arXiv subjects

A. Melatos

Publications and source records attributed to A. Melatos.

84 records · Page 5Linked to original sources

arXiv2006

Gravitational radiation from nonaxisymmetric spherical Couette flow in a neutron star

The gravitational wave signal generated by global, nonaxisymmetric shear flows in a neutron star is calculated numerically by integrating the incompressible Navier--Stokes equation in a spherical, differentially rotating shell. At Reynolds numbers $\Rey \gsim 3 \times 10^{3}$, the laminar Stokes flow is unstable and helical, oscillating Taylor--Görtler vortices develop. The gravitational wave strain generated by the resulting kinetic-energy fluctuations is computed in both $+$ and $\times$ polarizations as a function of time. It is found that the signal-to-noise ratio for a coherent, $10^{8}$-{\rm s} integration with LIGO II scales as $ 6.5 (Ω_*/10^{4} {\rm rad} {\rm s}^{-1})^{7/2}$ for a star at 1 {\rm kpc} with angular velocity $Ω_*$. This should be regarded as a lower limit: it excludes pressure fluctuations, herringbone flows, Stuart vortices, and fully developed turbulence (for $\Rey \gsim 10^{6}$).

gr-qc↗

arXiv2005

Frequency spectrum of gravitational radiation from global hydromagnetic oscillations of a magnetically confined mountain on an accreting neutron star

Recent time-dependent, ideal-magnetohydrodynamic (ideal-MHD) simulations of polar magnetic burial in accreting neutron stars have demonstrated that stable, magnetically confined mountains form at the magnetic poles, emitting gravitational waves at $f_{*}$ (stellar spin frequency) and $2 f_{*}$. Global MHD oscillations of the mountain, whether natural or stochastically driven, act to modulate the gravitational wave signal, creating broad sidebands (full-width half-maximum $\sim 0.2f_*$) in the frequency spectrum around $f_{*}$ and $2 f_{*}$. The oscillations can enhance the signal-to-noise ratio achieved by a long-baseline interferometer with coherent matched filtering by up to 15 per cent, depending on where $f_*$ lies relative to the noise curve minimum. Coherent, multi-detector searches for continuous waves from nonaxisymmetric pulsars should be tailored accordingly.

astro-ph↗

arXiv2005

Near-Infrared, Kilosecond Variability of the Wisps and Jet in the Crab Pulsar Wind Nebula

We present a time-lapse sequence of 20 near-infrared (J- and K'-band) snapshots of the central 20x20 arcsec of the Crab pulsar wind nebula, taken at subarcsecond resolution with the Hokupa'a/QUIRC adaptive optics camera on the Gemini North Telescope, and sampled at intervals of 10 min and 24 hr. It is observed that the equatorial wisps and polar knots in the termination shock of the pulsar wind appear to fluctuate in brightness on kilosecond time-scales. Maximum flux variations of +/-24 +/-4 and +/-14 +/-4 per cent relative to the mean (in 1.2 ks) are measured for the wisps and knots respectively, with greatest statistical significance in J band where the nebula background is less prominent. The J and K' flux densities imply different near-infrared spectra for the nonthermal continuum emission from the wisps and outermost polar knot (sprite), giving F_nu ~ nu^{-0.56+/-0.12} and F_nu ~ nu^{-0.21+/-0.13) respectively. The data are compared with existing optical and UV photometry and applied to constrain theories of the variability of the wisps (relativistic ion-cyclotron instability) and knots (relativistic fire hose instability).

astro-ph↗

arXiv2005

Global three-dimensional flow of a neutron superfluid in a spherical shell in a neutron star

We integrate for the first time the hydrodynamic Hall-Vinen-Bekarevich-Khalatnikov equations of motion of a $^{1}S_{0}$-paired neutron superfluid in a rotating spherical shell, using a pseudospectral collocation algorithm coupled with a time-split fractional scheme. Numerical instabilities are smoothed by spectral filtering. Three numerical experiments are conducted, with the following results. (i) When the inner and outer spheres are put into steady differential rotation, the viscous torque exerted on the spheres oscillates quasiperiodically and persistently (after an initial transient). The fractional oscillation amplitude ($\sim 10^{-2}$) increases with the angular shear and decreases with the gap width. (ii) When the outer sphere is accelerated impulsively after an interval of steady differential rotation, the torque increases suddenly, relaxes exponentially, then oscillates persistently as in (i). The relaxation time-scale is determined principally by the angular velocity jump, whereas the oscillation amplitude is determined principally by the gap width. (iii) When the mutual friction force changes suddenly from Hall-Vinen to Gorter-Mellink form, as happens when a rectilinear array of quantized Feynman-Onsager vortices is destabilized by a counterflow to form a reconnecting vortex tangle, the relaxation time-scale is reduced by a factor of $\sim 3$ compared to (ii), and the system reaches a stationary state where the torque oscillates with fractional amplitude $\sim 10^{-3}$ about a constant mean value. Preliminary scalings are computed for observable quantities like angular velocity and acceleration as functions of Reynolds number, angular shear, and gap width. The results are applied to the timing irregularities (e.g., glitches and timing noise) observed in radio pulsars.

astro-ph↗

arXiv2005

Particle-In-Cell Simulations of a Nonlinear Transverse Electromagnetic Wave in a Pulsar Wind Termination Shock

A 2.5-dimensional particle-in-cell code is used to investigate the propagation of a large-amplitude, superluminal, nearly transverse electromagnetic (TEM) wave in a relativistically streaming electron-positron plasma with and without a shock. In the freestreaming, unshocked case, the analytic TEM dispersion relation is verified, and the streaming is shown to stabilize the wave against parametric instabilities. In the confined, shocked case, the wave induces strong, coherent particle oscillations, heats the plasma, and modifies the shock density profile via ponderomotive effects. The wave decays over $\gtrsim 10^2$ skin depths; the decay length scale depends primarily on the ratio between the wave frequency and the effective plasma frequency, and on the wave amplitude. The results are applied to the termination shock of the Crab pulsar wind, where the decay length-scale (at least 0.05") might be comparable to the thickness of filamentary, variable substructure observed in the optical and X-ray wisps and knots.

astro-ph↗

arXiv2005

Kelvin Helmholtz Instability and Circulation Transfer at an Isotropic-Anisotropic Superfluid Interface in a Neutron Star

A recent laboratory experiment (Blaauwgeers et al. 2003) suggests that a Kelvin-Helmholtz (KH) instability at the interface between two superfluids, one rotating and anisotropic, the other stationary and isotropic, may trigger sudden spin-up of the stationary superfluid. This result suggests that a KH instability at the crust-core ($^1S_0$-$^3P_2$-superfluid) boundary of a neutron star may provide a trigger mechanism for pulsar glitches. We calculate the dispersion relation of the KH instability involving two different superfluids including the normal fluid components and their effects on stability, particularly entropy transport. We show that an entropy difference between the core and crust superfluids reduces the threshold differential shear velocity and threshold crust-core density ratio. We evaluate the wavelength of maximum growth of the instability for neutron star parameters and find the resultant circulation transfer to be within the range observed in pulsar glitches.

astro-ph↗

arXiv2005

Gravitational Radiation from an Accreting Millisecond Pulsar with a Magnetically Confined Mountain

The amplitude of the gravitational radiation from an accreting neutron star undergoing polar magnetic burial is calculated. During accretion, the magnetic field of a neutron star is compressed into a narrow belt at the magnetic equator by material spreading equatorward from the polar cap. In turn, the compressed field confines the accreted material in a polar mountain which is misaligned with the rotation axis in general, producing gravitational waves. The equilibrium hydromagnetic structure of the polar mountain, and its associated mass quadrupole moment, are computed as functions of the accreted mass, M_a, by solving a Grad-Shafranov boundary value problem. The orientation- and polarization-averaged gravitational wave strain, h_c ~ 6e-24 for a 0.6 kHz source at 1 kpc with M_a > 10^-5 M_Sun, exceeds previous estimates that failed to treat equatorward spreading and flux freezing self-consistently. It is concluded that an accreting millisecond pulsar emits a persistent, sinusoidal gravitational wave signal at levels detectable, in principle, by long baseline interferometers after phase-coherent integration, provided that the polar mountain is hydromagnetically stable. Magnetic burial also reduces the magnetic dipole moment, mu, monotonically, implying a novel, observationally testable scaling h_c(mu). The implications for the rotational evolution of (accreting) X-ray and (isolated) radio millisecond pulsars are explored.

astro-ph↗

arXiv2004

Torque bistability in the interaction between a neutron star magnetosphere and a thin accretion disc

We present a time-dependent model of the interaction between a neutron star magnetosphere and a thin (Shakura-Sunyaev) accretion disc, where the extent of the magnetosphere is determined by balancing outward diffusion and inward advection of the stellar magnetic field at the inner edge of the disc. The nature of the equilibria available to the system is governed by the magnetic Prandtl number Pm and the ratio ξof the corotation radius to the Alfven radius. For ξ> Pm^0.3, the system can occupy one of two stable states, where the torques are of opposite signs. If the star is spinning up initially, in the absence of extraneous perturbations, ξdecreases until the spin-up equilibrium vanishes, the star subsequently spins down, and the torque asymptotes to zero. Vortex-in-cell simulations of the Kelvin-Helmholtz instability suggest that the transport speed across the mixing layer between the disc and magnetosphere is less than the shear speed when the layer is thin, unlike in previous models.

astro-ph↗

arXiv2004

Burial of the polar magnetic field of an accreting neutron star. I. Self-consistent analytic and numerical equilibria

The hydromagnetic structure of a neutron star accreting symmetrically at both magnetic poles is calculated as a function of accreted mass, M_a, and polar cap radius,starting from a centered magnetic dipole and evolving through a quasistatic sequence of two-dimensional, Grad-Shafranov equilibria. The calculation is the first to track fully the growth of high-order magnetic multipoles, due to equatorward hydromagnetic spreading, while simultaneously preserving flux freezing and a self-consistent mass-flux distribution. Equilibria are constructed numerically by an iterative scheme and analytically by Green functions. Two key results are obtained, with implications for recycled pulsars. (i) The mass required to significantly reduce the magnetic dipole moment, 10^{-5} Msun, greatly exceeds previous estimates (~ 10^{-10} Msun), which ignored the confining stress exerted by the compressed equatorial magnetic field. (ii) Magnetic bubbles, disconnected from the stellar surface, form in the later stages of accretion (M_a > 10^{-4} Msun).

astro-ph↗

arXiv2002

Millimetre Science with the Upgraded Australia Telescope

A new astronomical window into the southern skies has been opened with the high-frequency upgrade to the Australia Telescope Compact Array (ATCA), which allows radio-interferometric mapping of sources at wavelengths as short as 3mm. In anticipation of the upgrade's completion, a two-day workshop was held at the University of Melbourne in November 2001. The workshop covered a diverse range of fields, tied together by a common theme of identifying key areas where ATCA observations can have an impact. More than half of the talks were concerned with molecular clouds and star formation, with the remainder covering topics such as molecular gas in the Galactic Centre, Seyfert nuclei, and high-redshift objects. Some early results from the 3mm and 12mm prototype systems were also presented. In consultation with the speakers, we are presenting in this article a summary of the talks. The original slides are available at http://www.atnf.csiro.au/whats_on/workshops/mm_science2001/ .

astro-ph↗

arXiv2000

Radiative Precession of an Isolated Neutron Star

Euler's equations of motion are derived exactly for a rigid, triaxial, internally frictionless neutron star spinning down electromagnetically in vacuo. It is shown that the star precesses, but not freely: its regular precession relative to the principal axes of inertia couples to the component of the radiation torque associated with the near-zone radiation fields and is modified into an anharmonic wobble. The wobble period τ_1 typically satisfies τ_1 < 10^{-2}τ_0, where τ_0 is the braking time-scale; the wobble amplitude evolves towards a constant non-zero value, oscillates, or decreases to zero, depending on the degree of oblateness or prolateness of the star and its initial spin state; and the (negative) angular frequency derivative dω/dt oscillates as well, exhibiting quasi-periodic spikes for triaxial stars of a particular figure. In light of these properties, a young, Crab-like pulsar ought to display fractional changes of order unity in the space of a few years in its pulse profile, magnetic inclination angle, and dω/dt. Such changes are not observed, implying that the wobble is damped rapidly by internal friction, if its amplitude is initially large upon crystallization of the stellar crust. If the friction is localized in the inner and outer crusts, the thermal luminosity of the neutron star increases by a minimum amount ΔL = 3*10^{31} (ε/ 10^{-12}) (ω/ 10^3 rad s^{-1})^2 (τ_d / 1 yr)^{-1} erg s^{-1}, where epsilon is the ellipticity and τ_d is the damping time-scale, with the actual value of ΔL determined in part by the thermal conduction time τ_cond. The increased luminosity is potentially detectable as thermal X-rays lasting for a time max(tau_d,tau_cond) following crystallization of the crust.

astro-ph↗

arXiv1999

Bumpy Spin-Down of Anomalous X-Ray Pulsars: The Link with Magnetars

The two anomalous X-ray pulsars (AXPs) with well-sampled timing histories, 1E 1048.1-5937 and 1E 2259+586, are known to spin down irregularly, with `bumps' superimposed on an overall linear trend. Here we show that if AXPs are non-accreting magnetars, i.e. isolated neutron stars with surface magnetic fields B_0 > 10^{10} T, then they spin down electromagnetically in exactly the manner observed, due to an effect called `radiative precession'. Internal hydromagnetic stresses deform the star, creating a fractional difference epsilon=(I_3-I_1)/I_1 ~ 10^{-8} between the principal moments of inertia I_1 and I_3; the resulting Eulerian precession couples to an oscillating component of the electromagnetic torque associated with the near-zone radiation fields, and the star executes an anharmonic wobble with period tau_pr ~ 2 pi / epsilon Omega(t) ~ 10 yr, where Omega(t) is the rotation frequency as a function of time t. We solve Euler's equations for a biaxial magnet rotating in vacuo; show that the computed Omega(t) matches the measured timing histories of 1E 1048.1-5937 and 1E 2259+586; predict Omega(t) for the next 20 years for both objects; predict a statistical relation between and tau_pr, to be tested as the population of known AXPs grows; and hypothesize that radiative precession will be observed in future X-ray timing of soft gamma-ray repeaters (SGRs).

astro-ph↗