1,721,270 research outputs found
Gravitational waves from spinning neutron stars as not-quite-standard sirens
No full textAs is well known, gravitational wave detections of coalescing binaries are standard sirens, allowing a measurement of source distance by gravitational wave means alone. In this paper we explore the analogue of this capability for continuous gravitational wave emission from individual spinning neutron stars, whose spin-down is driven purely by gravitational wave emission. We show that in this case, the distance measurement is always degenerate with one other parameter, which can be taken to be the moment of inertia of the star. We quantify the accuracy to which such degenerate measurements can be made. We also discuss the practical application of this method to scenarios where one or other of distance or moment of inertia is constrained, breaking this degeneracy and allowing a measurement of the remaining parameter. We consider a broad range of possible, unknown parameters, as well as we present results for the aLIGO and Einstein Telescope sensitivities. Our results will be of use following the eventual detection of a neutron star spinning down through such gravitational wave emission
Gravitational radiation back-reaction from f-modes on neutron stars
No full textThe problem of the gravitational radiation damping of neutron star fundamental (f) mode oscillations has received considerable attention. Many studies have looked at the stability of such oscillations in rapidly rotating stars, calculating the growth or decay rate of the mode amplitude. In this paper, we look at the relatively neglected problem of the radiation reaction on the spin of the star. We specialize greatly to the so-called Kelvin modes: The modes of oscillation of (initially) non-rotating incompressible stars. We find the unexpected result that the excitation of a mode of angular momentum δJ on an initially non-rotating star ends up radiating an angular momentum 2δJ to infinity, leaving the star itself with a bulk angular momentum of -δJ. This result is interesting in itself, and also will have implications for the angular momentum budgets of spinning down neutron stars, should such modes be excited.</p
Transient gravitational waves from pulsar post-glitch recoveries
No full textThis work explores whether gravitational waves (GWs) from neutron star (NS) mountains can be detected with current second-generation and future third-generation GW detectors. In particular, we focus on a scenario where transient mountains are formed immediately after an NS glitch. In a glitch, an NS's spin frequency abruptly increases and then often exponentially recovers back to, but never quite reaches, the spin frequency prior to the glitch. If the recovery is ascribed to an additional torque due to a transient mountain, we find that GWs from that mountain are marginally detectable with Advanced LIGO at design sensitivity and is very likely to be detectable for third-generation detectors such as the Einstein Telescope. Using this model, we are able to find analytical expressions for the GW amplitude and its duration in terms of observables.</p
Gravitational waves from magnetically-induced thermal neutron star mountains
No full textMany low-mass X-ray binary (LMXB) systems are observed to contain rapidly spinning neutron stars. The spin frequencies of these systems may be limited by the emission of gravitational waves. This can happen if their mass distribution is sufficiently non-axisymmetric. It has been suggested that such `mountains' may be created via temperature non-axisymmetries, but estimates of the likely level of temperature asymmetry have been lacking. To remedy this, we examine a simple symmetry breaking mechanism, where an internal magnetic field perturbs the thermal conductivity tensor, making it direction-dependent. We find that the internal magnetic field strengths required to build mountains of the necessary size are very large, several orders of magnitude larger than the inferred external field strengths, pushing into the regime where our assumption of the magnetic field having a perturbative effect on the thermal conductivity breaks down. We also examine how non-axisymmetric surface temperature profiles, as might be caused by magnetic funnelling of the accretion flow, lead to internal temperature asymmetries, but find that for realistic parameters the induced non-axisymmetries are very small. We conclude that, in the context of this work at least, very large internal magnetic fields are required to generate mountains of the necessary size
Does elasticity stabilize a magnetic neutron star?
No full textThe configuration of the magnetic field in the interior of a neutron star is mostly unknown from observations. Theoretical models of the interior magnetic field geometry tend to be oversimplified to avoid mathematical complexity and tend to be based on axisymmetric barotropic fluid systems. These static magnetic equilibrium configurations have been shown to be unstable on a short time-scale against an infinitesimal perturbation. Given this instability, it is relevant to consider how more realistic neutron star physics affects the outcome. In particular, it makes sense to ask if elasticity, which provides an additional restoring force on the perturbations, may stabilize the system. It is well known that the matter in the neutron star crust forms an ionic crystal. The interactions between the crystallized nuclei can generate shear stress against any applied strain. To incorporate the effect of the crust on the dynamical evolution of the perturbed equilibrium structure, we study the effect of elasticity on the instability of an axisymmetric magnetic star. In particular, we determine the critical shear modulus required to prevent magnetic instability and consider the corresponding astrophysical consequences.</p
Modelling neutron star mountains
Full text linkAs the era of gravitational-wave astronomy has well and truly begun,
gravitational radiation from rotating neutron stars remains elusive. Rapidly
spinning neutron stars are the main targets for continuous-wave searches since,
according to general relativity, provided they are asymmetrically deformed,
they will emit gravitational waves. It is believed that detecting such
radiation will unlock the answer to why no pulsars have been observed to spin
close to the break-up frequency. We review existing studies on the maximum
mountain that a neutron star crust can support, critique the key assumptions
and identify issues relating to boundary conditions that need to be resolved.
In light of this discussion, we present a new scheme for modelling neutron star
mountains. The crucial ingredient for this scheme is a description of the
fiducial force which takes the star away from sphericity. We consider three
examples: a source potential which is a solution to Laplace's equation, another
solution which does not act in the core of the star and a thermal pressure
perturbation. For all the cases, we find that the largest quadrupoles are
between a factor of a few to two orders of magnitude below previous estimates
of the maximum mountain size.Comment: 13 pages, 8 figures. Accepted for publication in MNRA
Neutron-star spindown and magnetic inclination-angle evolution
No full textA rotating fluid star, endowed with a magnetic field, can undergo a form of precessional motion: a sum of rigid-body free precession and a non-rigid response. On secular timescales this motion is dissipated by bulk and shear viscous processes in the stellar interior and magnetospheric braking in the exterior, changing the inclination angle between the rotation and magnetic axes. Using our recent solutions for the non-rigid precessional dynamics, and viscous dissipation integrals derived in this paper, we make the only self-consistent calculation to date of these dissipation rates. We present the first results for the full coupled evolution of spindown and inclination angle for a model of a late-stage proto-neutron star with a strong toroidal magnetic field, allowing for both electromagnetic torques and internal dissipation when evolving the inclination angle. We explore this coupled evolution for a range of initial inclination angles, rotation rates and magnetic field strengths. For fixed initial inclination angle, our results indicate that the neutron-star population naturally evolves into two classes: near-aligned and near-orthogonal rotators -- with typical pulsars falling into the latter category. Millisecond magnetars can evolve into the near-aligned rotators which mature magnetars appear to be, but only for small initial inclination angle and internal toroidal fields stronger than roughly G. Once any model has evolved to either an aligned or orthogonal state, there appears to be no further evolution away from this state at later times
The early life of millisecond magnetars
No full textSome neutron stars may be born spinning fast and with strong magnetic fields - the so-called millisecond magnetars. It is important to understand how a star's magnetic axis moves with respect to the spin axis in the star's early life, as this effects both electromagnetic and gravitational wave emission. Previous studies have highlighted the importance of viscous dissipation within the star in this process. We advance this program by additionally considering the effect of the electromagnetic torque. We find an interesting interplay between the viscous dissipation, which makes the magnetic axis orthogonalise with respect to the spin, verses magnetic torques that tend to make the magnetic axis align with the spin axis. We present some results, and highlight areas where our model needs to be made more realistic.</p
Magnetar birth: rotation rates and gravitational-wave emission
Full text linkUnderstanding the evolution of the angle χ between a magnetar's rotation and magnetic axes sheds light on the star's birth properties. This evolution is coupled with that of the stellar rotation ω, and depends on the competing effects of internal viscous dissipation and external torques. We study this coupled evolution for a model magnetar with a strong internal toroidal field, extending previous work by modelling-for the first time in this context-the strong protomagnetar wind acting shortly after birth. We also account for the effect of buoyancy forces on viscous dissipation at late times. Typically, we find that χ → 90° shortly after birth, then decreases towards 0° over hundreds of years. From observational indications that magnetars typically have small χ, we infer that these stars are subject to a stronger average exterior torque than radio pulsars, and that they were born spinning faster than ∼100-300 Hz. Our results allow us to make quantitative predictions for the gravitational and electromagnetic signals from a newborn rotating magnetar. We also comment briefly on the possible connection with periodic fast radio burst sources.</p
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