172 research outputs found
Three evolutionary paths for magnetar oscillations
Quasi-periodic oscillations have been seen in the light curves following
several magnetar giant flares. These oscillations are of great interest as they
probably provide our first ever view of the normal modes of oscillation of
neutron stars. The state-of-the-art lies in the study of the oscillations of
elastic-magnetic stellar models, mainly with a view to relating the observed
frequencies to the structure and composition of the star itself. We advance
this programme by considering several new physical mechanisms that are likely
to be important for magnetar oscillations. These relate to the
superfluid/superconducting nature of the stellar interior, and the damping of
the modes, both through internal dissipation mechanisms and the launching of
waves into the magnetosphere. We make simple order-of-magnitude estimates to
show that both the frequencies and the damping time of magnetar oscillations
can evolve in time, identifying three distinct `pathways' that can be followed,
depending upon the initial magnitude of the mode excitation. These results are
interesting as they show that the information buried in magnetar QPOs may be
even richer than previously thought, and motivate more careful examination of
magnetar light curves, to search for signatures of the different types of
evolution that we have identified.Comment: To appear in MNRAS. This version reflects changes made in response to
referee's comments, mainly extra discussion in Section 2.
Magnetic neutron star equilibria with stratification and type-II superconductivity
We construct two-fluid equilibrium configurations for neutron stars with
magnetic fields, using a self-consistent and nonlinear 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.Comment: 21 pages, 14 figures; some minor changes to match published versio
Superfluid instability of r-modes in "differentially rotating" neutron stars
Superfluid hydrodynamics affects the spin-evolution of mature neutron stars,
and may be key to explaining timing irregularities such as pulsar glitches.
However, most models for this phenomenon exclude the global instability
required to trigger the event. In this paper we discuss a mechanism that may
fill this gap. We establish that small scale inertial r-modes become unstable
in a superfluid neutron star that exhibits a rotational lag, expected to build
up due to vortex pinning as the star spins down. Somewhat counterintuitively,
this instability arises due to the (under normal circumstances dissipative)
vortex-mediated mutual friction. We explore the nature of the superfluid
instability for a simple incompressible model, allowing for entrainment
coupling between the two fluid components. Our results recover a previously
discussed dynamical instability in systems where the two components are
strongly coupled. In addition, we demonstrate for the first time that the
system is secularly unstable (with a growth time that scales with the mutual
friction) throughout much of parameter space. Interestingly, large scale
r-modes are also affected by this new aspect of the instability. We analyse the
damping effect of shear viscosity, which should be particularly efficient at
small scales, arguing that it will not be sufficient to completely suppress the
instability in astrophysical systems.Comment: RevTex, 11 figure
Eikonal quasinormal modes of black holes beyond general relativity III: scalar Gauss-Bonnet gravity
In a recent series of papers we have shown how the eikonal/geometrical optics approximation can be used to calculate analytically the fundamental quasinormal mode frequencies associated with coupled systems of wave equations, which arise, for instance, in the study of perturbations of black holes in gravity theories beyond General Relativity. As a continuation to this series, we here focus on the quasinormal modes of nonrotating black holes in scalar Gauss-Bonnet gravity assuming a small-coupling expansion. We show that the axial perturbations are purely tensorial and are described by a modified Regge-Wheeler equation, while the polar perturbations are of mixed scalar-tensor character and are described by a system of two coupled wave equations. When applied to these equations, the eikonal machinery leads to axial modes that deviate from the general relativistic results at quadratic order in the Gauss-Bonnet coupling constant. We show that this result is in agreement with an analysis of unstable circular null orbits around blackholes in this theory, allowing us to establish the geometrical optics-null geodesic correspondence for the axial modes. For the polar modes the small-coupling approximation forces us to consider the ordering between eikonal and small-coupling perturbative parameters; one of which we show, by explicit comparison against numerical data, yields the correct identification of the quasinormal modes of the scalar-tensor coupled system of wave equations. These corrections lift the general relativistic degeneracy between scalar and tensorial eikonal quasinormal modes at quadratic order in Gauss-Bonnet coupling in a way reminiscent of the Zeeman effect. In general, our analytic, eikonal quasinormal mode frequencies (normalized to the General Relativity ones) agree with numerical results with an error of in the regime of small coupling constant. (abridged
Transition from inspiral to plunge for eccentric equatorial Kerr orbits
Ori and Thorne have discussed the duration and observability (with LISA) of
the transition from circular, equatorial inspiral to plunge for stellar-mass
objects into supermassive () Kerr black holes. We
extend their computation to eccentric Kerr equatorial orbits. Even with orbital
parameters near-exactly determined, we find that there is no universal length
for the transition; rather, the length of the transition depends sensitively --
essentially randomly -- on initial conditions. Still, Ori and Thorne's
zero-eccentricity results are essentially an upper bound on the length of
eccentric transitions involving similar bodies (e.g., fixed). Hence the
implications for observations are no better: if the massive body is
, the captured body has mass , and the process occurs at
distance from LISA, then , with the precise constant depending on
the black hole spin. For low-mass bodies () for which the
event rate is at least vaguely understood, we expect little chance (probably
[much] less than 10%, depending strongly on the astrophysical assumptions) of
LISA detecting a transition event with during its run; however, even a
small infusion of higher-mass bodies or a slight improvement in LISA's noise
curve could potentially produce transition events during LISA's
lifetime.Comment: Submitted to PR
Perturbative Approach to an orbital evolution around a Supermassive black hole
A charge-free, point particle of infinitesimal mass orbiting a Kerr black
hole is known to move along a geodesic. When the particle has a finite mass or
charge, it emits radiation which carries away orbital energy and angular
momentum, and the orbit deviates from a geodesic.
In this paper we assume that the deviation is small and show that the
half-advanced minus half-retarded field surprisingly provides the correct
radiation reaction force, in a time-averaged sense, and determines the orbit of
the particle.Comment: accepted for publication in the Physical Revie
Rotating black hole orbit functionals in the frequency domain
In many astrophysical problems, it is important to understand the behavior of
functions that come from rotating (Kerr) black hole orbits. It can be
particularly useful to work with the frequency domain representation of those
functions, in order to bring out their harmonic dependence upon the fundamental
orbital frequencies of Kerr black holes. Although, as has recently been shown
by W. Schmidt, such a frequency domain representation must exist, the coupled
nature of a black hole orbit's and motions makes it difficult to
construct such a representation in practice. Combining Schmidt's description
with a clever choice of timelike coordinate suggested by Y. Mino, we have
developed a simple procedure that sidesteps this difficulty. One first Fourier
expands all quantities using Mino's time coordinate . In particular,
the observer's time is decomposed with . The frequency domain
description is then built from the -Fourier expansion and the
expansion of . We have found this procedure to be quite simple to implement,
and to be applicable to a wide class of functionals. We test the procedure
using a simple test function, and then apply it in a particularly interesting
case, the Weyl curvature scalar used in black hole perturbation
theory.Comment: 16 pages, 2 figures. Submitted to Phys Rev D. New version gives a
vastly improved algorithm due to Drasco for computing the Fourier transforms.
Drasco has been added as an author. Also fixed some references and
exterminated a small herd of typos; final published versio
Gravitational waveforms from a point particle orbiting a Schwarzschild black hole
We numerically solve the inhomogeneous Zerilli-Moncrief and Regge-Wheeler
equations in the time domain. We obtain the gravitational waveforms produced by
a point-particle of mass traveling around a Schwarzschild black hole of
mass M on arbitrary bound and unbound orbits. Fluxes of energy and angular
momentum at infinity and the event horizon are also calculated. Results for
circular orbits, selected cases of eccentric orbits, and parabolic orbits are
presented. The numerical results from the time-domain code indicate that, for
all three types of orbital motion, black hole absorption contributes less than
1% of the total flux, so long as the orbital radius r_p(t) satisfies r_p(t)> 5M
at all times.Comment: revtex4, 24 pages, 23 figures, 3 tables, submitted to PR
- …