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2020-09-28
|
Comparison of proton shower developments in the BGO calorimeter of the Dark Matter Particle Explorer between GEANT4 and FLUKA simulations
|
The DArk Matter Particle Explorer (DAMPE) is a satellite-borne detector for
high-energy cosmic rays and $\gamma$-rays. To fully understand the detector
performance and obtain reliable physical results, extensive simulations of the
detector are necessary. The simulations are particularly important for the data
analysis of cosmic ray nuclei, which relies closely on the hadronic and nuclear
interactions of particles in the detector material. Widely adopted simulation
softwares include the GEANT4 and FLUKA, both of which have been implemented for
the DAMPE simulation tool. Here we describe the simulation tool of DAMPE and
compare the results of proton shower properties in the calorimeter from the two
simulation softwares. Such a comparison gives an estimate of the most
significant uncertainties of our proton spectral analysis.
|
2009.13036v1
|
2021-01-19
|
Sub-damped Lyman alpha systems in the XQ-100 survey II -- Chemical evolution at 2.4<z<4.3
|
We present the measured gas-phase metal column densities in 155 sub-damped
Lyman alpha systems (subDLAs) with the aim to investigate the contribution of
subDLAs to the chemical evolution of the Universe. The sample was identified
within the absorber-blind XQ-100 quasar spectroscopic survey over the redshift
range 2.4<=z<=4.3. Using all available column densities of the ionic species
investigated (mainly CIV, SiII, MgII, SiIV, AlII, FeII, CII, and OI; in order
of decreasing detection frequency), we estimate the ionization-corrected
gas-phase metallicity of each system using Markov Chain Monte Carlo techniques
to explore a large grid of Cloudy ionization models. Without accounting for
ionization and dust depletion effects, we find that the HI-weighted gas-phase
metallicity evolution of subDLAs are consistent with damped Lyman alpha systems
(DLAs). When ionization corrections are included, subDLAs are systematically
more metal-poor than DLAs (between ~0.5 sigma and ~3 sigma significance) by up
to ~1.0 dex over the redshift range 3<=z<=4.3. The correlation of gas-phase
[Si/Fe] with metallicity in subDLAs appears to be consistent with that of DLAs,
suggesting that the two classes of absorbers have a similar relative dust
depletion pattern. As previously seen for Lyman limit systems, the gas-phase
[C/O] in subDLAs remains constantly solar for all metallicities indicating that
both subDLAs and Lyman limit systems could trace carbon-rich ejecta,
potentially in circumgalactic environments.
|
2101.07821v1
|
2021-04-24
|
Compressive oscillations in hot coronal loops: Are sloshing oscillations and standing slow waves independent?
|
Employing high-resolution EUV imaging observations from SDO/AIA, we analyse a
compressive plasma oscillation in a hot coronal loop triggered by a C-class
flare near one of its foot points as first studied by Kumar et al. We
investigate the oscillation properties in both the 131{\,}{\AA} and 94{\,}{\AA}
channels and find that what appears as a pure sloshing oscillation in the
131{\,}{\AA} channel actually transforms into a standing wave in the
94{\,}{\AA} channel at a later time. This is the first clear evidence of such
transformation confirming the results of a recent numerical study which
suggests that these two oscillations are not independent phenomena. We
introduce a new analytical expression to properly fit the sloshing phase of an
oscillation and extract the oscillation properties. For the AIA 131{\,}{\AA}
channel, the obtained oscillation period and damping time are 608$\pm$4{\,}s
and 431$\pm$20{\,}s, respectively during the sloshing phase. The corresponding
values for the AIA 94{\,}{\AA} channel are 617$\pm$3{\,}s and 828$\pm$50{\,}s.
During the standing phase that is observed only in the AIA 94{\,}{\AA} channel,
the oscillation period and damping time have increased to 791$\pm$5{\,}s and
1598$\pm$138{\,}s, respectively. The plasma temperature obtained from the DEM
analysis indicates substantial cooling of the plasma during the oscillation.
Considering this, we show that the observed oscillation properties and the
associated changes are compatible with damping due to thermal conduction. We
further demonstrate that the absence of a standing phase in the 131{\,}{\AA}
channel is a consequence of cooling plasma besides the faster decay of
oscillation in this channel.
|
2104.12038v1
|
2021-08-09
|
Synchronization of Power Systems under Stochastic Disturbances
|
The synchronization of power generators is an important condition for the
proper functioning of a power system, in which the fluctuations in frequency
and the phase angle differences between the generators are sufficiently small
when subjected to stochastic disturbances. Serious fluctuations can prompt
desynchronization, which may lead to widespread power outages. Here, we model
the stochastic disturbance by a Brownian motion process in the linearized
system of the non-linear power systems and characterize the fluctuations by the
variances of the frequency and the phase angle differences in the invariant
probability distribution. We propose a method to calculate the variances of the
frequency and the phase angle differences. For the system with uniform
disturbance-damping ratio, we derive explicit formulas for the variance
matrices of the frequency and the phase angle differences. It is shown that the
fluctuation of the frequency at a node depends on the disturbance-damping ratio
and the inertia at this node only, and the fluctuations of the phase angle
differences in the lines are independent of the inertia. In particular, the
synchronization stability is related to the cycle space of the network. We
reveal the influences of constructing new lines and increasing capacities of
lines on the fluctuations in the phase angle differences in the existing lines.
The results are illustrated for the transmission system of Shandong Province of
China. For the system with non-uniform disturbance-damping ratio, we further
obtain bounds of the variance matrices.
|
2108.04667v2
|
2021-10-12
|
Two-body collapse model for self-gravitating flow of dark matter and generalized stable clustering hypothesis for pairwise velocity
|
Analytical tools are extremely hard to find for non-linear gravitational
collpase. Only a few simple but powerful tools exist so far. Two examples are
the spherical collapse model (SCM) and stable clustering hypothesis (SCH). We
present a new analytical tool, a two-body collapse model (TBCM), that plays the
same fundamental role as harmonic oscillator in dynamics. For convenience, TBCM
is formulated for gravity with any potential exponent $n$ in a static
background with a fixed damping ($n$=-1 for Newtonian gravity). The competition
between gravity, expanding background (or damping), and angular momentum
classifies two-body collapse into: 1) free fall collapse, where free fall time
is greater if same system starts to collapse at earlier time; 2) equilibrium
collapse that persists longer in time, whose perturbative solutions lead to
power-law evolution of system energy and momentum. Two critical values
$\beta_{s1}=1$ and $\beta_{s2}=1/3\pi$ are identified that quantifies the
competition between damping and gravity. Value $\beta_{s2}$ only exists for
discrete values of potential exponent $n=(2-6m)/(1+3m)=$ -1,-10/7... for
integer $m$. Critical density ratio ($\Delta_c=18\pi^2$) is obtained for $n$=-1
that is consistent with SCM. TBCM predicts angular velocity $\propto Hr^{-3/2}$
for two-body system of size $r$. The isothermal density is a result of
extremely fast mass accretion. TBCM is able to demonstrate SCH, i.e. mean
pairwise velocity (first moment) $\langle\Delta u\rangle=-Hr$. A generalized
SCH is developed for higher order moments $\langle\Delta
u^{2m+1}\rangle=-(2m+1)\langle\Delta u^{2m}\rangle Hr$ that is validated by
N-body simulation. Energy evolution in TBCM is independent of particle mass and
energy equipartition does not apply. TBCM can be considered as a non-radial
SCM. Both models predict the same critical density ratio, while TBCM contains
much richer information.
|
2110.05784v2
|
2021-10-25
|
Capillary gravity water waves linearized at monotone shear flows: eigenvalues and inviscid damping
|
This paper is concerned with the eigenvalues and linear inviscid damping of
the 2D capillary gravity water waves of finite depth $x_2\in(-h,0)$ linearized
at a monotone shear flow $U(x_2)$. Unlike the linearized Euler equation in a
fixed channel where eigenvalues exist only in low horizontal wave number $k$,
we first prove the linearized capillary gravity wave has two branches of
eigenvalues $-ikc^\pm(k)$, where the wave speeds $c^\pm(k)=O(\sqrt{|k|})$ for
$|k|\gg1$ have the same asymptotics as the those of the linear irrotational
capillary gravity waves. Under the additional assumption of $U"\ne0$, we obtain
the complete continuation of these two branches, which are all the eigenvalues
in this (and some other) case(s). Particularly $-ikc^-(k)$ could bifurcate into
unstable eigenvalues at $c^-(k)=U(-h)$. The bifurcation of unstable eigenvalues
from inflection values of $U$ is also proved. Assuming no singular modes, i.e.
no embedded eigenvalues for any wave number $k$, linear solutions
$(v(t,x),\eta(t,x_1))$ are studieded in both periodic-in-$x_1$ and $x_1\in R$
cases, where $v$ is the velocity and $\eta$ the surface profile. Solutions can
be split into $(v^p,\eta^p)$ and $(v^c,\eta^c)$ whose $k$-th Fourier mode in
$x_1$ correspond to the eigenvalues and the continuous spectra of wave number
$k$, respectively. The component $(v^p,\eta^p)$ is governed by a (possibly
unstable) dispersion relation given by the eigenvalues, which are simply
$k\to-ikc^\pm(k)$ in the case of $x_1\in R$. The other component $(v^c,\eta^c)$
satisfies the inviscid damping as fast as
$|v_1^c|_{L_x^2},|\eta^c|_{L_x^2}=O(|t|^{-1})$ and $|v_2^c|_{L_x^2}=O(t^{-2})$
as $|t|\gg1$. Additional decay of $tv_1^c,t^2v_2^c$ in $L_x^2L_t^q$,
$q\in(2,\infty]$, is obtained after leading asymptotic terms are removed, which
are in the forms of $t$-dependent translations in $x_1$ of certain functions of
$x$.
|
2110.12604v3
|
2021-12-01
|
Damped Ly-alpha Absorbers in Star-forming Galaxies at z < 0.15 Detected with the Hubble Space Telescope and Implications for Galaxy Evolution
|
We report {\it HST} COS spectroscopy of 10 quasars with foreground
star-forming galaxies at 0.02$<$$z$$<$ 0.14 within impact parameters of
$\sim$1-7 kpc. We detect damped/sub-damped Ly$\alpha$ absorption in 100$\%$ of
cases where no higher-redshift Lyman-limit systems extinguish the flux at the
expected wavelength of Ly$\alpha$ absorption, obtaining the largest targeted
sample of DLA/sub-DLAs in low-redshift galaxies. We present absorption
measurements of neutral hydrogen and metals. Additionally, we present GBT 21-cm
emission measurements for 5 of the galaxies (including 2 detections). Combining
our sample with the literature, we construct a sample of 115 galaxies
associated with DLA/sub-DLAs spanning 0$<$$z$$<$4.4, and examine trends between
gas and stellar properties, and with redshift. The H~I column density is
anti-correlated with impact parameter and stellar mass. More massive galaxies
appear to have gas-rich regions out to larger distances. The specific SFR
(sSFR) of absorbing galaxies increases with redshift and decreases with
$M^{\ast}$, consistent with evolution of the star-formation main sequence
(SFMS). However, $\sim$20$\%$ of absorbing galaxies lie below the SFMS,
indicating that some DLA/sub-DLAs trace galaxies with longer-than-typical
gas-depletion time-scales. Most DLA/sub-DLA galaxies with 21-cm emission have
higher H I masses than typical galaxies with comparable $M^{\ast}$. High
$M_{\rm H I}/M^{\ast}$ ratios and high sSFRs in DLA/sub-DLA galaxies with
$M^{\ast}$$<$$10^{9}$$M_{\odot}$ suggest these galaxies may be gas-rich because
of recent gas accretion rather than inefficient star formation. Our study
demonstrates the power of absorption and emission studies of DLA/sub-DLA
galaxies for extending galaxy-evolution studies to previously under-explored
regimes of low $M^{\ast}$ and low SFR.
|
2112.00870v1
|
2022-01-06
|
Parameter-free quantum hydrodynamic theory for plasmonics: Electron density-dependent damping rate and diffusion coefficient
|
Plasmonics is a rapid growing field, which has enabled both fundamental
science and inventions of various quantum optoelectronic devices. An accurate
and efficient method to calculate the optical response of metallic structures
with feature size in the nanoscale plays an important role. Quantum
hydrodynamic theory (QHT) provides an efficient description of the
free-electron gas, where quantum effects of nonlocality and spill-out are taken
into account. In this work, we introduce a general QHT that includes diffusion
to account for the broadening, which is a key problem in practical applications
of surface plasmon. We will introduce a density-dependent diffusion coefficient
to give very accurate linewidth. It is a self-consistent method, in which both
the ground and excited states are solved by using the same energy functional,
with the kinetic energy described by the Thomas-Fermi and von Weizs\"{a}cker
(vW) formalisms. In addition, our QHT method is stable by introduction of an
electron density-dependent damping rate. For sodium nanosphere of various
sizes, the plasmon energy and broadening by our QHT method are in excellent
agreement with those by density functional theory and Kreibig formula. By
applying our QHT method to sodium jellium nanorods, we clearly show that our
method enables a parameter-free simulation, i.e. without resorting to any
empirical parameter, such as size-dependent damping rate and diffusing
coefficient. It is found that there exists a perfect linear relation between
the resonance wavelength and aspect radio. The width decreases with increasing
aspect ratio and height. The calculations show that our QHT method provides an
explicit and unified way to account for size-dependent frequency shifts and
broadening of arbitrarily shaped geometries. It is reliable and robust with
great predicability, and hence provides a general and efficient platform to
study plasmonics.
|
2201.03426v3
|
2022-01-12
|
Neutrino effective potential and damping in a fermion and scalar background in the resonance region
|
We consider the propagation of a neutrino or an antineutrino in a medium
composed of fermions $f$ and scalars $\phi$ interacting via a Yukawa-type
coupling of the form $\bar f\nu\phi$, for neutrino energies at which the
processes like $\nu + \phi \leftrightarrow f$ or $\nu + \bar f \leftrightarrow
\bar\phi$, and the corresponding ones for the antineutrino, are kinematically
accessible. The relevant energy values are around $|m^2_\phi - m^2_f|/2m_\phi$
or $|m^2_\phi - m^2_f|/2m_f$, where $m_\phi$ and $m_f$ are the masses of $\phi$
and $f$, respectively. We refer to either one of these regions as a resonance
energy range. Near these points, the one-loop formula for the neutrino
self-energy has a singularity. From a technical point of view, that feature is
indicative that the self-energy acquires an imaginary part, which is associated
with damping effects and cannot be neglected, while the integral formula for
the real part must be evaluated using the principal value of the integral. We
carry out the calculations explicitly for some cases that allow us to give
analytic results. Writing the dispersion relation in the form $\omega = \kappa
+ V_{\text{eff}} - i\gamma/2$, we give the explicit formula for
$V_{\text{eff}}$ and $\gamma$ for the cases considered. When the neutrino
energy is either much larger or much smaller than the resonance energy,
$V_{\text{eff}}$ reduces to the effective potential that has been already
determined in the literature in the high or low momentum regime, respectively.
The virtue of the formula we give for $V_{\text{eff}}$ is that it is valid also
in the \emph{resonance energy range}, which is outside the two limits
mentioned. As a guide to possible applications we give the relevant formulas
for $V_{\text{eff}}$ and $\gamma$, and consider the solution to the oscillation
equations including the damping term, in a simple two-generation case.
|
2201.04661v2
|
2022-01-19
|
Transverse Coronal-Loop Oscillations Induced by the Non-radial Eruption of a Magnetic Flux Rope
|
We investigate the transverse coronal-loop oscillations induced by the
eruption of a prominence-carrying flux rope on 7 December 2012. The flux rope
originating from NOAA Active Region (AR) 11621 was observed in EUV wavelengths
by the SDO/AIA and in H$\alpha$ line center by the ground-based telescope at
the BBSO. The early evolution of the flux rope is divided into two steps: a
slow rise phase at a speed of $\approx$230\,km\,s$^{-1}$ and a fast rise phase
at a speed of $\approx$706\,km\,s$^{-1}$. The eruption generates a C5.8 flare
and the onset of the fast rise is consistent with the HXR peak time of the
flare. The embedded prominence has a lower speed of $\approx$452\,km\,s$^{-1}$.
During the early eruption of the flux rope, the nearby coronal loops are
disturbed and experience independent kink-mode oscillations in the horizontal
and vertical directions. The oscillation in the horizontal direction has an
initial amplitude of $\approx$3.1\,Mm, a period of $\approx$294\,seconds, and a
damping time of $\approx$645\,seconds. It is most striking in 171\,{\AA} and
lasts for three to four cycles. The oscillations in the vertical directions are
observed mainly in 171, 193, and 211\,{\AA}. The initial amplitudes lie in the
range of 3.4\,--\,5.2\,Mm, with an average value of 4.5\,Mm. The periods are
between 407\,seconds and 441\,seconds, with an average value of 423\,seconds.
The oscillations are damping and last for nearly four cycles. The damping times
lie in the range of 570\,--\,1012\,seconds, with an average value of
741\,seconds. Assuming a semi-circular shape of the vertically oscillating
loops, we calculate the loop lengths according to their heights. Using the
observed periods, we carry out coronal seismology and estimate the internal
Alfv\'{e}n speeds (988\,--\,1145\,km\,s$^{-1}$) and the magnetic-field
strengths (12\,--\,43\,G) of the oscillating loops.
|
2201.07389v1
|
2022-03-16
|
Snowmass Whitepaper AF6: Plasma-Based Particle Sources
|
High-brightness beams generated by particle sources based on advanced
accelerator concepts have the potential to become an essential part of future
accelerator technology. High-gradient accelerators can generate and rapidly
accelerate particle beams to relativistic energies while minimizing
irreversible detrimental effects to the beam brightness that occur at low beam
energies. Due to the high accelerating gradients, these novel accelerators are
also significantly more compact than conventional technology. The beam
parameters of these particle sources are largely determined by the injection
and subsequent acceleration processes. While there has been significant
progress crucial parameters that are required for a future collider or more
near-term applications, including X-ray free-electron lasers (XFELs), such as a
sufficiently small energy spread and small emittance for bunches with a high
charge and at high pulse repetition rate. Major research and development
efforts are required to realize these approaches for a front-end injector for a
future collider in order to address these limitations. In particular, this
includes methods to control and manipulate the phase-space and spin
degrees-of-freedom of ultrashort LWFA electron bunches with high accuracy,
methods that increase the laser-to-electron beam efficiency and increased
repetition rate. This also includes the development of high-resolution
diagnostics, such as full 6D phase-space measurements, beam polarimetry and
high-fidelity simulation tools. A further increase in beam luminosity can be
achieve through emittance damping. For future colliders, the damping rings
might be replaced by a substantially more compact plasma-based approach. Here,
plasma wigglers are used to achieve similar damping performance but over a two
orders of magnitude reduced length.
|
2203.08379v2
|
2022-04-04
|
Staring at the Shadows of Archaic Galaxies: Damped Ly$α$ and Metal Absorbers toward a Young $z \sim 6$ Weak-line Quasar
|
We characterize the Ly$\alpha$ halo and absorption systems toward PSO
J083+11, a unique $z=6.3401$ weak-line quasar, using Gemini/GNIRS,
Magellan/FIRE, and VLT/MUSE data. Strong absorptions by hydrogen and several
metal lines (e.g., CII, MgII, and OI) are discovered in the spectrum, which
indicates the presence of: (i) a proximate sub-damped Ly$\alpha$ (sub-DLA)
system at $z=6.314$ and (ii) a MgII absorber at $z=2.2305$. To describe the
observed damping wing signal, we model the Ly$\alpha$ absorption with a
combination of a sub-DLA with the neutral hydrogen column density of $\log
N_\mathrm{HI} = 20.03 \pm 0.30$ cm$^{-2}$ and absorption from the intergalactic
medium with a neutral fraction of around 10 percent. The sub-DLA toward PSO
J083+11 has an abundance ratio of [C/O] $=-0.04 \pm 0.33$ and metallicity of
[O/H] $=-2.19 \pm 0.44$, similar to those of low-redshift metal-poor DLAs.
These measurements suggest that the sub-DLA might truncate PSO J083+11's
proximity zone size and complicate the quasar lifetime measurement. However,
this quasar shows no sign of a Ly$\alpha$ halo in the MUSE data cube, where the
estimated $1\sigma$ limit of surface brightness is $2.76 \times 10^{-18}$ erg
s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$ at aperture size of 1 arcsecond, or equivalent
to a Ly$\alpha$ luminosity of $\leq 43.46$ erg s$^{-1}$. This non-detection,
while being only weak independent evidence on its own, is at least consistent
with a young quasar scenario, as expected for a quasar with a short accretion
timescale.
|
2204.01245v2
|
2022-04-28
|
Viscous inertial modes on a differentially rotating sphere: Comparison with solar observations
|
In a previous paper we studied the effect of latitudinal rotation on solar
equatorial Rossby modes in the beta-plane approximation. Since then, a rich
spectrum of inertial modes has been observed on the Sun, which is not limited
to the equatorial Rossby modes and includes high-latitude modes.
Here we extend the computation of toroidal modes in 2D to spherical geometry,
using realistic solar differential rotation and including viscous damping. The
aim is to compare the computed mode spectra with the observations and to study
mode stability.
At fixed radius, we solve the eigenvalue problem numerically using a
spherical harmonics decomposition of the velocity stream function. Due to the
presence of viscous critical layers, the spectrum consists of four different
families: Rossby modes, high-latitude modes, critical-latitude modes, and
strongly damped modes. For each longitudinal wavenumber m<4, up to three
Rossby-like modes are present on the sphere, in contrast to the equatorial beta
plane where only the equatorial Rossby mode is present. The least damped modes
in the model have eigenfrequencies and eigenfunctions that resemble the
observed modes; the comparison improves when the radius is taken in the lower
half of the convection zone. For radii above 0.75R and Ekman numbers E<10^{-4},
at least one mode is unstable. For either m=1 or m=2, up to two Rossby modes
are unstable when the radial dependence of the Ekman number follows a quenched
diffusivity model (E=2. 10^{-5} at the base of the convection zone). For m=3,
up to two Rossby modes can be unstable, including the equatorial Rossby mode.
Although the 2D model discussed here is highly simplified, the spectrum of
toroidal modes appears to include many of the observed solar inertial modes.
The self-excited modes in the model have frequencies close to those of the
observed modes with the largest amplitudes.
|
2204.13412v1
|
2022-10-18
|
Magnetohydrodynamic instabilities in a self-gravitating rotating cosmic plasma
|
The generation of magnetohydrodynamic (MHD) waves and their instabilities are
studied in galactic gaseous rotating plasmas with the effects of the magnetic
field, the self gravity, the diffusion-convection of cosmic rays as well as the
gas and cosmic-ray pressures. The coupling of the Jeans, Alfv{\'e}n and
magnetosonic waves, and the conditions of damping or instability are studied in
three different cases, namely when the propagation direction is perpendicular,
parallel and oblique to the static magnetic field, and are shown to be
significantly modified by the effects of the Coriolis force due to the rotation
of cosmic fluids and the cosmic-ray diffusion. The coupled modes can be damped
or anti-damped depending on the wave number is above or below the Jeans
critical wave number that is reduced by the effects of the Coriolis force and
the cosmic-ray pressure. It is found that the deviation of the axis of rotation
from the direction of the static magnetic field gives rise to the coupling
between the Alfv{\'e}n wave and the classical Jeans mode which otherwise
results into the modified slow and fast Alfv{\'e}n waves as well as the
modified classical Jeans modes. Furthermore, due to the effects of the cosmic
rays diffusion, there appears a new wave mode (may be called the fast Jeans
mode) in the intermediate frequency regimes of the slow and fast Alfv{\'e}n
waves, which seems to be dispersionless in the long-wavelength propagation and
has a lower growth rate of instability in the high density regimes of galaxies.
The dispersion properties and the instabilities of different kinds of MHD waves
reported here can play pivotal roles in the formation of various galactic
structures at different length scales.
|
2210.09595v1
|
2023-04-17
|
Theoretical study of the Alfven Eigenmode stability in CFETR steady state discharges
|
The aim of this study is to analyze the stability of Alfven Eigenmodes (AE)
in the China Fusion Engineering Test Reactor (CFETR) plasma for steady state
operations. The analysis is done using the gyro-fluid code FAR3d including the
effect of the acoustic modes, EP Finite Larmor radius damping effects and
multiple energetic particle populations. Two high poloidal beta scenarios are
studied with respect to the location of the internal transport barrier (ITB) at
r/a = 0.45 (case A) and r/a = 0.6 (case B). Both operation scenarios show a
narrow TAE gap between the inner-middle plasma region and a wide EAE gap all
along the plasma radius. The AE stability of CFETR plasmas improves if the ITB
is located inwards, case A, showing AEs with lower growth rates with respect to
the case B. The AEs growth rate is smaller in the case A because the modes are
located in the inner-middle plasma region where the stabilizing effect of the
magnetic shear is stronger with respect to the case B. Multiple EP populations
effects (NBI driven EP + alpha articles) are negligible for the case A,
although the simulations for the case B show a stabilizing effect of the NBI EP
on the n=1 BAE caused by alpha particles during the thermalization process. If
the FLR damping effects are included in the simulations, the growth rate of the
EAE/NAE decreases up to 70 %, particularly for n > 3 toroidal families. Low n
AEs (n<6) show the largest growth rates. On the other hand, high $n$ modes (n=6
to 15) are triggered in the frequency range of the NAE, strongly damped by the
FLR effects.
|
2304.08412v1
|
2023-05-21
|
Small-amplitude Compressible Magnetohydrodynamic Turbulence Modulated by Collisionless Damping in Earth's Magnetosheath: Observation Matches Theory
|
Plasma turbulence is a ubiquitous dynamical process that transfers energy
across many spatial and temporal scales and affects energetic particle
transport. Recent advances in the understanding of compressible
magnetohydrodynamic (MHD) turbulence demonstrate the important role of damping
in shaping energy distributions on small scales, yet its observational evidence
is still lacking. This study provides the first observational evidence of
substantial collisionless damping (CD) modulation on small-amplitude
compressible MHD turbulence cascade in Earth's magnetosheath using four Cluster
spacecraft. Based on an improved compressible MHD decomposition algorithm,
turbulence is decomposed into three eigenmodes: incompressible Alfv\'en modes,
and compressible slow and fast (magnetosonic) modes. Our observations
demonstrate that CD enhances the anisotropy of compressible MHD modes because
CD has a strong dependence on wave propagation angle. The wavenumber
distributions of slow modes are mainly stretched perpendicular to the
background magnetic field ($\mathbf{B_0}$) and weakly modulated by CD. In
contrast, fast modes are subjected to a more significant CD modulation. Fast
modes exhibit a weak, scale-independent anisotropy above the CD truncation
scale. Below the CD truncation scale, the anisotropy of fast modes enhances as
wavenumbers increase. As a result, fast mode fractions in the total energy of
compressible modes decrease with the increase of perpendicular wavenumber (to
$\mathbf{B_0}$) or wave propagation angle. Our findings reveal how the
turbulence cascade is shaped by CD and its consequences to anisotropies in the
space environment.
|
2305.12507v3
|
2023-07-14
|
PIC simulations of stable surface waves on a subcritical fast magnetosonic shock front
|
We study with particle-in-cell (PIC) simulations the stability of fast
magnetosonic shocks. They expand across a collisionless plasma and an
orthogonal magnetic field that is aligned with one of the directions resolved
by the 2D simulations. The shock speed is 1.6 times the fast magnetosonic speed
when it enters a layer with a reduced density of mobile ions, which decreases
the shock speed by up to 15\% in 1D simulations. In the 2D simulations, the
density of mobile ions in the layer varies sinusoidally perpendicularly to the
shock normal. We resolve one sine period. This variation only leads to small
changes in the shock speed evidencing a restoring force that opposes a shock
deformation. As the shock propagates through the layer, the ion density becomes
increasingly spatially modulated along the shock front and the magnetic field
bulges out where the mobile ion density is lowest. The perturbed shock
eventually reaches a steady state. Once it leaves the layer, the perturbations
of the ion density and magnetic field oscillate along its front at a frequency
close to the lower-hybrid frequency; the shock is mediated by a standing wave
composed of obliquely propagating lower-hybrid waves. We perform three 2D
simulations with different box lengths along the shock front. The shock front
oscillations are aperiodically damped in the smallest box with the fastest
variation of the ion density, strongly damped in the intermediate one, and
weakly damped in the largest box. The shock front oscillations perturb the
magnetic field in a spatial interval that extends by several electron skin
depths upstream and downstream of the shock front and could give rise to
Whistler waves that propagate along the shock's magnetic field overshoot.
Similar waves were observed in hybrid and PIC simulations and by the MMS
satellite mission.
|
2307.07435v1
|
2023-08-03
|
Part I: Rebuttal to "Uniform stabilization for the Timoshenko beam by a locally distributed damping"
|
A paper, entitled "Uniform stabilization for the Timoshenko beam by a locally
distributed damping" was published in 2003, in the journal Electronic Journal
of Differential Equations. Its title concerns exclusively its Section 3,
devoted to the case of equal speeds of propagation and to its main theorem,
namely Theorem 3.1. It states that the solutions of the Timoshenko system (see
(1.3) in [1]) decays exponentially when the damping coefficient b is locally
distributed. The proof of Theorem 3.1 is crucially based on Lemma 3.6, which
states the existence of a strict Lyapunov function along which the solutions of
(1.3) decay when the speeds of propagation are equal. This rebuttal shows the
major gap and flaws in the proof of Lemma 3.6, which invalidate the proofs of
Lemma 3.6 and Theorem 3.1. Lemma 3.6 is stated at the top of page 12. The main
part of its proof is given in the pages 12 and 13. In the last eight lines of
page 13, eight inequalities are requested to hold together for the proof of
Lemma 3.6. They don't appear in the statements of Lemma 3.6. The subsequent
flaws come from the evidence that several of them are contradictory either
between them or with claims in the title of the article. We also point in this
rebuttal other flaws, or gaps in the proofs of Theorem 2.2 related to strong
stability and non uniform stability for the case of distinct speeds of
propagation. In [3], we correct and complete the proof of strong stability. We
also correct, set up the missing functional frames, fill the gaps in the proof
of non uniform stability in the cases of different speeds of propagation, and
complete a missing argument in the proof of Theorem A in [4] (see Remark 4.3),
the result of Theorem A being used in the paper [1] on which this rebuttal is
mainly devoted.
|
2308.01611v1
|
2023-09-01
|
f-mode oscillations of anisotropic neutron stars in full general relativity
|
We investigate f-mode oscillations of static anisotropic stable neutron stars
within the framework of full general relativity. We present equations governing
unperturbed stellar structures and oscillations with an ansatz to account for
the anisotropy. We solve those equations for two different equations of states.
We see that, moderately anisotropic neutron stars with the tangential pressure
larger than the radial pressure can give more massive neutron stars than the
isotropic or very anisotropic ones. We find that the frequency of the f-mode
exhibits a linear relationship with the square root of the average density of
the stars and the slope of the fit depends on the anisotropic strength. For any
given value of the anisotropic strength, the frequency increases with the
increase of the mass of the neutron star, linearly for lower masses, and
rapidly at higher masses. However, this non-linear rise in the frequency with
the mass is not prominent when the radial pressure is larger than the
tangential pressure. For a fixed value of a small mass, higher anisotropy leads
to a larger value of the frequency, but when the fixed mass is above a
threshold value, higher anisotropy leads to a smaller value of the frequency.
The nature of the variation in the frequency with the change in the anisotropic
strength is similar for the two equations of state, but for a fixed mass and
the same amount of the anisotropy, the softer equations of state gives higher
frequency. We also find that the damping time of the f-mode oscillation
decreases as the mass of the neutron star increases for all values of the
anisotropic strength. For a fixed mass of the neutron star and for the same
amount of the anisotropy, the value of the damping time is lower for the softer
equation of state, but the nature of the variation in the damping time with the
change in the anisotropic strength is similar.
|
2309.00439v2
|
2023-09-10
|
Stability and Regularity for Double Wall Carbon Nanotubes Modeled as Timoshenko Beams with Thermoelastic Effects and Intermediate Damping
|
This research studies two systems composed by the Timoshenko beam model for
double wall carbon nanotubes, coupled with the heat equation governed by
Fourier's law. For the first system, the coupling is given by the speed the
rotation of the vertical filament in the beam $\beta\psi_t$ from the first beam
of Tymoshenko and the Laplacian of temperature $\delta\theta_{xx}$, where we
also consider the damping terms fractionals
$\gamma_1(-\partial_{xx})^{\tau_1}\phi_t$, $\gamma_2(-\partial_{xx})^{\tau_2}
y_t$ and $\gamma_3(-\partial_{xx})^{\tau_3} z_t$, where $(\tau_1, \tau_2,
\tau_3) \in [0,1]^3$. For this first system we proved that the semigroup
$S_1(t)$ associated to system decays exponentially for all $(\tau_1 , \tau_2 ,
\tau_3 ) \in [0,1]^3$. The second system also has three fractional damping
$\gamma_1(-\partial_{xx})^{\beta_1}\phi_t$, $\gamma_2(-\partial_{xx})^{\beta_2}
y_t$ and $\gamma_3(-\partial_{xx})^{\beta_3} z_t$, with $(\beta_1, \beta_2,
\beta_3) \in [0,1]^3$. Furthermore, the couplings between the heat equation and
the Timoshenko beams of the double wall carbon nanotubes for the second system
is given by the Laplacian of the rotation speed of the vertical filament in the
beam $\beta\psi_{xxt}$ of the first beam of Timoshenko and the Lapacian of the
temperature $\delta\theta_{xx}$. For the second system, we prove the
exponential decay of $S_2(t)$ for $(\beta_1, \beta_2, \beta_3) \in [0,1]^3$ and
also show that $S_2(t)$ admits Gevrey classes $s>(\phi+1)/(2\phi)$ for
$\phi=\min\{\beta_1,\beta_2,\beta_3\}, \forall (\beta_1,\beta_2,\beta_3)\in
(0,1)^3$, and proving that $S_2(t)$ is analytic when the parameters $(\beta_1,
\beta_2, \beta_3) \in [1/2,1]^3$. One of the motivations for this research was
the work; Ramos et al. \cite{Ramos2023CNTs}, whose partial results are part of
our results obtained for the first system for $(\tau_1, \tau_2, \tau_3) = (0,
0, 0)$.
|
2309.04906v1
|
2023-11-03
|
Probing the disc-jet coupling in S4 0954+65, PKS 0903-57, & 4C +01.02 with $γ$-rays
|
We present a comprehensive variability study on three blazars, S4 0954+65,
PKS 0903-57, and 4C +01.02 covering a mass range of log(M/M$_{\odot}$) = 8--9,
by using $\sim$15 years-long $\gamma$-ray light curves from \textit{Fermi}-LAT.
The variability level is characterized by the fractional variability amplitude
which is higher for $\gamma$-rays compared to optical/UV and X-rays emissions.
A power spectral density (PSD) study and damped random walk (DRW) modeling are
done to probe the characteristic timescale. The PSD is fitted with a single
power-law (PL) and bending power-law models and the corresponding success
fraction was estimated. In the case of PKS 0903-57, We observed a break in the
$\gamma$-ray PSD at 256 days which is comparable to the viscous timescale in
the accretion disc suggesting a possible disk-jet coupling. The non-thermal
damping timescale from the DRW modeling is compared with the thermal damping
timescale for AGNs including our three sources. Our sources lie on the best-fit
of the $\mathrm{\tau^{rest}_{damping}} - M_{BH}$ plot derived for AGN
suggesting a possible accretion disc-jet connection. If the jet's variability
is linked to the disc's variability, we expect a log-normal flux distribution,
often connected to the accretion disc's multiplicative processes. Our study
observed a double log-normal flux distribution, possibly linked to long and
short-term variability from the accretion disk and the jet. In summary, PSD and
DRW modeling results for these three sources combined with blazars and AGNs
studied in literature favor a disc-jet coupling scenario. However, more such
studies are needed to refine this understanding.
|
2311.01738v1
|
2023-11-30
|
Compton scattering of electrons in the intergalactic medium
|
This paper investigates the distribution and implications of cosmic ray
electrons within the intergalactic medium (IGM). Utilizing a synthesis model of
the extragalactic background, we evolve the spectrum of Compton-included cosmic
rays. The energy density distribution of cosmic ray electrons peaks at redshift
$z \approx2$, and peaks in the $\sim$MeV range. The fractional contribution of
cosmic ray pressure to the general IGM pressure progressively increases toward
lower redshift. At mean density, the ratio of cosmic ray electron to thermal
pressure in the IGM $ P_{\rm CRe} / P_{\rm th}$ is 0.3% at $z=2$, rising to
1.0% at $z=1$, and 1.8% at $z=0.1$ (considering only the cosmic rays produced
locally by Compton scattering). We compute the linear Landau damping rate of
plasma oscillations in the IGM caused by the $\sim$MeV cosmic ray electrons,
and find it to be of order $\sim 10^{-6}\,\rm s^{-1}$ for wavenumbers
$1.2\lesssim ck/\omega_{\rm p}\lesssim 5$ at $z=2$ and mean density (where
$\omega_{\rm p}$ is the plasma frequency). This strongly affects the fate of
TeV $e^+e^-$ pair beams produced by blazars, which are potentially unstable to
oblique instabilities involving plasma oscillations with wavenumber
$ck/\omega_{\rm p}\approx\sec\theta$ ($\theta$ being the angle between the beam
and wave vector). Linear Landau damping is at least thousands of times faster
than either pair beam instability growth or collisional effects; it thus turns
off the pair beam instability except for modes with very small $\theta$
($ck/\omega_{\rm p}\rightarrow 1$, where linear Landau damping is kinematically
suppressed). This leaves open the question of whether the pair beam instability
is turned off entirely, or can still proceed via the small-$\theta$ modes.
|
2311.18721v2
|
2024-01-18
|
Chronicling the reionization history at $6\lesssim z \lesssim 7$ with emergent quasar damping wings
|
The spectra of high-redshift ($z\gtrsim 6$) quasars contain valuable
information on the progression of the Epoch of Reionization (EoR). At redshifts
$z<6$, the observed Lyman-series forest shows that the intergalactic medium
(IGM) is nearly ionized, while at $z>7$ the observed quasar damping wings
indicate high neutral gas fractions. However, there remains a gap in neutral
gas fraction constraints at $6\lesssim z \lesssim 7$ where the Lyman series
forest becomes saturated but damping wings have yet to fully emerge. In this
work, we use a sample of 18 quasar spectra at redshifts $6.0<z<7.1$ to close
this gap. We apply neural networks to reconstruct the quasars' continuum
emission around the partially absorbed Lyman $\alpha$ line to normalize their
spectra, and stack these continuum-normalized spectra in three redshift bins.
To increase the robustness of our results, we compare the stacks to a grid of
models from two hydrodynamical simulations, ATON and CROC, and we measure the
volume-averaged neutral gas fraction, $\bar{x}_{\rm HI}$, while jointly fitting
for the mean quasar lifetime, $t_{\rm Q}$, for each stacked spectrum. We
chronicle the evolution of neutral gas fraction using the ATON (CROC) models as
follows: $\bar{x}_{\rm HI} = 0.21_{-0.07}^{+0.17}$ ($\bar{x}_{\rm HI} =
0.10_{<10^{-4}}^{+0.73}$) at $\langle z \rangle =6.10$, $\bar{x}_{\rm HI} =
0.21_{-0.07}^{+0.33}$ ($\bar{x}_{\rm HI} =0.57_{-0.47}^{+0.26}$) at $\langle z
\rangle =6.46$, and $\bar{x}_{\rm HI} = 0.37_{-0.17}^{+0.17}$ ($\bar{x}_{\rm
HI} =0.57_{-0.21}^{+0.26}$) at $\langle z \rangle =6.87$. At the same time we
constrain the average quasar lifetime to be $t_{\rm Q} \lesssim 7\ {\rm Myr}$
across all redshift bins, in good agreement with previous studies.
|
2401.10328v1
|
2024-03-04
|
Exploring Standing and Reflected Slow-mode Waves in Flaring Coronal Loops: A Parametric Study Using 2.5D MHD Modeling
|
Recent observations of reflected propagating and standing slow-mode waves in
hot flaring coronal loops have spurred our investigation into their underlying
excitation and damping mechanisms. To understand these processes, we conduct
2.5D magnetohydrodynamic (MHD) simulations using an arcade active region model
that includes a hot and dense loop. Our simulations allow for in-depth
parametric investigations complementing and expanding our previous 3D MHD
modeling results. We excite these waves in two distinct models as motivated by
observations from the SDO/AIA. Model 1 incorporates classical compressive
viscosity coefficient, while Model 2 adopts a 10-times enhanced viscosity
coefficient. We find that: (1) Our 2.5D MHD simulations reinforce previous
conclusions derived from 1D and 3D MHD models that significantly enhanced
viscosity is crucial for the rapid excitation of standing slow waves with
damping times consistent with observations by Wang et al. (2015). (2) We
uncover that nonlinearity in Model 1 delays the conversion of a reflected
propagating wave into a standing wave. In contrast, Model 2 exhibits a much
weak influence of nonlinearity. (3) Our results reveal that the transverse
temperature structure holds more influence on wave behavior than the density
structure. In Model 1, increased loop temperature contrast significantly
enhances wave trapping within the structure, mitigating the impact of
temperature-dependent viscous damping. Conversely, in Model 2, the impact of
temperature structure on wave behavior weakens in comparison to the effect of
viscosity. (4) Model 1 displays evident nonlinear coupling to the fast and kink
magnetoacoustic waves and pronounced wave leakage into the corona. However,
analyzing three observed wave events by SDO/AIA aligns with Model 2
predictions, providing further support for the substantial viscosity increase.
|
2403.02464v1
|
2024-04-05
|
Searching for Emission Lines at $z>11$: The Role of Damped Lyman-$α$ and Hints About the Escape of Ionizing Photons
|
We describe new ultra-deep James Webb Space Telescope (JWST) NIRSpec PRISM
and grating spectra for the galaxies JADES-GS-z11-0 ($z_{\mathrm{spec}} =
11.122^{+0.005}_{-0.003}$) and JADES-GS-z13-0 ($z_{\mathrm{spec}} =
13.20^{+0.03}_{-0.04}$), the most distant spectroscopically-confirmed galaxy
discovered in the first year of JWST observations. The extraordinary depth of
these observations (75 hours and 56 hours, respectively) provides a unique
opportunity to explore the redshifts, stellar properties, UV magnitudes, and
slopes for these two sources. For JADES-GS-z11-0, we find evidence for multiple
emission lines, including [OII]3726,3729 and [NeIII]3869, resulting in a
spectroscopic redshift we determine with 94% confidence. At this spectroscopic
redshift, the Lyman-$\alpha$ break in JADES-GS-z11-0 can be fit with a damped
Lyman-$\alpha$ absorber with $\log{(N_\mathrm{HI}/\mathrm{cm}^{-2})} =
22.42^{+0.093}_{-0.120}$. We present stringent upper limits on the emission
line fluxes and line equivalent widths for JADES-GS-z13-0. These results
demonstrate how neutral hydrogen fraction and Lyman-damping wings may impact
the recovery of spectroscopic redshifts for sources like these, providing
insight into the overprediction of the photometric redshifts seen for distant
galaxies observed with JWST. In addition, we analyze updated NIRCam photometry
to calculate the morphological properties of these resolved sources, and find a
secondary source $0.3^{\prime\prime}$ south of JADES-GS-z11-0 at a similar
photometric redshift, hinting at how galaxies grow through interactions in the
early Universe.
|
2404.04325v1
|
2022-04-17
|
Sharper Bounds on Four Lattice Constants
|
The Korkine--Zolotareff (KZ) reduction, and its generalisations, are widely
used lattice reduction strategies in communications and cryptography. The KZ
constant and Schnorr's constant were defined by Schnorr in 1987. The KZ
constant can be used to quantify some useful properties of KZ reduced matrices.
Schnorr's constant can be used to characterize the output quality of his block
$2k$-reduction and is used to define his semi block $2k$-reduction, which was
also developed in 1987. Hermite's constant, which is a fundamental constant
lattices, has many applications, such as bounding the length of the shortest
nonzero lattice vector and the orthogonality defect of lattices. Rankin's
constant was introduced by Rankin in 1953 as a generalization of Hermite's
constant. It plays an important role in characterizing the output quality of
block-Rankin reduction, proposed by Gama et al. in 2006. In this paper, we
first develop a linear upper bound on Hermite's constant and then use it to
develop an upper bound on the KZ constant. These upper bounds are sharper than
those obtained recently by the authors, and the ratio of the new linear upper
bound to the nonlinear upper bound, developed by Blichfeldt in 1929, on
Hermite's constant is asymptotically 1.0047. Furthermore, we develop lower and
upper bounds on Schnorr's constant. The improvement to the lower bound over the
sharpest existing one developed by Gama et al. is around 1.7 times
asymptotically, and the improvement to the upper bound over the sharpest
existing one which was also developed by Gama et al. is around 4 times
asymptotically. Finally, we develop lower and upper bounds on Rankin's
constant. The improvements of the bounds over the sharpest existing ones, also
developed by Gama et al., are exponential in the parameter defining the
constant.
|
2204.08021v1
|
1998-09-23
|
An Unexpected Electrovac Solution with the Negative Cosmological Constant
|
An exact solution of the current-free Einstein-Maxwell equations with the
cosmological constant is presented. The solution is of Petrov type D, includes
the negative cosmological constant, and could be a ``background addition'' to
the present-day models of the universe. It has a surprising property such that
its electromagnetic field and cosmological constant are interdependent (this
constant is proportional to the energy density of this field), which may
suggest a new way of measuring the constant in question. The solution describes
a constant electromagnetic background with a preferred direction in the
universe, and defines the entire lifetime of the universe as a simple function
of the negative cosmological constant. According to our solution the absolute
value of this constant should be considerably lower than that recently
estimated, when astrophysical data are taken into account. Our solution is a
special case of that published by Bertotti in 1959. His solution (in terms of
which the cosmological constant and the background electromagnetic field are
independent) and its two other special cases, i.e. the conformally flat
Robinson solution (1959) and the one which is the counterpart of our solution
with the positive cosmological constant, are briefly discussed.
|
9809066v2
|
2006-05-24
|
New Universal Flavor-Electroweak Physical Constant and Neutrino Type
|
By analogy with the solution of the classical physics problems achieved not
by an extension of the established dynamic and symmetry knowledge but by the
emergence of a fundamentally new empirical physical constant h leading to
quantum mechanics, I address in this paper the known basic problems of lepton
mass ratios, neutrino type and electroweak charges in terms of an emerging new
dimensionless flavor-electroweak physical constant of a new sort. A special
value of that new universal constant is suggested av0 = exp(-5). Like the Plank
constant h, initially closely related to suggested discrete radiation energy,
the new constant a_o is related here to the problem of discrete electric
charge. With known data, it is observed that the constant a_o determines the
mass ratios of charged leptons, the mass ratios and absolute mass scale and
oscillation mass squared differences of quasidegenerate neutrinos on the one
hand, and the low energy fine structure constant and the second electroweak
constant on the other hand. I gathered, organized and commented here an
interesting system of primary observations made on particle mass and
electroweak experimental data. As a result, the new physical constant a_o
describes a fundamental aspect of low energy phenomenology uniting the
electroweak theory with the necessary idea of anthropic selection of the free
interaction constant values and particle flavor freedom mass values.
|
0605267v6
|
2023-11-10
|
Robust Constant-Time Cryptography
|
The constant-time property is considered the security standard for
cryptographic code. Code following the constant-time discipline is free from
secret-dependent branches and memory accesses, and thus avoids leaking secrets
through cache and timing side-channels. The constant-time property makes a
number of implicit assumptions that are fundamentally at odds with the reality
of cryptographic code. Constant-time is not robust. The first issue with
constant-time is that it is a whole-program property: It relies on the entirety
of the code base being constant-time. But, cryptographic developers do not
generally write whole programs; rather, they provide libraries and specific
algorithms for other application developers to use. As such, developers of
security libraries must maintain their security guarantees even when their code
is operating within (potentially untrusted) application contexts. Constant-time
requires memory safety. The whole-program nature of constant-time also leads to
a second issue: constant-time requires memory safety of all the running code.
Any memory safety bugs, whether in the library or the application, will wend
their way back to side-channel leaks of secrets if not direct disclosure. And
although cryptographic libraries should (and are) written to be memory-safe, it
is unfortunately unrealistic to expect the same from every application that
uses each library. We formalize robust constant-time and build a RobustIsoCrypt
compiler that transforms the library code and protects the secrets even when
they are linked with untrusted code. Our evaluation with SUPERCOP benchmarking
framework shows that the performance overhead is less than five percent on
average.
|
2311.05831v1
|
2023-11-24
|
Constant-Time Wasmtime, for Real This Time: End-to-End Verified Zero-Overhead Constant-Time Programming for the Web and Beyond
|
We claim that existing techniques and tools for generating and verifying
constant-time code are incomplete, since they rely on assumptions that compiler
optimization passes do not break constant-timeness or that certain operations
execute in constant time on the hardware. We present the first end-to-end
constant-time-aware compilation process that preserves constant-time semantics
at every step from a high-level language down to microarchitectural guarantees,
provided by the forthcoming ARM PSTATE.DIT feature. First, we present a new
compiler-verifier suite based on the JIT-style runtime Wasmtime, modified to
compile ct-wasm, a preexisting type-safe constant-time extension of
WebAssembly, into ARM machine code while maintaining the constant-time property
throughout all optimization passes. The resulting machine code is then fed into
an automated verifier that requires no human intervention and uses static
dataflow analysis in Ghidra to check the constant-timeness of the output. Our
verifier leverages characteristics unique to ct-wasm-generated code in order to
speed up verification while preserving both soundness and wide applicability.
We also consider the resistance of our compilation and verification against
speculative timing leakages such as Spectre. Finally, in order to expose
ct-Wasmtime at a high level, we present a port of FaCT, a preexisting
constant-time-aware DSL, to target ct-wasm.
|
2311.14246v1
|
1994-11-01
|
Symplectic Computation of Lyapunov Exponents
|
A recently developed method for the calculation of Lyapunov exponents of
dynamical systems is described. The method is applicable whenever the
linearized dynamics is Hamiltonian. By utilizing the exponential representation
of symplectic matrices, this approach avoids the renormalization and
reorthogonalization procedures necessary in usual techniques. It is also easily
extendible to damped systems. The method is illustrated by considering two
examples of physical interest: a model system that describes the beam halo in
charged particle beams and the driven van der Pol oscillator.
|
9411001v1
|
1993-10-28
|
Modes of Elliptical Galaxies
|
Long lived modes of elliptical galaxies can exist {\it \`a la} van Kampen.
Specific systems may possess long lived oscillations which Landau damp on time
scales longer than a Hubble time. Some physical processes such as a close
encounter, tidal forces from a cluster or an orbiting satellite could
preferentially excite a coherent mode. These may relate to the observed faint
structure in elliptical galaxies such as shells and ripples. Their detection in
projected phase space would ultimately provide a detailed probe of the
underlying potential. I give an overview of linear perturbations to stationary
solutions of the Vlasov equation, including a discretized Hermite polynomial
expansion which explicitly demonstrates completeness and orthogonality of
solutions. Some exact solutions are shown, which implies the feasibility of
such a procedure and suggest future fully numerical studies.
|
9310050v1
|
1994-12-19
|
Massive Neutrinos and Galaxy Formation
|
We report the most recent results from high-resolution numerical simulations
of structure formation in two flat cold+hot dark matter models with neutrino
mass densities $\onu=0.2$ and 0.3. We find that structure forms too late in all
CDM+HDM models with $\onu>0.2$ to account for the amount of dense neutral gas
in high-redshift damped Lyman-$\alpha$ systems. The $\onu=0.2$ model at
$z\approx0$ provides a better match to observations than the pure CDM model.
|
9412068v1
|
1995-01-30
|
Element Abundances at High Redshifts: The N/O Ratio at Low Metallicity
|
Our knowledge of galactic chemical evolution is currently limited to
observations of Milky Way stars and H II regions of nearby galaxies. Damped
Lyman~$\alpha$ systems offer a new approach for tracking the evolution of
normal galaxies from early epochs to the present day. Here we report the first
measurements of nitrogen abundances in galaxies with less than 1/100 of solar
metallicity, a range unexplored by previous observations.
|
9501105v1
|
1996-06-21
|
Measuring the Curvature of the Universe
|
We discuss how the curvature of the universe can be robustly measured
employing only the gross features of the CMB anisotropy spectrum. Though the
position of the first peak is not robust, uncertainties in the model for
structure formation can be removed by using the spacing of the acoustic peaks
and the location of the damping tail. Combined these provide important
consistency tests that can be used to discriminate against a truly exotic
model.
|
9606140v1
|
1996-10-24
|
Small-angle anisotropies in the CMBR from active sources
|
We consider the effects of photon diffusion on the small-angle microwave
background anisotropies due to active source models. We find that fluctuations
created just before the time of last scattering allow anisotropy to be created
on scales much smaller than allowed by standard Silk damping. Using simple
models for string and texture structure functions as examples, we illustrate
the differences in the angular power spectrum at scales of order a few
arcminutes. In particular, we find that the Doppler peak heights are modified
by 10-50% and the small-angle fall-off is power law rather than exponential.
|
9610197v1
|
1996-11-06
|
Interactions Between Massive Dark Halos And Warped Disks
|
The normal mode theory for warping of galaxy disks, in which disks are
assumed to be tilted with respect to the equator of a massive, flattened dark
halo, assumes a rigid, fixed halo. However, consideration of the back-reaction
by a misaligned disk on a massive particle halo shows there to be strong
coupling leading to efficient damping (or in some circumstances excitation) of
the misalignment, and hence the warp. We therefore discuss possible alternative
explanations of the warp phenomenon, with emphasis on the effect of a
responsive, gravitationally live massive galactic halo.
|
9611050v1
|
1996-11-19
|
Reheating and causal thermodynamics
|
The reheating process in inflationary universe models is considered as an
out-of-equilibrium mixture of two interacting and reacting fluids, and studied
within the framework of causal, irreversible thermodynamics. The evolution of
the temperature and the decay rate as determined by causal thermodynamics are
estimated at different stages of the process. A simple model is also used to
find the perturbations of the expansion rate, including the possibility of
damped oscillations.
|
9611147v1
|
1996-12-04
|
UV HST Spectroscopy of Star-Forming Galaxies
|
HST spectroscopical observations of 8 HII galaxies are reported. Ly alpha
emission was detected in 4 of them. We find that the velocity structure of the
gas is the main determining factor for the escape of the Ly alpha photons, and
not the abundance of dust. The rest of the sample shows broad damped Ly alpha
absorption attributed to large HI column densities, that is static with respect
to the emitted Ly alpha photons, emerging from the HII regions. The
star-forming galaxies IZW18 and even more SBS0335-052 may have extremely metal
deficient HI clouds, the latter with [O/H] as low as -7.2.
|
9612043v1
|
1996-12-05
|
Modified Artificial Viscosity in Smooth Particle Hydrodynamics
|
Artificial viscosity is needed in Smooth Particle Hydrodynamics to prevent
interparticle penetration, to allow shocks to form and to damp post shock
oscillations. Artificial viscosity may, however, lead to problems such as
unwanted heating and unphysical solutions. A modification of the standard
artificial viscosity recipe is proposed which reduces these problems. Some test
cases discussed.
|
9612050v2
|
1997-02-16
|
Barnett Relaxation in Thermally-Rotating Grains
|
We present an exact formulation of the physics of Barnett relaxation. Our
formulation is based on a realistic kinetic model of the relaxation mechanism
which includes the alignment of the grain angular momentum in body coordinates
by Barnett dissipation, disalignment by thermal fluctuations, and coupling of
the angular momentum to the gas via gas damping. We solve the Fokker-Planck
equation for the measure of internal alignment using numerical integration of
the equivalent Langevin equation for Brownian rotation. The accuracy of our
results is calibrated by comparing our numerical solutions with exact analytic
results obtained for special cases.We describe an analytic approximation for
the measure of alignment which fits our numerical results for cases of
practical interest.
|
9702138v1
|
1997-04-16
|
Gravitational Waves from Phase Transition of Accreting Neutron Stars
|
We propose that when neutron stars in low-mass X-ray binaries accrete
sufficient mass and become millisecond pulsars, the interiors of these stars
may undergo phase transitions, which excite stellar radial oscillations. We
show that the radial oscillations will be mainly damped by gravitational-wave
radiation instead of internal viscosity. The gravitational waves can be
detected by the advanced Laser Interferometer Gravitational-Wave Observatory at
a rate of about three events per year.
|
9704161v1
|
1997-07-08
|
Collisionless Relaxation in Galactic Dynamics and the Evolution of Long Range Order
|
This talk provides a critical assessment of collisionless galactic dynamics,
focusing on the interpretation and limitations of the collisionless Boltzmann
equation and the physical mechanisms associated with collisionless relaxation.
Numerical and theoretical arguments are presented to motivate the idea that the
evolution of a system far from equilibrium should be interpreted as involving
nonlinear gravitational Landau damping, which implies a greater overall
coherence and remembrance of initial conditions than is implicit in the
conventional theory of violent relaxation.
|
9707103v1
|
1997-10-31
|
On Breaking Cosmic Degeneracy
|
It has been argued that the power spectrum of the anisotropies in the Cosmic
Microwave Background (CMB) may be effectively degenerate, namely that the
observable spectrum does not determine a unique set of cosmological parameters.
We describe the physical origin of this degeneracy and show that at small
angular scales it is broken by gravitational lensing: effectively degenerate
spectra become distinguishable at l ~ 3000 because lensing causes their damping
tails to fall at different rates with increasing l. This effect also helps in
distinguishing nearly degenerate power spectra such as those of mixed dark
matter models. Forthcoming interferometer experiments should provide the means
of measuringotherwise degenerate parameters at the 5-25% level.
|
9710364v1
|
1998-01-05
|
Model Predictions for Clustering and Morphologies at HDF depths
|
The current status of numerical simulations of the formation of galaxies is
reviewed. Success and failure of modeling galaxies at low and high redshift is
demonstrated using a variety of examples, such as the Tully-Fisher relation,
the appearance of high-redshift galaxies and the kinematics of damped Lya
systems. The relationship between the clustering properties of high-z galaxies
and the present generation of galaxies is emphasized.
|
9801025v1
|
1998-05-26
|
Do Soft Gamma Repeaters Emit Gravitational Waves?
|
Soft gamma repeaters are identified as highly magnetized (B$\approx 10^{14}$
Gauss) neutron stars. Magnetic stresses induce tectonic activity, and field
annihilation in faults is the ultimate energy source for the observed
$\gamma$-ray emission. As a consequence of the crustal cracking, the stored
elastic energy is converted into high frequency (kHz) shear waves, that excite
nonradial oscillation modes damped by gravitational wave emission. This class
of objects should certainly be considered as potential sources of gravitational
waves that could be detected by the present planned interferometric antennas
like VIRGO or LIGO.
|
9805321v1
|
1998-06-29
|
The impact on cosmology of a primordial scaling field
|
A scalar field with an exponential potential has the particular property that
it is attracted into a solution in which its energy scales as the dominant
component (radiation or matter) of the Universe, contributing a fixed fraction
of the total energy density. We briefly discuss the dynamics of such a scalar
field and its impact on Big Bang nucleosynthesis, the growth of large scale
structure and abundance of damped Lyman$-\alpha$ systems at high redshift.
Given the simplicity of the model, its theoretical motivation, and its success
in matching observations, we argue that it should be taken on par with other
currently viable models of structure formation.
|
9806373v1
|
2000-01-05
|
Cold gas, the HI 21cm line and evolving galactic potentials
|
Neutral hydrogen traces gravitational potentials. In the nearby universe,
21cm emission-line surveys show that the bulk of the HI resides in well-formed,
optically-luminous galaxies. At high redshift, 21cm line absorption against
background radio quasars occurs in gas-rich systems identified with the highest
HI column densities -- the ``damped Lyman alpha'' quasar absorption-line
systems. High spatial-resolution observations of the redshifted 21cm line
absorbers measure sizes and kinematics of the neutral absorbers.
|
0001070v1
|
2000-01-19
|
Stochastic Acceleration and Non-Thermal Radiation in Clusters of Galaxies
|
We calculate the distribution of electrons in clusters of galaxies, resulting
from thermalization processes in the presence of stochastic acceleration due to
plasma waves. We show that the electron distribution can deviate from a
Maxwell-Boltzmann distribution, due to the effect of the stochastic energy
gain, provided waves can be sustained against damping. The non-thermal tail of
electrons can generate as bremsstrahlung emission a flux of hard X-rays
compatible with the ones recently detected in some clusters of galaxies.
|
0001344v1
|
2000-02-29
|
Models of Wave Supported Clumps in Giant Molecular Clouds
|
We present plane-parallel equilibrium models of molecular clumps that are
supported by Alfven waves damped by the linear process of ion-neutral friction.
We used a WKB approximation to treat the inward propagation of waves and
adopted a realistic ionization structure influenced by dissociation and
ionization due to photons of external origin. The model clumps are larger and
less centrally condensed than those obtained for an assumed ionization
structure, used in some previous studies, that is more appropriate for dark
regions.
|
0002531v1
|
2000-03-20
|
Old models for Cygnus X-1 and AGN
|
Recently, there appeared many papers devoted to the modeling of X-ray
properties of Cygnus X-1 and other black hole accretion disk candidates: e.g.
J.Poutanen, J.Krolik, F.Ryde, MNRAS 292 (1997) L21; E.Agol, J.Krolik, ApJ 507
(1998) 304; A.Beloborodov, in ``High Energy Processes in Accreting Black
Holes'', ASP Conf. Series, 161, (1999), p.295. The goal of this electronic
publication is to draw attention to our old papers where many ideas of recent
discussions were anticipated (hot coronae, Comptonization, photon damping of
waves, particle acceleration and matter ejection from accretion disks with
large scale poloidal magnetic fields, etc.)
|
0003275v1
|
2000-05-08
|
X-ray Fluctuations from the Slim Disk
|
The responses of perturbations added into the optically thick,
advection-dominated accretion disk (ADAD), what we call the slim disk (SD), are
investigated through numerical simulations. Although it is proposed that the SD
is thermally stable, I find that a perturbation added into the disk is not
rapidly damped and moves through the disk in its free-fall time. After the
perturbation moves, the global structure of the disk does not vary very much.
These facts may account for the substantial variability of the X-ray
luminosities of stellar super-luminal jet sources (SLJSs) and Narrow-Line
Seyfert 1s (NLS1s).
|
0005162v1
|
2000-10-18
|
Disks at High Redshift: Interactions, Mergers, and Starbursts
|
Do disk galaxies exist at redshifts much greater than unity, and how might
they look different from local disks? How does the morphological mix of
galaxies change with redshift? What can we learn from current observations
about the properties of high redshift galactic disks? I present theoretical
predictions based on semi-analytic hierarchical models, focussing on the role
played by interactions, mergers, and starbursts in determining the observable
properties of disk galaxies at high redshift, and discuss the interpretation of
high redshift observations of possible proto-disks (damped Lyman-$\alpha$
systems and Lyman-break galaxies) in light of these predictions.
|
0010350v1
|
2001-01-10
|
Fast Zonal Field Dynamo in Collisionless Kinetic Alfven Wave Turbulence
|
The possibility of fast dynamo action by collisionless kinetic Alfven Wave
turbulence is demonstrated. The irreversibility necessary to lock in the
generated field is provided by electron Landau damping, so the induced electric
field does not vanish with resistivity. Mechanisms for self-regulation of the
system and the relation of these results to the theory of alpha quenching are
discussed. The dynamo-generated fields have symmetry like to that of zonal
flows, and thus are termed zonal fields.
|
0101159v1
|
2001-09-27
|
The theory of CMB anisotropies
|
This is a review of the theory of CMB anisotropies, an updated version of a
course given at the troisieme cycle de la Suisse Romande. An introduction to
gauge invariant cosmological perturbation theory is given and the theory CMB
anisotropies is develiped in this context. Simple analytical approximations for
the acoustic peak positions for adiabatic and isocurvature perturbations are
derived. Silk damping is discussed by an analytic approximation. A short
description of the present status of observations and parameter estimation
followed by a critical discussion terminate the review. The full system of
differential equations for CMB anisotropies and polarization needed in a
numerical treatment is also developed and given in an appendix.
|
0109522v1
|
2001-11-30
|
Chemical Abundances in High-Redshift Neutral Clouds
|
Neutral hydrogen clouds with high column density detected towards distant
quasars are unique probes of elemental nucleosynthesis and chemical evolution
in the low metallicity regime. They provide measurements for several elements
at very early times which are unfeasible in other astrophysical environments.
Comparison between refractory and non-refractory elements provides evidence for
the presence of dust, and the recently measured Ar probes photoionization. A
prominent characteristic is the dominance of a solar abundance pattern, which
is somewhat unexpected at low metallicities. It is argued that this property
and Nitrogen observations can be used to constrain the age of the Damped
Ly_alpha systems and the epoch of star formation.
|
0111592v1
|
2001-12-21
|
Quasar Absorption Lines
|
The absorption lines observed in quasar spectra have given us a detailed
picture of the intergalactic medium and the metal abundance and kinematics of
high redshift galaxies. In this review, we present an introduction to the
field, starting with the techniques used for interpreting absorption line
spectra. We then survey the observational and theoretical development of our
understanding of the Lyman-alpha forest, the metal absorbers, and the damped
Ly-alpha absorbers. We conclude with a discussion of some of the remaining
outstanding issues, and prospects for the future.
|
0112521v1
|
2002-01-25
|
Numerical studies of galaxy formation using special purpose hardware
|
I review recent progress in numerically simulating the formation and
evolution of galaxies in hierarchically clustering universes. Special emphasis
is given to results based on high-resolution gas dynamical simulations using
the N-body hardware integrator GRAPE. Applications address the origin of the
spin of disk galaxies, the structure and kinematics of damped Lyman-alpha
systems, and the origin of galaxy morphology and of galaxy scaling laws.
|
0201427v1
|
2002-09-10
|
Extrasolar Planets and Mean-Motion Resonances
|
The 2:1 orbital resonances of the GJ 876 system can be easily established by
the differential planet migration due to planet-nebula interaction. Significant
eccentricity damping is required to produce the observed orbital
eccentricities. The geometry of the GJ 876 resonance configuration differs from
that of the Io-Europa pair, and this difference is due to the magnitudes of the
eccentricities involved. We show that a large variation in the configuration of
2:1 and 3:1 resonances and, in particular, asymmetric librations can be
expected among future discoveries.
|
0209176v1
|
2002-09-17
|
Discrimination of Quark Stars from Neutron Stars in Quadrupole Oscillations
|
The frequencies and damping times due to gravitational radiation are
calculated for self-bound quark star models. The results are compared with
those for neutron star models. They are markedly contrasted in less
relativistic cases. The distinction derived here from a simple model of quark
stars may be relevant to the future theoretical and observational studies,
since the oscillation properties essentially depend on the mass and radius of
an equilibrium star.
|
0209320v1
|
2002-09-30
|
Perspectives in Galactic Chemical Evolution studies
|
In this review I focus on a few selected topics, where recent theoretical
and/or observational progress has been made and important developments are
expected in the future. They include: 1) Evolution of isotopic ratios, 2)
Mixing processes and dispersion in abundance ratios, 3) Abundance gradients in
the Galactic disk (and abundance patterns in the inner Galaxy), 4) The question
of primary Nitrogen and 5) Abundance patterns in extragalactic damped
Lyman-alpha systems (DLAs).
|
0209620v1
|
2003-04-10
|
Challenges in generating density perturbations from a fluctuating inflaton coupling
|
We discuss the possibility of generating adiabatic density perturbations from
spatial fluctuations in the inflaton decay rate which are due to quantum
fluctuations of light moduli fields coupling to the inflaton. We point out that
non-renormalizable operators, which lift the flatness of the moduli potential,
play an important role for the density perturbations. In particular, the
non-renormalizable terms give rise to a considerable damping of the
fluctuations and thereby pose an obstruction to the construction of possible
models.
|
0304187v1
|
2003-05-06
|
Depletion of small dust grains in the early stages of molecule formation
|
ISOCAM allowed the observation of diffuse interstellar regions where the
transition from atomic to molecular gas takes place. In this contribution we
report on spectacular variations of small dust grains abundance in such regions
observed with ISOCAM. In the four cirrus-like clouds observed, we note a
systematic disappearance of small dust grains in regions where CO emission
starts to appear. We suggest that the variations of the small dust grain
abundance observed here are related to the damping of the gas turbulent motions
that favors the coagulation of dust through grain-grain collisions.
|
0305084v1
|
2003-05-07
|
Constraining the curvaton scenario
|
We analyse the curvaton scenario in the context of supersymmetry.
Supersymmetric theories contain many scalars, and therefore many curvaton
candidates. To obtain a scale invariant perturbation spectrum, the curvaton
mass should be small during inflation $m \ll H$. This can be achieved by
invoking symmetries, which suppress the soft masses and non-renormalizable
terms in the potential. Other model-independent constraints on the curvaton
model come from nucleosynthesis, gravitino overproduction, and thermal damping.
The curvaton can work for masses $m \gtrsim 10^4 \GeV$, and very small
couplings (e.g. $h \lesssim 10^{-6}$ for $m \lesssim 10^8 \GeV$).
|
0305101v2
|
2003-05-18
|
Bulk viscosity of strange quark matter in density-dependent quark mass model and dissipation of r-modes in strange stars
|
We study the bulk viscosity of strange quark matter(SQM) in density dependent
quark mass model(DDQM) under the background of self-consistent thermodynamics.
The correct formulae, with which the viscosity can be evaluated, are derived.
We also find that the viscosity in DDQM can be higher by 2 to 3 orders of
magnitude than MIT bag model. We calculate the damping time scales due to
viscosity coupled to r-modes. The numerical results show the time scale can't
be shorter than $10^{-1}$s.
|
0305320v1
|
2004-11-02
|
Spurious contribution to CR scattering calculations
|
The quasilinear theory for cosmic ray propagation is a well known and widely
accepted theory. In this paper, we discuss the different contributions to the
pitch-angle Fokker-Planck coefficient from large and small scales for slab
geometry using the damping model of dynamical turbulence. These examinations
will give us a hint on the limitation range where quasilinear approximation is
a good approximation.
|
0411074v1
|
2005-06-07
|
Resonant absorption in dissipative flux tubes
|
We study the resonant absorption of MHD waves in magnetized flux tubes with a
radial density inhomogeneity. Within the approximation that resistive and
viscous processes are operative in thin layers surrounding the singularities of
the MHD equations, we give the full spectrum of the eigenfrequencies and
damping rates of the MHD quasi modes of the tube. Both surface and body modes
are analyzed. \keywords{Sun -- corona: magnetohydrodynamics (MHD) -- Sun:
magnetic fields -- Sun: oscillations
|
0506141v3
|
2005-06-28
|
Probing the interior of neutron stars with gravitational waves
|
We show here how the internal structure of a neutron star can be inferred
from its gravitational wave spectrum. Under the premise that the frequencies
and damping rates of a few $w$-mode oscillations are found, we apply an
inversion scheme to determine its mass, radius and density distribution. In
addition, accurate equation of state of nuclear matter can also be determined.
|
0506681v1
|
2005-07-30
|
The Largest Scale We Can Detect in the Universe and the Inflation
|
From the damping of the Cosmic Microwave Background Radiation
(CMB) anisotropy power spectrum at large scale and the recent accelerating
expansion of the Universe, we find that, there may be a largest scale which we
can detect in the Universe. From this, we can get the inflation parameters as
spectrum index $n_s$, e-fold
$N$, Hubble parameter $H$, the ratio of tensor and scalar $r$, the lasting
time of reheating time $\alpha$ for special inflation models. We do them in
three inflation models, and find that all the results fit very well with the
observations and the inflation theory.
|
0508008v3
|
2005-12-15
|
Galaxy Formation
|
I summarize current knowledge of galaxy formation with emphasis on the
initial conditions provided by the Lambda CDM cosmology, integral constraints
from cosmological quantities, and the demographics of high-redshift
protogalaxies. Tables are provided summarizing the number density, star
formation rate and stellar mass per object, cosmic star formation rate and
stellar mass densities, clustering length and typical dark matter halo masses
for Lyman break galaxies, Lyman alpha emitting galaxies, Distant red galaxies,
Sub-millimeter galaxies, and Damped Lyman alpha absorption systems. I also
discuss five key unsolved problems in galaxy formation and prognosticate
advances that the near future will bring.
|
0512384v1
|
2006-01-16
|
Bistability in Interstellar Gas-Phase Chemistry
|
We present an analysis of "bistability" in gas-phase chemical models of dark
interstellar clouds. We identify the chemical mechanisms that allow high- and
low-ionization solutions to the chemical rate-equations to coexist. We derive
simple analytic scaling relations for the gas densities and ionization rates
for which the chemistry becomes bistable. We explain why bistability is
sensitive to the H3+ dissociative recombination rate coefficient, and why it is
damped by gas-grain neutralization.
|
0601323v1
|
2006-01-25
|
Primordial magnetic fields and CMB anisotropies
|
Possible signatures of primordial magnetic fields on the Cosmic Microwave
Background (CMB) temperature and polarization anisotropies are reviewed. The
signals that could be searched for include excess temperature anisotropies
particularly at small angular scales below the Silk damping scale, B-mode
polarization, and non-Gaussian statistics. A field at a few nG level produces
temperature anisotropies at the 5 micro Kelvin level, and B-mode polarization
anisotropies 10 times smaller, and is therefore potentially detectable via the
CMB anisotropies. An even smaller field, with B_0 < 0.1 nG, could lead to
structure formation at high redshift z > 15, and hence naturally explain an
early re-ionization of the Universe.
|
0601570v1
|
2006-09-01
|
Implication of Existence of Hybrid stars and Theoretical Expectation of Submillisecond Pulsars
|
We derive the bulk viscous damping timescale of hybrid stars, neutron stars
with quark matter core. The r-mode instability windows of the stars show that
the theoretical results are consistent with the rapid rotation pulsar data,
which may give an indication for the existence of quark matter in the interior
of neutron stars. Hybrid stars instead of neutron or strange stars may lead to
submillisecond pulsars.
|
0609011v2
|
2006-11-07
|
Viscosity in X-ray clusters: Braginskii over 5
|
We argue that it is currently impossible to simulate X-ray clusters using
correct equations, because even the MHD description is not applicable. But
since fluid simulations actually reproduce observations quite well, one may try
to improve the fluid codes by including molecular transport of heat and
momentum. We calculate the effective molecular viscosity for the simplest model
of magnetic field and obtain 1/5 of the Braginskii value, similar to 1/3 of
Spitzer for the heat conduction. This is large enough to noticeably damp the
X-ray cluster turbulence.
|
0611243v1
|
2006-11-29
|
Cosmological constraints on Neutrino - Dark Matter interactions
|
I summarize the results of a recent analysis where the cosmological effects
of interactions of neutrinos with cold Dark Matter (DM) is investigated. This
interaction produces diffusion-damped oscillations in the matter power
spectrum, analogous to the acoustic oscillations in the baryon-photon fluid. I
discuss the bounds from the Sloan Digital Sky Survey on the corresponding
opacity defined as the ratio of neutrino-DM scattering cross section over DM
mass, and compare with the constraint from observation of neutrinos from
supernova 1987A.
|
0611887v1
|
2006-12-20
|
Oscillations and Waves in coronal loops
|
In the past few years observations by high-resolution space imaging
telescopes and spectrometers have confirmed that a great variety of MHD waves
are supported in the solar corona of a low-beta plasma and fine structure. MHD
waves are an important diagnostic tool for the determination of the physical
parameters of coronal loops, dubbed {\em coronal seismology}. In this paper, I
will review recent results of both propagating and standing waves observed with
SOHO and TRACE, and discuss the wave damping and excitation mechanisms as well
as some applications of coronal seismology based on recent numerical
simulations and theories in relation to the observations.
|
0612605v1
|
1994-06-26
|
Symplectic Calculation of Lyapunov Exponents
|
The Lyapunov exponents of a chaotic system quantify the exponential
divergence of initially nearby trajectories. For Hamiltonian systems the
exponents are related to the eigenvalues of a symplectic matrix. We make use of
this fact to develop a new method for the calculation of Lyapunov exponents of
such systems. Our approach avoids the renormalization and reorthogonalization
of usual techniques. It is also easily extendible to damped systems. We apply
our method to two examples of physical interest: a model system that describes
the beam halo in charged particle beams and the driven van der Pol oscillator.
|
9406010v1
|
1994-12-17
|
When are vector fields Hamiltonian?
|
Dynamical systems can be quantised only if they are Hamiltonian. This prompts
the question from which our talk gets its title. We show how the simple
predator-prey equation and the damped harmonic oscillator can be considered to
be Hamiltonian with respect to an infinite number of non-standard Poisson
brackets. This raises some interesting questions about the nature of
quantisation. Questions which are valid even for flows which possess a
canonical structure.
|
9412010v1
|
1996-07-14
|
Statistical Description of Acoustic Turbulence
|
We develop expressions for the nonlinear wave damping and frequency
correction of a field of random, spatially homogeneous, acoustic waves. The
implications for the nature of the equilibrium spectral energy distribution are
discussed
|
9607007v1
|
1996-12-09
|
Chaotic hysteresis in an adiabatically oscillating double well
|
We consider the motion of a damped particle in a potential oscillating slowly
between a simple and a double well. The system displays hysteresis effects
which can be of periodic or chaotic type. We explain this behaviour by
computing an analytic expression of a Poincar'e map.
|
9612015v1
|
1998-04-23
|
Adiabatic geometric phases and response functions
|
Treating a many-body Fermi system in terms of a single particle in a
deforming mean field. We relate adiabatic geometric phase to susceptibility for
the noncyclic case, and to its derivative for the cyclic case. Employing the
semiclassical expression of susceptibility, the expression for geometric phase
for chaotic quantum system immediately follows. Exploiting the well-known
association of the absorptive part of susceptibility with dissipation, our
relations may provide a quantum mechanical origin of the damping of collective
excitations in Fermi systems.
|
9804037v1
|
1999-05-14
|
Noise-induced flow in quasigeostrophic turbulence with bottom friction
|
Randomly-forced fluid flow in the presence of scale-unselective dissipation
develops mean currents following topographic contours. Known mechanisms based
on the scale-selective action of damping processes are not at work in this
situation. Coarse-graining reveals that the phenomenon is a kind of
noise-rectification mechanism, in which lack of detailed balance and the
symmetry-breaking provided by topography play an important role.
|
9905022v1
|
1999-07-30
|
Free Decay of Turbulence and Breakdown of Self-Similarity
|
It has been generally assumed, since the work of von Karman and Howarth in
1938, that free decay of fully-developed turbulence is self-similar. We present
here a simple phenomenological model of the decay of 3D incompressible
turbulence, which predicts breakdown of self-similarity for low-wavenumber
spectral exponents $n$ in the range $n_c<n<4$, where $n_c$ is some threshold
wavenumber. Calculations with the eddy-damped quasi-normal Markovian
approximation give the value as $n_c\approx 3.45$. The energy spectrum for this
range of exponents develops two length-scales, separating three distinct
wavenumber ranges.
|
9908006v1
|
1992-12-25
|
Spatial Correlation of Conduction Electrons in Metal with Complicated Geometry Of The Fermi Surface
|
The "density-density" correlation function of conduction electrons in metal
is investigated. It is shown, that the asymptotic behaviour of the CF depends
on the shape and the local geometry of the Fermi surface. In particular, the
exponent of power law which describes the damping of Friedel oscillations at
large r (-4 for an isotropic Fermi gas) is determined by local geometry of the
FS. The applications of the obtained results to calculations of the CF in a
metal near the electron topological transition and of the RKKY exchange
integral are considered as well.
|
9212032v1
|
1993-10-13
|
Magneto-Optics of type-II superconductors
|
The magneto-optical activity of superconducting ${\rm YBa}_{2}{\rm
Cu}_{3}{\rm O}_{7}$ observed by Karrai {\it et al.} is not present in many
commonly employed models of vortex dynamics. Here we propose a simple, unifying
picture for the frequency dependent magneto-optic response of type-II
superconductors at low temperatures. We bring together Kohn's theorem, vortex
core excitations, and vortex pinning and damping into a single expression for
the conductivity tensor. The theory describes magneto-optical activity observed
in infrared transmission measurements of thin films of ${\rm YBa}_{2}{\rm
Cu}_{3}{\rm O}_{7}$.
|
9310026v1
|
1993-12-02
|
Theory of the Eigler-swith
|
We suggest a simple model to describe the reversible field-induced transfer
of a single Xe-atom in a scanning tunneling microscope, --- the Eigler-switch.
The inelasticly tunneling electrons give rise to fluctuating forces on and
damping of the Xe-atom resulting in an effective current dependent temperature.
The rate of transfer is controlled by the well-known Arrhenius law with this
effective temperature. The directionality of atom transfer is discussed, and
the importance of use of non-equlibrium-formalism for the electronic
environment is emphasized. The theory constitutes a formal derivation and
generalization of the so-called Desorption Induced by Multiple Electron
Transitions (DIMET) point of view.
|
9312008v1
|
1994-07-25
|
Diffusion Processes and Coherent States
|
It is shown that stochastic processes of diffusion type possess, in all
generality, a structure of uncertainty relations and of coherent and squeezed
states. This fact is used to obtain, via Nelson stochastic formulation of
quantum mechanics, the harmonic-oscillator coherent and squeezed states. The
method allows to derive new minimum uncertainty states in time-dependent
oscillator potentials and for the Caldirola-Kanai model of quantum damped
oscillator.
|
9407100v1
|
1994-11-01
|
Cyclotron resonance lineshape in a Wigner crystal
|
The cyclotron resonance absorption spectrum in a Wigner crystal is
calculated. Effects of spin-splitting are modelled by substitutional disorder,
and calculated in the coherent potential approximation. Due to the increasing
strength of the dipole-dipole interaction, the results show a crossover from a
double-peak spectrum at small filling factors to a single-peak spectrum at
filling factors $\agt 1/6$. Radiation damping and magnetophonon scattering can
also influence the cyclotron resonance. The results are in very good agreement
with experiments.
|
9411003v2
|
1995-01-23
|
de Haas-van Alphen Oscillations in a Superconducting State at High Magnetic Fields
|
Low-temperature quantum oscillations of the dHvA amplitude are shown to
persist far below the upper critical field of a strongly type-II
superconductor, due to the gapless nature of the BCS quasiparticle spectrum in
high fields. The dHvA amplitude in the superconducting state is smaller than
its normal state countpart by factor ~[max(T,Gamma]/delta]**2, where Gamma is
the damping. This factor reflects the presence of a small gapless portion of
the fermi surface, surrounded by regions where the BCS gap is large. The
agreement with recent experimental data on V3Si is very good.
|
9501109v1
|
1995-08-14
|
Density-dependent phonoriton states in highly excited semiconductors
|
The dynamical aspects of the phonoriton state in highly-photoexcited
semiconductors is studied theoretically. The effect of the exciton-exciton
interaction and nonbosonic character of high-density excitons are taken into
account. Using Green's function method and within the Random Phase
Approximation it is shown that the phonoriton dispersion and damping are very
sensitive to the exciton density, characterizing the excitation degree of
semiconductors.
|
9508046v1
|
1995-12-21
|
Collective Modes in a Symmetry-Broken Phase: Antiferromagnetically Correlated Quantum Wells
|
We investigate the intersubband spin-density-excitation spectrum of a double
quantum well in a low-density symmetry-broken phase with interwell
antiferromagnetic correlations. This spectrum is related to the intensity
measured in depolarized inelastic light scattering (ILS) experiments and
therefore provides a means of empirically identifying the antiferromagnetic
phase. Our computations reveal the existence of two collective modes, a damped
Nambu-Goldstone (NG) mode arising from the broken spin symmetry and an undamped
optical mode. Since the NG mode contains most of the spectral weight, ILS
experiments will need to examine the low-frequency response for signatures of
the antiferromagnetic phase.
|
9512145v1
|
1996-03-15
|
Boson-fermion model beyond mean-field approximation
|
A model of hybridized bosons and fermions is studied beyond the mean field
approximation. The divergent boson self-energy at zero temperature makes the
Cooper pairing of fermions impossible.The frequency and momentum dependence of
the self- energy and the condensation temperature $T_{c}$ of initially
localized bosons are calculated analytically. The value of the boson
condensation temperature $T_{c}$ is below $1K$ which rules out the
boson-fermion model with the initially localized bosons as a phenomenological
explanation of high-temperature superconductivity. The intra-cell
density-density fermion-boson interaction dominates in the fermion self-energy.
The model represents a normal metal with strongly damped bosonic excitations.
The latter play the role of normal impurities.
|
9603111v1
|
1996-05-15
|
Internal Excitations and Dissipative Damping of Quantum Hall Skyrmions
|
We propose an intrinsic maximum speed for dissipationless E cross B drift of
Skyrmion quasiparticles in quantum Hall ferromagnets. When this speed is
exceeded, Skyrmions can radiate spin-waves by making internal excitations which
allow total spin to be conserved. Our proposal is illustrated by a
time-dependent Hartree-Fock approximation calculation of the excitation
spectrum for a Skyrmion bound to an impurity.
|
9605091v1
|
1996-07-19
|
Spectral properties of the planar t-J model
|
The single-particle spectral functions $A({\bf k},\omega)$ and self-energies
$\Sigma({\bf k},\omega)$ are calculated within the $t-J$ model using the
finite-temperature Lanczos method for small systems. A remarkable asymmetry
between the electron and hole part is found. The hole (photoemission) spectra
are overdamped, with ${\rm Im} \Sigma \propto \omega$ in a wide energy range,
consistent with the marginal Fermi liquid scenario, and in good agreement with
experiments on cuprates. In contrast, the quasiparticles in the electron part
of the spectrum show weak damping.
|
9607140v1
|
1996-11-19
|
Quasi-particle behavior of composite fermions in the half-filled Landau level
|
We calculate the effect of infrared fluctuations of the Chern-Simons gauge
field on the single-particle Green's function of composite fermions in the
half-filled Landau level via higher-dimensional bosonization on a curved Fermi
surface. We find that composite fermions remain well-defined quasi-particles,
with an effective mass given by the mean-field value, but with anomalously
large damping and a spectral function that contains considerable weight away
from the quasi-particle peak.
|
9611139v2
|
1996-11-22
|
Flux Noise near the Berezinskii-Kosterlitz-Thouless Transition
|
We study the flux noise in Josephson junction arrays in the critical regime
above the Berezinskii-Kosterlitz-Thouless transition. In proximity coupled
arrays a local ohmic damping for the phases is relevant, giving rise to
anomalous vortex diffusion and a dynamic scaling of the flux noise in the
critical region. It shows a crossover from white to $1/f$-noise at a frequency
$\omega_\xi\propto\xi^{-z}$ with a dynamic exponent $z=2$.
|
9611177v2
|
1996-11-29
|
Electron Green's Function in the Planar t-J Model
|
The electron Green's functions $G({\bf k},\omega)$ within the t-J model and
in the regime of intermediate doping is studied analytically using equations of
motion for projected fermionic operators and the decoupling of the self energy
into the single-particle and spin fluctuations. It is shown that the assumption
of marginal spin dynamics at T=0 leads to an anomalous quasiparticle damping.
Numerical result show also a pronounced asymmetry between the hole ($\omega<0$)
and the electron ($\omega>0$) part of the spectral function, whereby hole-like
quasiparticles are generally overdamped.
|
9612002v1
|
1997-02-21
|
One-Particle Excitation of the Two-Dimensional Hubbard Model
|
The real part of the self-energy of interacting two-dimensional electrons has
been calculated in the t-matrix approximation. It is shown that the forward
scattering results in an anomalous term leading to the vanishing
renormalization factor of the one-particle Green function, which is a
non-perturbative effect of the interaction U. The present result is a
microscopic demonstration of the claim by Anderson based on the conventional
many-body theory. The effect of the damping of the interacting electrons, which
has been ignored in reaching above conclusion, has been briefly discussed.
|
9702200v1
|
1997-05-03
|
Free induction signal from biexcitons and bound excitons
|
A theory of the free induction signal from biexcitons and bound excitons is
presented. The simultaneous existence of the exciton continuum and a bound
state is shown to result in a new type of time dependence of the free
induction. The optically detected signal increases in time and oscillates with
increasing amplitude until damped by radiative or dephasing processes.
Radiative decay is anomalously fast and can result in strong picosecond pulses.
The expanding area of a coherent exciton polarization (inflating antenna),
produced by the exciting pulse, is the underlying physical mechanism. The
developed formalism can be applied to different biexciton transients.
|
9705024v1
|
1997-06-06
|
Dynamics of viscous amphiphilic films supported by elastic solid substrates
|
The dynamics of amphiphilic films deposited on a solid surface is analyzed
for the case when shear oscillations of the solid surface are excited. The two
cases of surface- and bulk shear waves are studied with film exposed to gas or
to a liquid. By solving the corresponding dispersion equation and the wave
equation while maintaining the energy balance we are able to connect the
surface density and the shear viscocity of a fluid amphiphilic overlayer with
experimentally accessible damping coefficients, phase velocity, dissipation
factor and resonant frequency shifts of shear waves.
|
9706058v1
|
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