1,027 research outputs found
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
Effects of Patch Size, Fragmentation, and Invasive Species on Plant and Lepidoptera Communities in Southern Texas
Simple Summary Human land use has removed habitats, separated habitats into small and disconnected fragments, and introduced foreign species, which all harm wildlife. South Texas is highly diverse and home to many endangered species, but human disturbance threatens its wildlife. In south Texas, we poorly understand how different aspects of human land use influence wildlife diversity and abundance. We studied this by surveying plants and butterflies in 24 habitat fragments in south Texas that differed in size, shape, type, and land use history. Human disturbance was extensive, and foreign and weedy species were dominant in most habitats. Habitat types had distinctive sets of plants and butterflies, but habitats with the most human disturbance were the least distinct and had the most foreign or weedy species. Usually, larger and less-fragmented habitats have fewer foreign and weedy species and have higher diversity, and habitats with more foreign and weedy species have lower diversity, but only the first of these was true in our study. This suggests that historic sets of native plants are very rare, most areas are actively recovering from disturbance, and foreign species are now a normal part of communities. This study helps us understand how human land use impacts wildlife and how we can better manage land to protect and enhance wildlife. Abstract Habitat loss, fragmentation, and invasive species are major threats to biodiversity. In the Lower Rio Grande Valley (LRGV) of southern Texas, a conservation hotspot, few studies have examined how land use change and biotic disturbance influence biodiversity, particularly among Lepidoptera. We surveyed 24 habitat fragments on private lands in the LRGV and examined how patch size, edge to interior ratio (EIR), prevalence of invasive, exotic, and pest (IEP) plant species, and other environmental factors influenced plant and Lepidoptera communities within four habitat classes. Biotic disturbance was widespread and intense. IEP plants represented three of the four most common species in all but one habitat class; yet, classes largely had distinctive plant and Lepidoptera communities. Larger habitat patches had lower IEP prevalence but also lower plant richness and lower Lepidoptera richness and abundance. Conversely, patches with higher EIRs had greater IEP prevalence, plant richness, and Lepidoptera richness and abundance. IEP prevalence was negatively related to plant diversity and positively related to woody dominance, blooming plant abundance, and, surprisingly, both plant cover and richness. However, plant richness, abundance, and diversity were higher where a greater proportion of the plants were native. Lepidoptera diversity increased with plant cover, and Lepidoptera richness and abundance increased with plant richness. More individual Lepidoptera species were influenced by habitat attributes than by availability of resources such as host plants or nectar sources. Our results illustrate extensive landscape alteration and biotic disturbance and suggest that most regional habitats are at early successional stages and populated by a novel species pool heavy in IEP species; these factors must be considered together to develop effective and realistic management plans for the LRGV
Combined Effects of Scarification, Phytohormones, Stratification, and Soil Type on the Germination and/or Seedling Performance of Three Tamaulipan Thornscrub Forest Species
Tamaulipan thornforests in south Texas and northeast Mexico are a conservation hotspot. Shortages of native seedlings limit regional restoration and are largely driven by knowledge gaps regarding propagation of the 75+ thornforest species planted during restorations. We previously investigated three thornforest species with low or inconsistent germination or seedling survival: Ebenopsis ebano (Fabaceae), Cordia boissieri (Boraginaceae), and Zanthoxylum fagara (Rutaceae), and identified the types and dosages of chemical seed treatments that maximized germination. However, chemical treatments were performed in isolation and combinational treatments may be required to break dormancy or maximize germination. This study builds on prior work by investigating the effects of all possible combinations of sulfuric acid (SA), gibberellic acid (GA), and indole-3-butyric acid (IBA) treatments on germination of the same focal species, and further quantified the combined effects of five chemical treatments, three stratification treatments, and six soil mixture types on the germination and seedling performance of the focal species. Ebenopsis ebano germination peaked with SA and was not improved with additional chemical treatments. Cordia boissieri germination was highest with GA only in our indoor experiment but peaked with GA + IBA + SA in our outdoor experiment. Zanthoxylum fagara germination was near zero in all treatments. Stratification treatments marginally reduced E. ebano germination and reduced C. boissieri seedling height. Soil type had significant impacts on E. ebano germination and leaf abundance (residual differences up to 40% or 4 leaves, respectively) and influenced some of the effects of chemical treatments. These results enhance our understanding of thornforest seed ecology and best practices for nursery propagation of seedlings
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
Comparative Characterization of Gluten and Hydrolyzed Wheat Proteins
Hydrolyzed wheat proteins (HWPs) are widely used as functional ingredients in foods and cosmetics, because of their emulsifying and foaming properties. However, in individuals suffering from celiac disease or wheat allergy, HWPs may have a modified immunoreactivity compared to native gluten due to changes in molecular structures. Although a variety of HWPs are commercially available, there are no in-depth comparative studies that characterize the relative molecular mass (M) distribution, solubility, and hydrophilicity/hydrophobicity of HWPs compared to native gluten. Therefore, we aimed to fill this gap by studying the above characteristics of different commercial HWP and gluten samples. Up to 100% of the peptides/proteins in the HWP were soluble in aqueous solution, compared to about 3% in native gluten. Analysis of the Mr distribution indicated that HWPs contained high percentages of low-molecular-weight peptides/proteins and also deamidated glutamine residues. We also found considerable differences between the seven HWPs studied, so that each HWP needs to be studied in detail to help explain its potential immunoreactivit
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