178 research outputs found
Spatial distribution of radionuclides in 3D models of SN 1987A and Cas A
Fostered by the possibilities of multi-dimensional computational modeling, in
particular the advent of three-dimensional (3D) simulations, our understanding
of the neutrino-driven explosion mechanism of core-collapse supernovae (SNe)
has experienced remarkable progress over the past decade. First
self-consistent, first-principle models have shown successful explosions in 3D,
and even failed cases may be cured by moderate changes of the microphysics
inside the neutron star (NS), better grid resolution, or more detailed
progenitor conditions at the onset of core collapse, in particular large-scale
perturbations in the convective Si and O burning shells. 3D simulations have
also achieved to follow neutrino-driven explosions continuously from the
initiation of the blast wave, through the shock breakout from the progenitor
surface, into the radioactively powered evolution of the SN, and towards the
free expansion phase of the emerging remnant. Here we present results from such
simulations, which form the basis for direct comparisons with observations of
SNe and SN remnants in order to derive constraints on the still disputed
explosion mechanism. It is shown that predictions based on hydrodynamic
instabilities and mixing processes associated with neutrino-driven explosions
yield good agreement with measured NS kicks, light-curve properties of SN
1987A, and asymmetries of iron and 44Ti distributions observed in SN 1987A and
Cassiopeia A.Comment: 9 pages, 6 figures; submitted to: "SN 1987A, 30 years later",
Proceedings IAU Symposium No. 331, 2017; eds. M. Renaud et a
Search for quasi-periodic signals in magnetar giant flares
Quasi-periodic oscillations (QPOs) discovered in the decaying tails of giant
flares of magnetars are believed to be torsional oscillations of neutron stars.
These QPOs have a high potential to constrain properties of high-density
matter. In search for quasi-periodic signals, we study the light curves of the
giant flares of SGR 1806-20 and SGR 1900+14, with a non-parametric Bayesian
signal inference method called DPO. The DPO algorithm models the raw
photon counts as a continuous flux and takes the Poissonian shot noise as well
as all instrument effects into account. It reconstructs the logarithmic flux
and its power spectrum from the data. Using this fully noise-aware method, we
do not confirm previously reported frequency lines at Hz
because they fall into the noise-dominated regime. However, we find two new
potential candidates for oscillations at Hz (SGR 1806-20) and Hz
(SGR 1900+14). If these are real and the fundamental magneto-elastic
oscillations of the magnetars, current theoretical models would favour
relatively weak magnetic fields G
(SGR 1806-20) and a relatively low shear velocity inside the crust compared to
previous findings
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
Magneto-elastic oscillations modulating the emission of magnetars
Magneto-elastic oscillations of neutron stars are believed to explain
observed quasi-periodic oscillations (QPOs) in the decaying tail of the giant
flares of highly magnetized neutron stars (magnetars). Strong efforts of the
theoretical modelling from different groups have increased our understanding of
this phenomenon significantly. Here, we discuss some constraints on the matter
in neutron stars that arise if the interpretation of the observations in terms
of superfluid, magneto-elastic oscillations is correct. To explain the observed
modulation of the light curve of the giant flare, we describe a model that
allows the QPOs to couple to the stellar exterior through the magnetic field.
In this magnetosphere, the shaking magnetic field induces currents that provide
scattering targets for resonant cyclotron scattering of photons, which is
calculated with a Monte-Carlo approach and coupled to a code that calculates
the momentum distribution of the charge carriers as a one-dimensional
accelerator problem. We show first results of a simplified, but self-consistent
momentum distribution, i.e. a waterbag distribution, and of the corresponding
spectra.Comment: 7 pages, 4 figures, proceedings of stars2017 and 2017smfn
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