1,721,064 research outputs found
Gravitational waves from rotating strained neutron stars
No full textIn this review we examine the dynamics and gravitational wave detectability of rotating strained neutron stars. The discussion is divided into two halves: triaxial stars and precessing stars. We summarize recent studies on how crustal strains and magnetic fields can sustain triaxiality, and suggest that Magnus forces connected with pinned superfluid vortices might contribute to deformation also. The conclusions that could be drawn following the successful gravitational wave detection of a triaxial star are discussed, and areas requiring further study identified. The latest ideas regarding free precession are then outlined, and the recent suggestion of Middleditch et al (Middleditch et al 2000 New Astronomy 5 243; 2000 Preprint astro-ph/0010044) that the remnant of SN1987A contains a freely precessing star, spinning down by gravitational wave energy loss, is examined critically. We describe what we would learn about neutron stars should the gravitational wave detectors prove this hypothesis to be correct
Gravitational wave emission from rotating superfluid neutron stars
No full textIn this paper, we investigate the effect of a pinned superfluid component on the gravitational wave emission of a steadily rotating deformed neutron star. We show that the superfluid pinning allows the possibility for there to be gravitational wave emission at both the stellar spin frequency ? and its first harmonic, 2?. This contrasts with the conventional case where there is no pinned superfluidity, where either only the 2? harmonic is present or else the star undergoes precession, a feature which is not believed to be common in the known pulsar population. This work motivates the carrying out of gravitational wave searches where both the ? and 2? harmonics are searched for, even in targeted searches for waves from known pulsars which are not observed to precess. Observation of such a two-component signal would provide evidence in favour of pinned superfluidity inside the star
Is timing noise important in the gravitational wave detection of neutron stars?
No full textIn this paper we ask whether the phenomenon of timing noise long known in electromagnetic pulsar astronomy is likely to be important in gravitational wave (GW) observations of spinning-down neutron stars. We find that timing noise is strong enough to be of importance only in the young pulsars, which must have larger triaxialities than theory predicts for their GW emission to be detectable. However, assuming that their GW emission is detectable, we list the pulsars for which timing noise is important, either because it is strong enough that its neglect by the observer would render the source undetectable or else because it is a measurable feature of the GW signal. We also find that timing noise places a limit on the observation duration of a coherent blind GW search, and suggest that hierarchical search techniques might be able to cope with this problem. Demonstration of the presence or absence of timing noise in the GW channel would give a new probe of neutron star physics
Bounding the mass of the graviton using eccentric binaries
No full textWe describe a method by which gravitational wave observations of eccentric binary systems could be used to test general relativity's prediction that gravitational waves are dispersionless. We present our results in terms of the graviton having a nonzero rest mass, or equivalently a noninfinite Compton wavelength. We make a rough estimate of the bounds that might be obtained following gravitational wave detections by the Laser Interferometer Space Antenna. The bounds we find are comparable to those obtainable from a method proposed by C. M. Will and several orders of magnitude stronger than other dynamic (i.e., gravitational wave-based) tests that have been proposed. The method described here has the advantage over those proposed previously of being simple to apply, as it does not require the in-spiral to be in the strong field regime nor correlation with electromagnetic signals. We compare our results with those obtained from static (i.e., non-gravitational wave-based) tests
Multimessenger observations and the science enabled: continuous waves and their progenitors, equation of state of dense matter
No full textRotating and oscillating neutron stars can give rise to long-lived Continuous Gravitational Waves (CGWs). Despite many years of searching, the detection of such a CGW signal remains elusive. In this article we describe the main astrophysical uncertainties regarding such emission, and their relation to the behaviour of matter at extremely high density. We describe the main challenges in searching for CGWs, and the prospects of detecting them using third-generation gravitational wave detectors. We end by describing some pressing issues in the field, whose resolution would help turn the detection and exploitation of CGWs into reality
Freely precessing neutron stars: model and observations
No full textWe present a model of a freely precessing neutron star, which is then compared against pulsar observations. The aim is to draw conclusions regarding the structure of the star, and to test theoretical ideas of crust–core coupling and superfluidity. We argue that, on theoretical grounds, it is likely that the core neutron superfluid does not participate in the free precession of the crust. We apply our model to the handful of proposed observations of free precession that have appeared in the literature. Assuming crust-only precession, we find that all but one of the observations are consistent with there being no pinned crustal superfluid at all; the maximum amount of pinned superfluid consistent with the observations is about 10-10 of the total stellar moment of inertia. However, the observations do not rule out the possibility that the crust and neutron superfluid core precess as a single unit. In this case the maximum amount of pinned superfluid consistent with the observations is about 10-8 of the total stellar moment of inertia. Both of these values are many orders of magnitude less than the 10-2 value predicted by many theories of pulsar glitches. We conclude that superfluid pinning, at least as it affects free precession, needs to be reconsidered
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
Astrophysical input for gravitational wave searches
No full textWe describe several areas where the newly emerging field of gravitational wave astronomy would benefit from exploiting the expertise of the broader astrophysics community. We deal specifically with searches for long-lived gravitational wave signals from neutron stars, paying particular attention to the known radio pulsar population and supernova remnants
Strange stars as persistent sources of gravitational waves
No full textWe investigate the relevance of the gravitational-wave driven r-mode instability for strange stars. We find that the unstable r-modes affect strange stars in a way that is quite distinct from the neutron star case. For accreting strange stars, we show that the onset of r-mode instability does not lead to the thermo-gravitational runaway that is likely to occur in neutron stars. Instead, the strange star evolves towards a quasi-equilibrium state on a time-scale of about a year. This mechanism could thus explain the clustering of spin frequencies inferred from kHz quasi-periodic oscillation data in low-mass X-ray binaries. For young strange stars, we show that the r-mode driven spin-evolution is also distinct from the neutron star case. In a young strange star, the r-mode undergoes short cycles of instability during the first few months. This is followed by a quasi-adiabatic phase where the r-mode remains at a small, roughly constant, amplitude for thousands of years. Another distinguishing feature from the neutron star case is that the r-modes in a strange star never grow to amplitudes of the order of unity. Our results suggest that the r-modes in a strange star emit a persistent gravitational-wave signal that should be detectable with large-scale interferometers given an observation time of a few months. If detected, these signals would provide unique evidence for the existence of strange stars, which would put useful constraints on the parameters of quantum chromodynamics
The nature of low T/|W| dynamical instabilities in differentially rotating stars
No full textRecent numerical simulations indicate the presence of dynamical instabilities of the f-mode in differentially rotating stars even at very low values of T/|W|, the ratio of kinetic to potential energy. In this Letter we argue that these may be shear instabilities that occur when the degree of differential rotation exceeds a critical value and when the f-mode develops a corotation point associated with the presence of a continuous spectrum. Our explanation, which is supported by detailed studies of a simple shell model, offers a straightforward way of understanding all of the key features of these instabilities
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