1,721,207 research outputs found
Adiabatic oscillations of non-rotating superfluid neutron stars
Full text linkWe present results concerning the linear (radial and non-radial) oscillations of non-rotating superfluid neutron stars in Newtonian physics. We use a simple two-fluid model to describe the superfluid neutron star, where one fluid consists of the superfluid neutrons, while the second fluid contains all the comoving constituents (protons, electrons). The two fluids are assumed to be "free" in the sense of absence of vortex-mediated forces like mutual friction or pinning, but they can be coupled by the equation of state, in particular by entrainment. We calculate numerically the eigen-frequencies and -modes of adiabatic oscillations, neglecting beta-reactions that would lead to dissipation. We find a doubling of all acoustic-type modes (f-modes, p-modes), and confirm the absence of g-modes in these superfluid models. We show analytically and numerically that only in the case of non-stratified background models (i.e. with no composition gradient) can these doublets of acoustic modes be separated into two distinct families, which are characterized by either co- or counter-moving fluids respectively, and which are sometimes referred to as "ordinary"- and "superfluid" modes. In the general, stratified case, however, this separation is not possible, and these acoustic modes can not be classified as being either purely "ordinary" or "superfluid". We show how the properties of the two-fluid modes change as functions of the coupling by entrainment. We find avoided mode-crossings for the stratified models, while the crossings are not avoided in the non-stratified, separable case. The oscillations of normal-fluid neutron stars are recovered as a special case simply by locking the two fluids together. In this effective one-fluid case we find the usual singlet f- and p-modes, and we also find the expected g-modes of stratified neutron star models
Hierarchical multistage MCMC follow-up of continuous gravitational wave candidates
Full text linkLeveraging Markov chain Monte Carlo (MCMC) optimization of the F-statistic, we introduce a method for the hierarchical follow-up of continuous gravitational wave candidates identified by wide-parameter space semi-coherent searches. We demonstrate parameter estimation for continuous wave sources and develop a framework and tools to understand and control the effective size of the parameter space, critical to the success of the method. Monte Carlo tests of simulated signals in noise demonstrate that this method is close to the theoretical optimal performance
Comparing models of the periodic variations in spin-down and beamwidth for PSR B1828-11
Full text linkWe build a framework using tools from Bayesian data analysis to evaluate models explaining the periodic variations in spin-down and beamwidth of PSR B1828?11. The available data consist of the time-averaged spin-down rate, which displays a distinctive double-peaked modulation, and measurements of the beamwidth. Two concepts exist in the literature that are capable of explaining these variations; we formulate predictive models from these and quantitatively compare them. The first concept is phenomenological and stipulates that the magnetosphere undergoes periodic switching between two metastable states as first suggested by Lyne et al. The second concept, precession, was first considered as a candidate for the modulation of B1828?11 by Stairs et al. We quantitatively compare models built from these concepts using a Bayesian odds ratio. Because the phenomenological switching model itself was informed by these data in the first place, it is difficult to specify appropriate parameter-space priors that can be trusted for an unbiased model comparison. Therefore, we first perform a parameter estimation using the spin-down data, and then use the resulting posterior distributions as priors for model comparison on the beamwidth data. We find that a precession model with a simple circular Gaussian beam geometry fails to appropriately describe the data, while allowing for a more general beam geometry provides a good fit to the data. The resulting odds between the precession model (with a general beam geometry) and the switching model are estimated as 102.7±0.5 in favour of the precession model
A semicoherent glitch-robust continuous-gravitational-wave search method
Full text linkIsolated nonaxisymmetric rotating neutron stars producing continuous-gravitational-wave signals may undergo occasional spin-up events known as glitches. If unmodeled by a search, these glitches can result in continuous wave signals being missed or misidentified as detector artifacts. We outline a semicoherent glitch-robust search method that allows identification of continuous wave signal candidates that contain glitches and inferences about the model parameters. We demonstrate how this can be applied to the follow-up of candidates found by wide-parameter space searches. We find that a Markov chain Monte Carlo method outperforms a grid-based method in speed and accuracy.</p
Inertial modes of non-stratified superfluid neutron stars
Full text linkWe present results concerning adiabatic inertial-mode oscillations of non-stratified superfluid neutron stars in Newtonian gravity, using the anelastic and slow-rotation approximations. We consider a simple two-fluid model of a superfluid neutron star, where one fluid consists of the superfluid neutrons and the second fluid contains all the comoving constituents (protons, electrons). The two fluids are assumed to be 'free' in the sense that vortex-mediated forces such as mutual friction or pinning are absent, but they can be coupled by the equation of state, in particular by entrainment. The stationary background consists of the two fluids rotating uniformly around the same axis with potentially different rotation rates. We study the special cases of corotating backgrounds, vanishing entrainment, and the purely toroidal r modes analytically. We calculate numerically the eigenfunctions and frequencies of inertial modes in the general case of non-corotating backgrounds, and study their dependence on the relative rotation rate and entrainment. In these non-stratified models, we find avoided crossings only between associated mode pairs, e.g. an 'ordinary' mode and its 'superfluid' counterpart, while other mode frequencies generally cross as the background parameters are varied. We confirm (for the first time in a mode calculation) the onset of a 'two-stream instability' at a critical relative background rotation rate, and we study some of the properties of this instability for the inertial modes
Effect of timing noise on targeted and narrow-band coherent searches for continuous gravitational waves from pulsars
Full text linkMost searches for continuous gravitational waves from pulsars use Taylor expansions in the phase to model the spin-down of neutron stars. Studies of pulsars demonstrate that their electromagnetic (EM) emissions suffer from timing noise, small deviations in the phase from Taylor expansion models. How the mechanism producing EM emission is related to any continuous gravitational-wave (CW) emission is unknown; if they either interact or are locked in phase, then the CW will also experience timing noise. Any disparity between the signal and the search template used in matched filtering methods will result in a loss of signal-to-noise ratio, referred to as “mismatch.” In this work we assume the CW suffers a level of timing noise similar to its EM counterpart. We inject and recover fake CW signals, which include timing noise generated from observational data on the Crab pulsar. Measuring the mismatch over durations of order ?10 months, the effect is, for the most part, found to be small. This suggests recent so-called “narrow-band” searches which placed upper limits on the signals from the Crab and Vela pulsars will not be significantly affected. At a fixed observation time, we find the mismatch depends upon the observation epoch. Considering the averaged mismatch as a function of observation time, we find that it increases as a power law with time, and so may become relevant in long baseline searche
On the free-precession candidate PSR B1828-11: Evidence for increasing deformation
Full text linkWe observe that the periodic variations in spin-down rate and beamwidth of the radio pulsar PSR B1828−11 are getting faster. In the context of a free precession model, this corresponds to a decrease in the precession period Pfp. We investigate how a precession model can account for such a decrease in Pfp, in terms of an increase over time in the absolute biaxial deformation (|εp| ∼ 10−8) of this pulsar. We perform a Bayesian model comparison against the ‘base’ precession model (with constant εp) developed in Ashton et al., and we obtain decisive odds in favour of a time-varying deformation. We study two types of time variation: (i) a linear drift with a posterior estimate of and odds of 1075 compared to the base model, and (ii) N discrete positive jumps in εp with very similar odds to the linear εp drift model. The physical mechanism explaining this behaviour is unclear, but the observation could provide a crucial probe of the interior physics of neutron stars. We also place an upper bound on the rate at which the precessional motion is damped, and translate this into a bound on a dissipative mutual friction-type coupling between the star's crust and core
Are pulsar glitches triggered by a superfluid two-stream instability?
Full text linkMature neutron stars are expected to have several superfluid components. Strong evidence for this is provided by the glitches that have been observed in dozens of pulsars. The underlying idea behind most glitch models is that, as the neutron star crust spins down due to the emission of electromagnetic radiation, the superfluid component lags behind until a critical point is reached and angular momentum is transferred from the superfluid to the crust, leading to the spin-up associated with the glitch. In this Letter we describe a superfluid analogue of the two-stream instability that is well known in plasma physics, and provide arguments that this instability is likely to be relevant for neutron stars. This is a new physical mechanism which may play a key role in explaining the glitch mechanism and which could also prove to be relevant in laboratory experiments on various superfluid systems
The superfluid two-stream instability
Full text linkThis paper provides the first study of a new dynamical instability in superfluids. This instability is similar to the two-stream instability known to operate in plasmas. It is analogous to the Kelvin-Helmholtz instability, but has the distinguishing feature that the two fluids are interpenetrating. The instability sets in once the relative flow between the two components of the system reaches a critical level. Our analysis is based on the two-fluid equations that have been used to model the dynamics of the outer core of a neutron star, where superfluid neutrons are expected to coexist with superconducting protons and relativistic electrons. These equations are analogous to the standard Landau model for superfluid Helium. We study this instability for two different model problems. First we analyze a local dispersion relation for waves in a system where one fluid is at rest while the other flows at a constant rate. This provides a proof of principle of the existence of the two-stream instability for superfluids. Our second model problem concerns two rotating fluids confined within an infinitesimally thin spherical shell. The aim of this model is to assess whether the two-stream instability may be relevant (perhaps as a trigger mechanism) for pulsar glitches. Our results for this problem show that the entrainment effect could provide a sufficiently strong couplin or the instability to set in at a relative flow small enough to be astrophysically plausible
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