1,720,962 research outputs found
Heat conduction in rotating relativistic stars
No full textIn the standard form of the relativistic heat equation used in astrophysics, information propagates instantaneously, rather than being limited by the speed of light as demanded by relativity. We show how this equation none the less follows from a more general, causal theory of heat propagation in which the entropy plays the role of a fluid. In deriving this result, however, we see that it is necessary to make some assumptions which are not universally valid: the dynamical time-scales of the process must be long compared with the explicitly causal physics of the theory, the heat flow must be sufficiently steady, and the space-time static. Generalizing the heat equation (e.g. restoring causality) would thus entail retaining some of the terms we neglected. As a first extension, we derive the heat equation for the space-time associated with a slowly-rotating star or black hole, showing that it only differs from the static result by an additional advection term due to the rotation, and as a consequence demonstrate that a hotspot on a neutron star will be seen to be modulated at the rotation frequency by a distant observer.</p
Non-rigid precession of magnetic stars
No full textStars are, generically, rotating and magnetized objects with a misalignment between their magnetic and rotation axes. Since a magnetic field induces a permanent distortion to its host, it provides effective rigidity even to a fluid star, leading to bulk stellar motion that resembles free precession. This bulk motion is, however, accompanied by induced interior velocity and magnetic field perturbations, which are oscillatory on the precession time-scale. Extending previous work, we show that these quantities are described by a set of second-order perturbation equations featuring cross-terms scaling with the product of the magnetic and centrifugal distortions to the star. For the case of a background toroidal field, we reduce these to a set of differential equations in radial functions, and find a method for their solution. The resulting magnetic field and velocity perturbations show complex multipolar structure and are strongest towards the centre of the star
Instabilities in neutron stars with toroidal magnetic fields
No full textWe study m= 1 oscillations and instabilities of magnetized neutron stars, by numerical time evolution of linear perturbations of the system. The background stars are stationary equilibrium configurations with purely toroidal magnetic fields. We find that an m= 1 instability of toroidal magnetic fields, already known from local analyses, may also be found in our relatively low-resolution global study. We present quantitative results for the instability growth rate and its suppression by rotation. The instability is discussed as a possible trigger mechanism for soft gamma repeater flares. Although our primary focus is evolutions of magnetized stars, we also consider perturbations about unmagnetized background stars in order to study m= 1 inertial modes. We track these modes up to break-up frequency ?K, extending known slow-rotation results.<br/
Oscillations and instabilities in neutron stars with poloidal magnetic fields
No full textWe study the time evolution of non-axisymmetric linear perturbations of a rotating magnetized neutron star, whose magnetic field is purely poloidal. The background stellar configurations are generated self-consistently, with multipolar field configurations and allowing for distortions to the density distribution from rotational and magnetic forces. The perturbations split into two symmetry classes, with perturbations in one class being dominated by an instability generic to poloidal fields, which is localized around the ‘neutral line’ where the background field vanishes. Rotation acts to reduce the effect of this instability. Perturbations in the other symmetry class do not suffer this instability and in this case we are able to resolve Alfvén oscillations, whose restoring force is the magnetic field; this is the first study of non-axisymmetric Alfvén modes of a star with a poloidal field. We find no evidence that these modes form a continuum. In a rotating magnetized star, we find that there are no pure Alfvén modes or pure inertial modes, but hybrids of these. We discuss the nature of magnetic instabilities and oscillations in magnetars and pulsars, finding the dominant Alfvén mode from our simulations has a frequency comparable with observed magnetar quasi-periodic oscillations (QPOs).<br/
Magnetic fields in axisymmetric neutron stars
No full textWe derive general equations for axisymmetric Newtonian magnetohydrodynamics and use these as the basis of a code for calculating equilibrium configurations of rotating magnetized neutron stars in a stationary state. We investigate the field configurations that result from our formalism, which include purely poloidal, purely toroidal and mixed fields. For the mixed-field formalism, the toroidal component appears to be bounded at less than 7 per cent. We calculate distortions induced both by magnetic fields and by rotation. From our non-linear work, we are able to look at the realm of validity of perturbative work: we find for our results that perturbative-regime formulae for magnetic distortions agree to within 10 per cent of the non-linear results if the ellipticity is less than 0.15 or the average field strength is less than 10^17 G. We also consider how magnetized equilibrium structures vary for different polytropic indices
Oscillations of rotating magnetized neutron stars with purely toroidal magnetic fields
No full textWe investigate the oscillation spectrum of rotating Newtonian neutron stars endowed with purely toroidal magnetic fields, using a time-evolution code to evolve linear perturbations in the Cowling approximation. The background star is generated by numerically solving the magnetohydrodynamics equilibrium equations and may be non-spherical by virtue of both rotation and magnetic effects; hence, our perturbations and background are fully consistent. Whilst the background field is purely toroidal, the perturbed field is mixed poloidal–toroidal. From Fourier analysis of the perturbations, we are able to identify a number of magnetically restored Alfvén (or a) modes. We show that in a rotating star pure inertial and a-modes are replaced by hybrid magneto-inertial modes, which reduce to a-modes in the non-rotating limit and inertial modes in the non-magnetic limit. We show that the r-mode instability is suppressed by magnetic fields in sufficiently slowly rotating stars. In addition, we determine magnetic frequency shifts in the f-mode. We discuss the astrophysical relevance of our results, in particular for magnetar oscillations
Magnetic neutron star equilibria with stratification and type II superconductivity
No full textWe construct two-fluid equilibrium configurations for neutron stars (NSs) with magnetic fields, using a self-consistent and non-linear numerical approach. The two-fluid approach - likely to be valid for large regions of all but the youngest NSs - provides us with a straightforward way to introduce stratification and allows for more realistic models than the ubiquitous barotropic assumption. In all our models, the neutrons are modelled as a superfluid, whilst for the protons we consider two cases: one where they are a normal fluid and another where they form a type II superconductor. We consider a variety of field configurations in the normal-proton case and purely toroidal fields in the superconducting case. We find that stratification allows for a stronger toroidal component in mixed-field configurations, though the poloidal component remains the largest in all our models. We provide quantitative results for magnetic ellipticities of NSs, both in the normal- and superconducting-proton cases
Magnetically driven crustquakes in neutron stars
Full text linkCrustquake events may be connected with both rapid spin-up `glitches' within the regular slowdown of neutron stars, and high-energy magnetar flares. We argue that magnetic-field decay builds up stresses in a neutron star's crust, as the elastic shear force resists the Lorentz force's desire to rearrange the global magnetic-field equilibrium. We derive a criterion for crust-breaking induced by a changing magnetic-field configuration, and use this to investigate strain patterns in a neutron star's crust for a variety of different magnetic-field models. Universally, we find that the crust is most liable to break if the magnetic field has a strong toroidal component, in which case the epicentre of the crustquake is around the equator. We calculate the energy released in a crustquake as a function of the fracture depth, finding that it is independent of field strength. Crust-breaking is, however, associated with a characteristic local field strength of 2.4 × 1014 G for a breaking strain of 0.001, or 2.4 × 1015 G at a breaking strain of 0.1. We find that even the most luminous magnetar giant flare could have been powered by crustal energy release alone
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
- …
