and source records attributed to Nils Andersson.

At least 73 records · Page 4Linked to original sources

arXiv2013

The hottest superfluid and superconductor in the Universe: Discovery and nuclear physics implications

We present recent work on using astronomical observations of neutron stars to reveal unique insights into nuclear matter that cannot be obtained from laboratories on Earth. First, we discuss our measurement of the rapid cooling of the youngest neutron star in the Galaxy; this provides the first direct evidence for superfluidity and superconductivity in the supra-nuclear core of neutron stars. We show that observations of thermonuclear X-ray bursts on neutron stars can be used to constrain properties of neutron superfluidity and neutrino emission. We describe the implications of rapid neutron star rotation rates on aspects of nuclear and superfluid physics. Finally, we show that entrainment coupling between the neutron superfluid and the nuclear lattice leads to a less mobile crust superfluid; this result puts into question the conventional picture of pulsar glitches as being solely due to the crust superfluid and suggests that the core superfluid also participates.

hep-ph↗

arXiv2012

Magnetars: super(ficially) hot and super(fluid) cool

We examine to what extent the inferred surface temperature of magnetars in quiescence can constrain the presence of a superfluid in the neutron star core and the role of magnetic field decay in the core. By performing detailed simulations of neutron star cooling, we show that extremely strong heating from field decay in the core cannot produce the high observed surface temperatures nor delay the onset of neutron superfluidity in the core. We verify the results of Kaminker et al., namely that the high magnetar surface temperatures require heating in the neutron star crust, and crust heating is decoupled from cooling/heating in the core. Therefore, because crust heating masks core heating, it is not possible to conclude that magnetar cores are in a non-superfluid state purely from high surface temperatures. From our interior temperature evolutions and after accounting for proton superconductivity in the core, we find that neutron superfluidity in the core occurs less than a few hundred years after neutron star formation. This onset time is unaffected by heating due to core field decay at fields < 10^16 G. Thus all known neutron stars, including magnetars, without a core containing exotic particles, should have a core of superfluid neutrons and superconducting protons.

astro-ph.HE↗

arXiv2012

Hydromagnetic equilibrium in non-barotropic multifluid neutron stars

Traditionally, the subject of hydromagnetic equilibrium in neutron stars has been addressed in the context of standard magnetohydrodynamics, with matter obeying a barotropic equation of state. In this paper we take a step towards a more realistic treatment of the problem by considering neutron stars with interior superfluid components. In this multifluid model stratification associated with a varying matter composition (the relative proton to neutron density fraction) enters as a natural ingredient, leading to a non-barotropic system. After formulating the hydromagnetic equilibrium of superfluid/superconducting neutron stars as a perturbation problem, we focus on the particular case of a three-fluid system consisting of superfluid neutrons and normal protons and electrons. We determine the equilibrium structure of dipolar magnetic fields with a mixed poloidal-toroidal composition. We find that, with respect to barotropic models, stratification has the generic effect of leading to equilibria with a higher fraction of magnetic energy stored in the toroidal component. However, even in models with strong stratification the poloidal and toroidal components are comparable, with the former contributing the bulk of the magnetic energy.

astro-ph.SR↗

arXiv2011

Magneto-rotational neutron star evolution: the role of core vortex pinning

We consider the pinning of superfluid (neutron) vortices to magnetic fluxtubes associated with a type II (proton) superconductor in neutron star cores. We demonstrate that core pinning affects the spin-down of the system significantly, and discuss implications for regular radio pulsars and magnetars. We find that magnetars are likely to be in the pinning regime, while most radio pulsars are not. This suggests that the currently inferred magnetic field for magnetars may be overestimated. We also obtain a new timescale for the magnetic field evolution which could be associated with the observed activity in magnetars, provided that the field has a strong toroidal component.

astro-ph.SR↗

arXiv2011

Revealing the physics of r-modes in low-mass X-ray binaries

We consider the astrophysical constraints on the gravitational-wave driven r-mode instability in accreting neutron stars in low-mass X-ray binaries. We use recent results on superfluid and superconducting properties to infer the core temperature in these neutron stars and show the diversity of the observed population. Simple theoretical models indicate that many of these systems reside inside the r-mode instability region. However, this is in clear disagreement with expectations, especially for the systems containing the most rapidly rotating neutron stars. The inconsistency highlights the need to re-evaluate our understanding of the many areas of physics relevant to the r-mode instability. We summarize the current status of our understanding, and we discuss directions for future research which could resolve this dilemma.

astro-ph.HE↗

arXiv2011

Cosmic recycling of millisecond pulsars

We compare the rotation rate of neutron stars in low-mass X-ray binaries (LMXBs) with the orbital period of the binaries. We find that, while short orbital period LMXBs span a range of neutron star rotation rates, all the long period LMXBs have fast rotators. We also find that the rotation rates are highest for the systems with the highest mean mass accretion rates, as can be expected if the accretion rate correlates with the orbital period. We show that these properties can be understood by a balance between spin-up due to accretion and spin-down due to gravitational radiation. Our scenario indicates that the gravitational radiation emitted by these systems may be detectable by future ground-based gravitational wave detectors.

astro-ph.HE↗

arXiv2010

Superfluid hyperon bulk viscosity and the r-mode instability of rotating neutron stars

In order to establish whether the unstable r-modes in a rotating neutron star provide a detectable source of gravitational waves, we need to understand the details of the many dissipative processes that tend to counteract the instability. It has been established that the bulk viscosity due to exotic particles, like hyperons, may be particularly important in this respect. However, the effects of hyperon superfluidity have so far not been fully accounted for. While the associated suppression of the reaction rates that give rise to the bulk viscosity has been estimated, superfluid aspects of the fluid dynamics have not been considered. In this paper we determine the r-mode instability window for a neutron star with a $Σ^{-}$ hyperon core, using the appropriate multifluid formalism including, for the first time, the effect of the "superfluid" bulk viscosity coefficients. We demonstrate that, even though the extra terms may increase the bulk viscosity damping somewhat, their presence does not affect the qualitative features of the r-mode instability window.

astro-ph.SR↗

arXiv2009

Nonlinear radial oscillations of neutron stars

The effects of nonlinear oscillations in compact stars are attracting considerable current interest. In order to study such phenomena in the framework of fully nonlinear general relativity, highly accurate numerical studies are required. We present a numerical scheme specifically tailored for studies, based on formulating the time evolution in terms of deviations from a stationary equilibrium configuration. Using this technique, we investigate nonlinear effects associated with radial oscillations of neutron stars for a wide range of amplitudes. In particular, we discuss mode coupling due to nonlinear interactions, the occurrence of resonance phenomena, shock formation near the stellar surface as well as the capacity of nonlinearities to stabilize perturbatively unstable neutron star models.

gr-qc↗

arXiv2009

Oscillations of Rapidly Rotating Superfluid Stars

Using time evolutions of the relevant linearised equations we study non-axisymmetric oscillations of rapidly rotating and superfluid neutron stars. We consider perturbations of Newtonian axisymmetric background configurations and account for the presence of superfluid components via the standard two-fluid model. Within the Cowling approximation, we are able to carry out evolutions for uniformly rotating stars up to the mass-shedding limit. This leads to the first detailed analysis of superfluid neutron star oscillations in the fast rotation regime, where the star is significantly deformed by the centrifugal force. For simplicity, we focus on background models where the two fluids (superfluid neutrons and protons) co-rotate, are in beta-equilibrium and coexist throughout the volume of the star. We construct sequences of rotating stars for two analytical model equations of state. These models represent relatively simple generalisations of single fluid, polytropic stars. We study the effects of entrainment, rotation and symmetry energy on non-radial oscillations of these models. Our results show that entrainment and symmetry energy can have a significant effect on the rotational splitting of non-axisymmetric modes. In particular, the symmetry energy modifies the inertial mode frequencies considerably in the regime of fast rotation.

astro-ph↗

arXiv2009

Axial quasi-normal modes of neutron stars: Accounting for the superfluid in the crust

We 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 magnetised 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.

astro-ph.SR↗

arXiv2007

A characteristic approach to the quasi-normal mode problem

In this paper we discuss a new approach to the quasinormal-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 generalise to the important problem of rapidly rotating neutron star spacetimes.

gr-qc↗

arXiv2007

Lagrangian perturbation theory for rotating magnetic stars

Motivated by the possibility of radiation driven instabilities in rotating magnetic stars, we study the stability properties of general linear perturbations of a stationary and axisymmetric, infinitely conducting perfect fluid configuration threaded by a magnetic field and surrounded by vacuum. We develop a Lagrangian perturbation framework which enables us to formulate a strict stability criterion based on the notion of a canonical energy (a functional of the fluid displacement $ξ$ and its first time derivative). For any given choice of $\{ξ,\partial_t ξ\} $, the sign of the canonical energy determines whether the configuration is stable or not at the linear level. Our analysis provides the first complete description of the stability problem for a magnetic star, allowing for both rotation and the presence of a magnetic field in the exterior vacuum region. A key feature of the Lagrangian formulation is the existence of so-called `trivial' fluid displacements, which do not represent true physical perturbations. In order for the stability criterion to make rigorous sense one has to isolate these trivials and consider only the physical `canonical' displacements. We discuss this problem and formulate a condition which must be satisfied by all canonical displacements. Having obtained a well-defined stability criterion we provide examples which indicate that the magnetic field has a stabilising effect on radiation driven instabilities.

astro-ph↗

arXiv2006

Neutron Star Asteroseismology. Axial Crust Oscillations in the Cowling Approximation

Recent 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.

astro-ph↗

arXiv2006

Ekman layer damping of r-modes revisited

We investigate the damping of neutron star r-modes due to the presence of a viscous boundary (Ekman) layer at the interface between the crust and the core. Our study is motivated by the possibility that the gravitational-wave driven instability of the inertial r-modes may become active in rapidly spinning neutron stars, eg. in low-mass X-ray binaries, and the fact that a viscous Ekman layer at the core-crust interface provides an efficient damping mechanism for these oscillations. We review various approaches to the problem and carry out an analytic calculation of the effects due to the Ekman layer for a rigid crust. Our analytic estimates support previous numerical results, and provide further insight into the intricacies of the problem. We add to previous work by discussing the effect that compressibility and composition stratification have on the boundary layer damping. We show that, while stratification is unimportant for the r-mode problem, composition suppresses the damping rate by about a factor of two (depending on the detailed equation of state).

astro-ph↗

arXiv2006

Elastic or magnetic? A toy model for global magnetar oscillations with implications for QPOs during flares

We use a simple toy-model to discuss global MHD modes of a neutron star, taking into account the magnetic coupling between the elastic crust and the fluid core. Our results suggest that the notion of pure torsional crust modes is not useful for the coupled system, all modes excite Alfven waves in the core. However, we also show that the modes that are most likely to be excited by a fractured crust, eg. during a magnetar flare, are such that the crust and the core oscillate in concert. For our simple model, the frequencies of these modes are similar to the ``pure crustal'' frequencies. In addition, our model provides a natural explanation for the presence of lower frequency ($< 30$~Hz) QPOs seen in the December 2004 giant flare of SGR 1806-20.

astro-ph↗

arXiv2005

A Flux-Conservative Formalism for Convective and Dissipative Multi-Fluid Systems, with Application to Newtonian Superfluid Neutron Stars

We develop a flux-conservative formalism for a Newtonian multi-fluid system, including dissipation and entrainment (i.e. allowing the momentum of one fluid to be a linear combination of the velocities of all fluids). Maximum use is made of mass, energy, and linear and angular momentum conservation to specify the equations of motion. Also used extensively are insights gleaned from a convective variational action principle, key being the distinction between each velocity and its canonically conjugate momentum. Dissipation is incorporated to second order in the ``thermodynamic forces'' via the approach pioneered by Onsager. An immediate goal of the investigation is to understand better the number, and form, of independent dissipation terms required for a consistent set of equations of motion in the multi-fluid context. A significant, but seemingly innocuous detail, is that one must be careful to isolate ``forces'' that can be written as total gradients, otherwise errors can be made in relating the net internal force to the net externally applied force. Our long-range aim is to provide a formalism that can be used to model dynamical multi-fluid systems both perturbatively and via fully nonlinear 3D numerical evolutions. To elucidate the formalism we consider the standard model for a heat-conducting, superfluid neutron star, which is believed to be dominated by superfluid neutrons, superconducting protons, and a highly degenerate, ultra-relativistic gas of normal fluid electrons. We determine that in this case there are, in principle, 19 dissipation coefficients in the final set of equations.

physics.flu-dyn↗

arXiv2004

Gravitational-wave astronomy: the high-frequency window

This contribution is divided in two parts. The first part provides a text-book level introduction to gravitational radiation. The key concepts required for a discussion of gravitational-wave physics are introduced. In particular, the quadrupole formula is applied to the anticipated ``bread-and-butter'' source for detectors like LIGO, GEO600, EGO and TAMA300: inspiralling compact binaries. The second part provides a brief review of high frequency gravitational waves. In the frequency range above (say) 100Hz, gravitational collapse, rotational instabilities and oscillations of the remnant compact objects are potentially important sources of gravitational waves. Significant and unique information concerning the various stages of collapse, the evolution of protoneutron stars and the details of the supranuclear equation of state of such objects can be drawn from careful study of the gravitational-wave signal. As the amount of exciting physics one may be able to study via the detections of gravitational waves from these sources is truly inspiring, there is strong motivation for the development of future generations of ground based detectors sensitive in the range from hundreds of Hz to several kHz.

gr-qc↗

arXiv2003

The traditional approximation in general relativity

We discuss the generalisation of the so-called traditional approximation, well known in geophysics, to general relativity. We show that the approximation is applicable for rotating relativistic stars provided that one focuses on relatively thin radial shells. This means that the framework can be used to study waves in neutron star oceans. We demonstrate that, once the effects of the relativistic frame-dragging are accounted for, the angular problem reduces to Laplace's tidal equation. We derive the dispersion relation for various classes of waves in a neutron star ocean and show that the combined effects of the frame-dragging and the gravitational redshift typically lower the frequency of a mode by about 20%.

astro-ph↗