63 research outputs found
Numerical viscosity in hydrodynamics simulations in general relativity
We present an alternative method to estimate the numerical viscosity in
simulations of astrophysical objects, which is based in the damping of fluid
oscillations. We apply the method to general relativistic hydrodynamic
simulations using a spherical coordinates. We perform 1D-spherical and 2D-
axisymmetric simulations of radial oscillations in spherical systems. We
calibrate first the method with simulations with added bulk viscosity and study
the differences between different numerical schemes. We apply the method to
radial oscillations of neutron stars and we conclude that the main source of
numerical viscosity in this case is the surface of the star. We expect that
this method could be useful to compute the resolution requirements and
limitations of the numerical simulations in different astrophysical scenarios
in the future.Comment: Accepted for publication in Class. Quantum Grav. 15 pages, 3 figure
Towards asteroseismology of core-collapse supernovae with gravitational-wave observations - I. Cowling approximation
Gravitational waves from core-collapse supernovae are produced by the
excitation of different oscillation modes in the proto-neutron star (PNS) and
its surroundings, including the shock. In this work we study the relationship
between the post-bounce oscillation spectrum of the PNS-shock system and the
characteristic frequencies observed in gravitational-wave signals from
core-collapse simulations. This is a fundamental first step in order to develop
a procedure to infer astrophysical parameters of the PNS formed in
core-collapse supernovae. Our method combines information from the oscillation
spectrum of the PNS, obtained through linear-perturbation analysis in general
relativity of a background physical system, with information from the
gravitational-wave spectrum of the corresponding non-linear, core-collapse
simulation. Using results from the simulation of the collapse of a 35
presupernova progenitor we show that both types of spectra are
indeed related and we are able to identify the modes of oscillation of the PNS,
namely g-modes, p-modes, hybrid modes, and standing-accretion-shock-instability
(SASI) modes, obtaining a remarkably close correspondence with the
time-frequency distribution of the gravitational-wave modes. The analysis
presented in this paper provides a proof-of-concept that asteroseismology is
indeed possible in the core-collapse scenario, and it may serve as a basis for
future work on PNS parameter inference based on gravitational-wave
observations
Coherent magneto-elastic oscillations in superfluid magnetars
We study the effect of superfluidity on torsional oscillations of highly
magnetised neutron stars (magnetars) with a microphysical equation of state by
means of two-dimensional, magnetohydrodynamical- elastic simulations. The
superfluid properties of the neutrons in the neutron star core are treated in a
parametric way in which we effectively decouple part of the core matter from
the oscillations. Our simulations confirm the existence of two groups of
oscillations, namely continuum oscillations that are confined to the neutron
star core and are of Alfv\'enic character, and global oscillations with
constant phase and that are of mixed magneto-elastic type. The latter might
explain the quasi-periodic oscillations observed in magnetar giant flares,
since they do not suffer from the additional damping mechanism due to phase
mixing, contrary to what happens for continuum oscillations. However, we cannot
prove rigorously that the coherent oscillations with constant phase are normal
modes. Moreover, we find no crustal shear modes for the magnetic field
strengths typical for magnetars.We provide fits to our numerical simulations
that give the oscillation frequencies as functions of magnetic field strength
and proton fraction in the core.Comment: 16 pages, 12 figures, accepted by MNRA
Modulating the magnetosphere of magnetars by internal magneto-elastic oscillations
We couple internal torsional, magneto-elastic oscillations of highly
magnetized neutron stars (magnetars) to their magnetospheres. The corresponding
axisymmetric perturbations of the external magnetic field configuration evolve
as a sequence of linear, force-free equilibria that are completely determined
by the background magnetic field configuration and by the perturbations of the
magnetic field at the surface. The perturbations are obtained from simulations
of magneto-elastic oscillations in the interior of the magnetar. While such
oscillations can excite travelling Alfv\'en waves in the exterior of the star
only in a very limited region close to the poles, they still modulate the near
magnetosphere by inducing a time-dependent twist between the foot-points of
closed magnetic field lines that exit the star at a polar angle rad. Moreover, we find that for a dipole-like background magnetic field
configuration the magnetic field modulations in the magnetosphere, driven by
internal oscillations, can only be symmetric with respect to the equator. This
is in agreement with our previous findings, where we interpreted the observed
quasi-periodic oscillations in the X-ray tail of magnetar bursts as driven by
the family of internal magneto-elastic oscillations with symmetric magnetic
field perturbations.Comment: 9 pages, 5 figures, 2 tables, Accepted by MNRA
Magneto-elastic oscillations of neutron stars with dipolar magnetic fields
By means of two dimensional, general-relativistic, magneto-hydrodynamical
simulations we investigate the oscillations of magnetized neutron star models
(magnetars) including the description of an extended solid crust. The aim of
this study is to understand the origin of the QPOs observed in the giant flares
of SGRs. We confirm the existence of three different regimes: (a) a weak
magnetic field regime B<5 x 10^13 G, where crustal shear modes dominate the
evolution; (b) a regime of intermediate magnetic fields 5 x 10^13 G<B< 10^15 G,
where Alfv\'en QPOs are mainly confined to the core of the neutron star and the
crustal shear modes are damped very efficiently; and (c) a strong field regime
B>10^15 G, where magneto-elastic oscillations reach the surface and approach
the behavior of purely Alfv\'en QPOs. When the Alfv\'en QPOs are confined to
the core of the neutron star, we find qualitatively similar QPOs as in the
absence of a crust. The lower QPOs associated with the closed field lines of
the dipolar magnetic field configuration are reproduced as in our previous
simulations without crust, while the upper QPOs connected to the open field
lines are displaced from the polar axis. Additionally, we observe a family of
edge QPOs. Our results do not leave much room for a crustal-mode interpretation
of observed QPOs in SGR giant flares, but can accommodate an interpretation of
these observations as originating from Alfv\'en-like, global, turning-point
QPOs in models with dipolar magnetic field strengths in the narrow range of 5 x
10^15 G < B < 1.4 x 10^16 G. This range is somewhat larger than estimates for
magnetic field strengths in known magnetars. The discrepancy may be resolved in
models including a more complicated magnetic field structure or with models
taking superfluidity of the neutrons and superconductivity of the protons in
the core into account.Comment: 25 pages, 17 figures, 7 tables, minor corrections to match published
version in MNRA
Constraining properties of high-density matter in neutron stars with magneto-elastic oscillations
We discuss torsional oscillations of highly magnetised neutron stars
(magnetars) using two-dimensional, magneto-elastic-hydrodynamical simulations.
Our model is able to explain both the low- and high-frequency quasi-periodic
oscillations (QPOs) observed in magnetars. The analysis of these oscillations
provides constraints on the breakout magnetic-field strength, on the
fundamental QPO frequency, and on the frequency of a particularly excited
overtone. More importantly, we show how to use this information to generically
constraint properties of high-density matter in neutron stars, employing
Bayesian analysis. In spite of current uncertainties and computational
approximations, our model-dependent Bayesian posterior estimates for SGR
1806-20 yield a magnetic-field strength G and a crust thickness of km, which are both in remarkable agreement with
observational and theoretical expectations, respectively (1- error bars
are indicated). Our posteriors also favour the presence of a superfluid phase
in the core, a relatively low stellar compactness, , indicating a
relatively stiff equation of state and/or low mass neutron star, and high shear
speeds at the base of the crust, cm/s. Although the
procedure laid out here still has large uncertainties, these constraints could
become tighter when additional observations become available.Comment: 14 pages, 8 figures, 6 tables, submitted to MNRA
Imprints of superfluidity on magneto-elastic QPOs of SGRs
Our numerical simulations show that axisymmetric, torsional, magneto-elastic
oscillations of magnetars with a superfluid core can explain the whole range of
observed quasi-periodic oscillations (QPOs) in the giant flares of soft
gamma-ray repeaters. There exist constant phase, magneto-elastic QPOs at both
low (f500 Hz), in full agreement with
observations. The range of magnetic field strengths required to match the
observed QPO frequencies agrees with that from spin-down estimates. These
results strongly suggest that neutrons in magnetar cores are superfluid.Comment: 5 pages, 4 figure
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