1,015 research outputs found
Magneto-elastic oscillations of neutron stars: exploring different magnetic field configurations
We study magneto-elastic oscillations of highly magnetized neutron stars
(magnetars) which have been proposed as an explanation for the quasi-periodic
oscillations (QPOs) appearing in the decaying tail of the giant flares of soft
gamma-ray repeaters (SGRs). We extend previous studies by investigating various
magnetic field configurations, computing the Alfv\'en spectrum in each case and
performing magneto-elastic simulations for a selected number of models. By
identifying the observed frequencies of 28 Hz (SGR 1900+14) and 30 Hz (SGR
1806-20) with the fundamental Alfv\'en QPOs, we estimate the required surface
magnetic field strength. For the magnetic field configurations investigated
(dipole-like poloidal, mixed toroidal-poloidal with a dipole-like poloidal
component and a toroidal field confined to the region of field lines closing
inside the star, and for poloidal fields with an additional quadrupole-like
component) the estimated dipole spin-down magnetic fields are between 8x10^14 G
and 4x10^15 G, in broad agreement with spin-down estimates for the SGR sources
producing giant flares. A number of these models exhibit a rich Alfv\'en
continuum revealing new turning points which can produce QPOs. This allows one
to explain most of the observed QPO frequencies as associated with
magneto-elastic QPOs. In particular, we construct a possible configuration with
two turning points in the spectrum which can explain all observed QPOs of SGR
1900+14. Finally, we find that magnetic field configurations which are entirely
confined in the crust (if the core is assumed to be a type I superconductor)
are not favoured, due to difficulties in explaining the lowest observed QPO
frequencies (f<30 Hz).Comment: 21 pages, 16 figures, 6 tables, matched to version accepted by MNRAS
with extended comparison/discussion to previous wor
Implementación de un sistema de tutorías académicas a nivel institucional dentro de un modelo de progresión del estudiante
La Universidad Central de Chile en el marco de promover la formación integral, la permanencia y el buen desempeño académico de sus estudiantes, ha desarrollado un modelo de progresión estudiantil. Este modelo pretende apoyar la trayectoria formativa de los estudiantes en base a una serie de estrategia, una de ellas corresponde a tutorías académicas, implementadas en una asignatura con alto porcentaje de reprobación en cada una de las carreras de la universidad, durante el año 2017, buscando disminuir la tasa de deserción producto del fracaso académico. El presente trabajo busca dar a conocer el sistema de tutorías, mostrando el trabajo colaborativo e institucional que se ha desarrollado en torno al mismo. Contiene una sistematización de los procedimientos involucrados, monitoreo del rendimiento y principales apreciaciones obtenidas por los tutorados y tutores, mediante la aplicación de encuestas y datos cuantitativos como son asistencia y notas. Se analizan y destacan las dificultades encontradas en la implementación de esta actividad bajo una mirada de mejora continua, con el objeto de aportar a otras instituciones de educación superior con la experiencia recabada, siendo así un aporte en la implementación de este tipo de tutorías
On the Quasi-Periodic Oscillations of Magnetars
We study torsional Alfv\'en oscillations of magnetars, i.e., neutron stars
with a strong magnetic field. We consider the poloidal and toroidal components
of the magnetic field and a wide range of equilibrium stellar models. We use a
new coordinate system (X,Y), where ,
and is the radial component of the magnetic
field. In this coordinate system, the 1+2-dimensional evolution equation
describing the quasi-periodic oscillations, QPOs, see Sotani et al. (2007), is
reduced to a 1+1-dimensional equation, where the perturbations propagate only
along the Y-axis. We solve the 1+1-dimensional equation for different boundary
conditions and open magnetic field lines, i.e., magnetic field lines that reach
the surface and there match up with the exterior dipole magnetic field, as well
as closed magnetic lines, i.e., magnetic lines that never reach the stellar
surface. For the open field lines, we find two families of QPOs frequencies; a
family of "lower" QPOs frequencies which is located near the X-axis and a
family of "upper" frequencies located near the Y-axis. According to Levin
(2007), the fundamental frequencies of these two families can be interpreted as
the turning points of a continuous spectrum. We find that the upper frequencies
are constant multiples of the lower frequencies with a constant equaling 2n+1.
For the closed lines, the corresponding factor is n+1 . By these relations, we
can explain both the lower and the higher observed frequencies in SGR 1806-20
and SGR 1900+14.Comment: 8 pages, 7 figure
What Magnetar Seismology can Teach us about the Magnetic Fields
The effect of magnetic fields on the frequencies of toroidal oscillations of
neutron stars is derived to lowest order. Interpreting the fine structure in
the QPO power spectrum of magnetars following giant flares reported by
Strohmayer and Watts (2006) to be "Zeeman splitting" of degenerate toroidal
modes, we estimate a crustal magnetic field of order 10^{15} Gauss or more. We
suggest that residual m, -m symmetry following such splitting might allow
beating of individual frequency components that is slow enough to be observed.Comment: 1 tabl
Magnetar Oscillations I: strongly coupled dynamics of the crust and the core
Quasi-Periodic Oscillations (QPOs) observed during Soft Gamma Repeaters giant
flares are commonly interpreted as the torsional oscillations of magnetars. The
oscillatory motion is influenced by the strong interaction between the shear
modes of the crust and Alfven-like modes in the core. We study the dynamics
which arises through this interaction, and present several new results: (1) We
show that global {\it edge modes} frequently reside near the edges of the core
Alfven continuum. (2) We compute the magnetar's oscillatory motion for
realistic axisymmetric magnetic field configurations and core density profiles,
but with a simplified model of the elastic crust. We show that one may
generically get multiple gaps in the Alfven continuum. One obtains discrete
global {\it gap modes} if the crustal frequencies belong to the gaps. (3) We
show that field tangling in the core enhances the role of the core discrete
Alfven modes and reduces the role of the core Alfven continuum in the overall
oscillatory dynamics of the magnetar. (4) We demonstrate that the system
displays transient and/or drifting QPOs when parts of the spectrum of the core
Alfven modes contain discrete modes which are densely and regularly spaced in
frequency. (5) We show that if the neutrons are coupled into the core Alfven
motion, then the post-flare crustal motion is strongly damped and has a very
weak amplitude. Thus magnetar QPOs give evidence that the proton and neutron
components in the core are dynamically decoupled and that at least one of them
is a quantum fluid. (6) We show that it is difficult to identify the
high-frequency 625 Hz QPO as being due to the physical oscillatory mode of the
magnetar, if the latter's fluid core consists of the standard
proton-neutron-electron mixture and is magnetised to the same extent as the
crust. (Abstract abridged)Comment: 22 pages, 22 figures, submitted to MNRA
An Optically Targeted Search for Gravitational Waves emitted by Core-Collapse Supernovae during the Third Observing Run of Advanced LIGO and Advanced Virgo
We present the results from a search for gravitational-wave transients associated with core-collapse supernovae observed optically within 30 Mpc during the third observing run of Advanced LIGO and Advanced Virgo. No gravitational wave associated with a core-collapse supernova has been identified. We then report the detection efficiency for a variety of possible gravitational-wave emissions. For neutrino-driven explosions, the distance at which we reach 50% detection efficiency is up to 8.9 kpc, while more energetic magnetorotationally-driven explosions are detectable at larger distances. The distance reaches for selected models of the black hole formation, and quantum chromodynamics phase transition are also provided. We then constrain the core-collapse supernova engine across a wide frequency range from 50 Hz to 2 kHz. The upper limits on gravitational-wave energy and luminosity emission are at low frequencies down to 10−4M⊙c2 and 5×10−4M⊙c2/s, respectively. The upper limits on the proto-neutron star ellipticity are down to 5 at high frequencies. Finally, by combining the results obtained with the data from the first and second observing runs of LIGO and Virgo, we improve the constraints of the parameter spaces of the extreme emission models. Specifically, the proto-neutron star ellipticities for the long-lasting bar mode model are down to 1 for long emission (1 s) at high frequency
Non-axisymmetric instabilities of neutron star with toroidal magnetic fields
The aim of this paper is to clarify the stabilities of neutron stars with
strong toroidal magnetic fields against non-axisymmetric perturbation. The
motivation comes from the fact that super magnetized neutron stars of G, magnetars, and magnetized proto-neutron stars born after the
magnetically-driven supernovae are likely to have such strong toroidal magnetic
fields. Long-term, three-dimensional general relativistic magneto-hydrodynamic
simulations are performed, preparing isentropic neutron stars with toroidal
magnetic fields in equilibrium as initial conditions. To explore the effects of
rotations on the stability, simulations are done for both non-rotating and
rigidly rotating models. We find the emergence of the Parker and/or Tayler
instabilities in both the non-rotating and rotating models. For both
non-rotating and rotating models, the Parker instability is the primary
instability as predicted by the local linear perturbation analysis. The
interchange instability also appears in the rotating models. It is found that
rapid rotation is not enough to suppress the Parker instability, and this
finding does not agree with the perturbation analysis. The reason for this is
that rigidly and rapidly rotating stars are marginally stable, and hence, in
the presence of stellar pulsations by which the rotational profile is deformed,
unstable regions with negative gradient of angular momentum profile is
developed. After the onset of the instabilities, a turbulence is excited.
Contrary to the axisymmetric case, the magnetic fields never reach an
equilibrium state after the development of the turbulence. This conclusion
suggests that three-dimensional simulation is indispensable for exploring the
formation of magnetars or prominence activities of magnetars such as giant
flares.Comment: 19 pages, 11 figures, to be published in A&
Analyzing the perceptions of pre-service teachers about the modules integración laboral v and vi during the practicum experience
Tesis (Profesor de Inglés para la Enseñanza Básica y Media, Licenciado en Educación)Este estudio tiene como objetivo analizar y entender las percepciones
de los estudiantes practicantes de pedagogía en Inglés de la Universidad
Nacional Andrés Bello (UNAB) sobre los contenidos teóricos cubiertos en los
módulos de Integración Laboral V y VI. Adicionalmente, este estudio también
tiene la intención de identificar las percepciones de los practicantes de
pedagogía en Inglés respecto al contenido enseñado en los módulos
previamente mencionados y determinar si los contenidos enseñados en
estos módulos son practicados antes de su implementación en la práctica.
Finalmente, este estudio tiene la intención de determinar la percepción de
los practicantes de pedagogía en inglés respecto a la reflexión realizada en
estos módulos. Con el fin de lograr estos objetivos, dos grupos de enfoque
fueron conducidos, la transcripción de estos fue analizada y codificada a
través de un análisis temático. Los principales hallazgos de esta
investigación fueron que los contenidos eran considerados repetitivos y no
contextualizados a la realidad del país y que los practicantes sentían que
hay una necesidad de haber tenido más instancias prácticas así también
como de ocasiones reflexivas.This study aims to analyse and understand the perceptions of Second
Language Teaching Education (SLTE) pre-service teachers from Universidad
Nacional Andres Bello (UNAB) about the theoretical contents covered in the
modules Integración Laboral V and VI. Additionally, this study also intends to
identify the perceptions of SLTE pre-service teachers regarding the content
taught in the previously mentioned modules, to determine (whether) the
contents taught in these modules are practiced before its implementation in
the practicum. Finally, this study also intends to determine the perception of
SLTE pre-service teachers regarding the reflection done in these modules. In
order to achieve these objectives, two focus groups were conducted, and the
transcriptions of these focus groups were analysed and coded through
thematic analysis. The main findings of this research were that the content
was considered to be repetitive and not contextualized to the country’s
reality, and that pre-service teachers felt that there is a need for more
practical instances as well as for reflective occasions
Constraints on Pasta Structure of Neutron Stars from Oscillations in Giant Flares
We show that the shear modes in the neutron star crust are quite sensitive to
the existence of nonuniform nuclear structures, so-called "pasta". Due to the
existence of pasta phase, the frequencies of shear modes are reduced, where the
dependence of fundamental frequency is different from that of overtones. Since
the torsional shear frequencies depend strongly on the structure of pasta
phase, through the observations of stellar oscillations, one can probe the
pasta structure in the crust, although that is quite difficult via the other
observations. Additionally, considering the effect of pasta phase, we show the
possibility to explain the all observed frequencies in the SGR 1806-20 with
using only crust torsional shear modes
- …