56 research outputs found
Sinking of a magnetically confined mountain on an accreting neutron star
We perform ideal-magnetohydrodynamic axisymmetric simulations of magnetically
confined mountains on an accreting neutron star, with masses less than ~0.12
solar masses. We consider two scenarios, in which the mountain sits atop a hard
surface or sinks into a soft, fluid base. We find that the ellipticity of the
star, due to a mountain grown on a hard surface, approaches ~2e-4 for accreted
masses greater than ~1.2e-3 solar masses, and that sinking reduces the
ellipticity by between 25% and 60%. The consequences for gravitational
radiation from low-mass x-ray binaries are discussed.Comment: 13 pages, 12 figures, and 3 tables; accepted for publication in MNRA
Continuous frequency spectrum of the global hydromagnetic oscillations of a magnetically confined mountain on an accreting neutron star
We compute the continuous part of the ideal-magnetohydrodynamic (ideal-MHD)
frequency spectrum of a polar mountain produced by magnetic burial on an
accreting neutron star. Applying the formalism developed by Hellsten & Spies
(1979), extended to include gravity, we solve the singular eigenvalue problem
subject to line-tying boundary conditions. This spectrum divides into an
Alfv\'{e}n part and a cusp part. The eigenfunctions are chirped and anharmonic
with an exponential envelope, and the eigenfrequencies cover the whole spectrum
above a minimum . For equilibria with accreted mass 1.2
\times 10^{-6} \la M_a/M_\odot \la 1.7 \times 10^{-4} and surface magnetic
fields 10^{11} \la B_\ast/\mathrm{G} \la 10^{13}, is
approximately independent of , and increases with . The results
are consistent with the Alfv\'{e}n spectrum excited in numerical simulations
with the \textsc{zeus-mp} solver. The spectrum is modified substantially by the
Coriolis force in neutron stars spinning faster than Hz. The
implications for gravitational wave searches for low-mass X-ray binaries are
considered briefly.Comment: accepted by MNRA
Improved estimate of the detectability of gravitational radiation from a magnetically confined mountain on an accreting neutron star
We give an improved estimate of the detectability of gravitational waves from
magnetically confined mountains on accreting neutron stars. The improved
estimate includes the following effects for the first time: three-dimensional
hydromagnetic ("fast") relaxation, three-dimensional resistive ("slow")
relaxation, realistic accreted masses M_a \la 2 \times 10^{-3} M_\odot,
(where the mountain is grown ab initio by injection), and verification of the
curvature rescaling transformation employed in previous work. Typically, a
mountain does not relax appreciably over the lifetime of a low-mass X-ray
binary. The ellipticity reaches for
. The gravitational wave spectrum for triaxial
equilibria contains an additional line, which, although weak, provides valuable
information about the mountain shape. We evaluate the detectability of magnetic
mountains with Initial and Advanced LIGO. For a standard, coherent matched
filter search, we find a signal-to-noise ratio of for Initial LIGO, where is the distance and is
the observation time. From the nondetection of gravitational waves from
low-mass X-ray binaries to date, and the wave strain limits implied by the spin
frequency distribution of these objects (due to gravitational wave braking), we
conclude that there are other, as yet unmodelled, physical effects that further
reduce he quadrupole moment of a magnetic mountain, most notably sinking into
the crust.Comment: accepted by MNRA
Tracing colliding winds in the UV line orbital variability of gamma-ray binaries
Gamma-ray binaries emit most of their radiated power beyond ~10 MeV. The
non-thermal emission is thought to arise from the interaction of the
relativistic wind of a rotation-powered pulsar with the stellar wind of its
massive (O or Be) companion star. A powerful pulsar creates an extended cavity,
filled with relativistic electrons, in the radiatively-driven wind of the
massive star. As a result, the observed P Cyg profiles of UV resonant lines
from the stellar wind should be different from those of single massive stars.
We propose to use UV emission lines to detect and constrain the colliding wind
region in gamma-ray binaries. We compute the expected orbital variability of P
Cyg profiles depending upon the interaction geometry (set by the ratio of
momentum fluxes from the winds) and the line-of-sight to the system. We predict
little or no variability for the case of LS 5039 and PSR B1259-63, in agreement
with currently available HST observations of LS 5039. However, variability
between superior and inferior conjunction is expected in the case of LS I+61
303.Comment: Accepted for publication in MNRA
Gravitational waves from rapidly rotating neutron stars
Rapidly rotating neutron stars in Low Mass X-ray Binaries have been proposed
as an interesting source of gravitational waves. In this chapter we present
estimates of the gravitational wave emission for various scenarios, given the
(electromagnetically) observed characteristics of these systems. First of all
we focus on the r-mode instability and show that a 'minimal' neutron star model
(which does not incorporate exotica in the core, dynamically important magnetic
fields or superfluid degrees of freedom), is not consistent with observations.
We then present estimates of both thermally induced and magnetically sustained
mountains in the crust. In general magnetic mountains are likely to be
detectable only if the buried magnetic field of the star is of the order of
G. In the thermal mountain case we find that gravitational
wave emission from persistent systems may be detected by ground based
interferometers. Finally we re-asses the idea that gravitational wave emission
may be balancing the accretion torque in these systems, and show that in most
cases the disc/magnetosphere interaction can account for the observed spin
periods.Comment: To appear in 'Gravitational Waves Astrophysics: 3rd Session of the
Sant Cugat Forum on Astrophysics, 2014', Editor: Carlos F. Sopuert
Gravitational-wave spin-down and stalling lower limits on the electrical resistivity of the accreted mountain in a millisecond pulsar
The electrical resistivity of the accreted mountain in a millisecond pulsar
is limited by the observed spin-down rate of binary radio millisecond pulsars
(BRMSPs) and the spins and X-ray fluxes of accreting millisecond pulsars
(AMSPs). We find (where is the
spin-down age) for BRMSPs and (where
and are the actual and Eddington
accretion rates) for AMSPs. These limits are inferred assuming that the
mountain attains a steady state, where matter diffuses resistively across
magnetic flux surfaces but is replenished at an equal rate by infalling
material. The mountain then relaxes further resistively after accretion ceases.
The BRMSP spin-down limit approaches the theoretical electron-impurity
resistivity at temperatures \ga 10^5 K for an impurity concentration of , while the AMSP stalling limit falls two orders of magnitude below the
theoretical electron-phonon resistivity for temperatures above K. Hence
BRMSP observations are already challenging theoretical resistivity calculations
in a useful way. Next-generation gravitational-wave interferometers will
constrain at a level that will be competitive with electromagnetic
observations.Comment: accepted for publication in ApJ
Continuous-wave gravitational radiation from pulsar glitch recovery
Nonaxisymmetric, meridional circulation inside a neutron star, excited by a
glitch and persisting throughout the post-glitch relaxation phase, emits
gravitational radiation. Here, it is shown that the current quadrupole
contributes more strongly to the gravitational wave signal than the mass
quadrupole evaluated in previous work. We calculate the signal-to-noise ratio
for a coherent search and conclude that a large glitch may be detectable by
second-generation interferometers like the Laser Interferometer
Gravitational-Wave Observatory. It is shown that the viscosity and
compressibility of bulk nuclear matter, as well as the stratification
length-scale and inclination angle of the star, can be inferred from a
gravitational wave detection in principle.Comment: 19 pages, 4 figures, accepted for publication in MNRA
Gravitational waves from single neutron stars: an advanced detector era survey
With the doors beginning to swing open on the new gravitational wave
astronomy, this review provides an up-to-date survey of the most important
physical mechanisms that could lead to emission of potentially detectable
gravitational radiation from isolated and accreting neutron stars. In
particular we discuss the gravitational wave-driven instability and
asteroseismology formalism of the f- and r-modes, the different ways that a
neutron star could form and sustain a non-axisymmetric quadrupolar "mountain"
deformation, the excitation of oscillations during magnetar flares and the
possible gravitational wave signature of pulsar glitches. We focus on progress
made in the recent years in each topic, make a fresh assessment of the
gravitational wave detectability of each mechanism and, finally, highlight key
problems and desiderata for future work.Comment: 39 pages, 12 figures, 2 tables. Chapter of the book "Physics and
Astrophysics of Neutron Stars", NewCompStar COST Action 1304. Minor
corrections to match published versio
Three-dimensional stability of magnetically confined mountains on accreting neutron stars
We examine the hydromagnetic stability of magnetically confined mountains,
which arise when material accumulates at the magnetic poles of an accreting
neutron star. We extend a previous axisymmetric stability analysis by
performing three-dimensional simulations using the ideal-magnetohydrodynamic
(ideal-MHD) code \textsc{zeus-mp}, investigating the role played by boundary
conditions, accreted mass, stellar curvature, and (briefly) toroidal magnetic
field strength. We find that axisymmetric equilibria are susceptible to the
undular sub-mode of the Parker instability but are not disrupted. The
line-tying boundary condition at the stellar surface is crucial in stabilizing
the mountain. The nonlinear three-dimensional saturation state of the
instability is characterized by a small degree of nonaxisymmetry (\la 0.1 per
cent) and a mass ellipticity of for an accreted mass of
. Hence there is a good prospect of detecting
gravitational waves from accreting millisecond pulsars with long-baseline
interferometers such as Advanced LIGO. We also investigate the ideal-MHD
spectrum of the system, finding that long-wavelength poloidal modes are
suppressed in favour of toroidal modes in the nonaxisymmetric saturation state.Comment: accepted by MNRA
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