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2021-05-16
|
Linear stability analysis of the Couette flow for the two dimensional non-isentropic compressible Euler equations
|
This note is devoted to the linear stability of the Couette flow for the
non-isentropic compressible Euler equations in a domain $\mathbb{T}\times
\mathbb{R}$. Exploiting the several conservation laws originated from the
special structure of the linear system, we obtain a Lyapunov type instability
for the density, the temperature, the compressible part of the velocity field,
and also obtain an inviscid damping for the incompressible part of the velocity
field.
|
2105.07395v1
|
2021-05-21
|
Effects of ambipolar diffusion on waves in the solar chromosphere
|
The chromosphere is a partially ionized layer of the solar atmosphere, the
transition between the photosphere where the gas motion is determined by the
gas pressure and the corona dominated by the magnetic field. We study the
effect of partial ionization for 2D wave propagation in a gravitationally
stratified, magnetized atmosphere with properties similar to the solar
chromosphere. We adopt an oblique uniform magnetic field in the plane of
propagation with strength suitable for a quiet sun region. The theoretical
model used is a single fluid magnetohydrodynamic approximation, where
ion-neutral interaction is modeled by the ambipolar diffusion term. Magnetic
energy can be converted into internal energy through the dissipation of the
electric current produced by the drift between ions and neutrals. We use
numerical simulations where we continuously drive fast waves at the bottom of
the atmosphere. The collisional coupling between ions and neutrals decreases
with the decrease of the density and the ambipolar effect becomes important.
Fast waves excited at the base of the atmosphere reach the equipartition layer
and reflect or transmit as slow waves. While the waves propagate through the
atmosphere and the density drops, the waves steepen into shocks. The main
effect of ambipolar diffusion is damping of the waves. We find that for the
parameters chosen in this work, the ambipolar diffusion affects the fast wave
before it is reflected, with damping being more pronounced for waves which are
launched in a direction perpendicular to the magnetic field. Slow waves are
less affected by ambipolar effects. The damping increases for shorter periods
and larger magnetic field strengths. Small scales produced by the nonlinear
effects and the superposition of different types of waves created at the
equipartition height are efficiently damped by ambipolar diffusion.
|
2105.10285v1
|
2021-05-26
|
Global Attractor for the Periodic Generalized Korteweg-de Vries Equation Through Smoothing
|
We establish a smoothing result for the generalized KdV (gKdV) on the torus
with polynomial non-linearity, damping, and forcing that matches the smoothing
level for the gKdV at $H^1$. As a consequence, we establish the existence of a
global attractor for this equation as well as its compactness in
$H^s(\mathbb{T})$, $s\in (1,2).$
|
2105.13405v2
|
2021-06-01
|
On the Well-Posedness of Two Driven-Damped Gross Pitaevskii-Type Models for Exciton-Polariton Condensates
|
We study the well-posedness of two systems modeling the non-equilibrium
dynamics of pumped decaying Bose-Einstein condensates. In particular, we
present the local theory for rough initial data using the Fourier restricted
norm method introduced by Bourgain. We extend the result globally for initial
data in $L^2$.
|
2106.00438v1
|
2021-06-11
|
Dynamics and Nonmonotonic Drag for Individually Driven Skyrmions
|
We examine the motion of an individual skyrmion driven through an assembly of
other skyrmions by a constant or increasing force in the absence of quenched
disorder. The skyrmion behavior is determined by the ratio of the damping and
Magnus terms, as expressed in terms of the intrinsic skyrmion Hall angle. For a
fixed driving force in the damping dominated regime, the effective viscosity
decreases monotonically with increasing skyrmion density, similar to what is
observed in overdamped systems where it becomes difficult for the driven
particle to traverse the surrounding medium at high densities. In contrast, in
the Magnus dominated regime the velocity dependence on the density is
nonmonotonic, and there is a regime in which the skyrmion moves faster with
increasing density, as well as a pronounced speed-up effect in which a skyrmion
traveling through a dense medium moves more rapidly than it would at low
densities or in the single particle limit. At higher densities, the effective
damping increases and the velocity decreases. The velocity-force curves in the
Magnus-dominated regime show marked differences from those in the
damping-dominated regimes. Under an increasing drive we find that there is a
threshold force for skyrmion motion which increases with density. Additionally,
the skyrmion Hall angle is drive dependent, starting near zero at the threshold
for motion and increasing with increasing drive before reaching a saturation
value, similar to the behavior found for skyrmions driven over quenched
disorder. We map dynamic phase diagrams showing the threshold for motion,
nonlinear flow, speed-up, and saturation regimes. We also find that in some
cases, increasing the density can reduce the skyrmion Hall angle while
producing a velocity boost, which could be valuable for applications.
|
2106.06093v1
|
2021-06-23
|
On generalized damped Klein-Gordon equation with nonlinear memory
|
In this paper we consider the Cauchy problem for linear dissipative
generalized Klein-Gordon equations with nonlinear memory in the right hand
side. Our goal is to study the effect of this nonlinearity on both the decay
estimates of global solutions as well as the admissible range of the exponent
p.
|
2106.12296v1
|
2021-07-21
|
Convergence rates for the Heavy-Ball continuous dynamics for non-convex optimization, under Polyak-Łojasiewicz condition
|
We study convergence of the trajectories of the Heavy Ball dynamical system,
with constant damping coefficient, in the framework of convex and non-convex
smooth optimization. By using the Polyak-{\L}ojasiewicz condition, we derive
new linear convergence rates for the associated trajectory, in terms of
objective function values, without assuming uniqueness of the minimizer.
|
2107.10123v2
|
2021-08-29
|
A note on the energy transfer in coupled differential systems
|
We study the energy transfer in the linear system $$ \begin{cases} \ddot
u+u+\dot u=b\dot v\\ \ddot v+v-\epsilon \dot v=-b\dot u \end{cases} $$ made by
two coupled differential equations, the first one dissipative and the second
one antidissipative. We see how the competition between the damping and the
antidamping mechanisms affect the whole system, depending on the coupling
parameter $b$.
|
2108.12776v1
|
2021-08-29
|
Well-posedness and stability for semilinear wave-type equations with time delay
|
In this paper we analyze a semilinear abstract damped wave-type equation with
time delay. We assume that the delay feedback coefficient is variable in time
and belonging to $L^1_{loc}([0, +\infty)).$ Under suitable assumptions, we show
well-posedness and exponential stability for small initial data. Our strategy
combines careful energy estimates and continuity arguments. Some examples
illustrate the abstract results.
|
2108.12786v1
|
2021-08-30
|
Application of Rothe's method to a nonlinear wave equation on graphs
|
We study a nonlinear wave equation on finite connected weighted graphs. Using
Rothe's and energy methods, we prove the existence and uniqueness of solution
under certain assumption. For linear wave equation on graphs, Lin and Xie
\cite{Lin-Xie} obtained the existence and uniqueness of solution. The main
novelty of this paper is that the wave equation we considered has the nonlinear
damping term $|u_t|^{p-1}\cdot u_t$ ($p>1$).
|
2108.12980v1
|
2021-09-08
|
Stabilisation of Waves on Product Manifolds by Boundary Strips
|
We show that a transversely geometrically controlling boundary damping strip
is sufficient but not necessary for $t^{-1/2}$-decay of waves on product
manifolds. We give a general scheme to turn resolvent estimates for impedance
problems on cross-sections to wave decay on product manifolds.
|
2109.03928v1
|
2021-09-10
|
Smoothing effect and large time behavior of solutions to nonlinear elastic wave equations with viscoelastic term
|
The Cauchy problem for a nonlinear elastic wave equations with viscoelastic
damping terms is considered on the 3 dimensional whole space. Decay and
smoothing properties of the solutions are investigated when the initial data
are sufficiently small; and asymptotic profiles as $t \to \infty$ are also
derived.
|
2109.04628v3
|
2021-10-04
|
Overdamped limit at stationarity for non-equilibrium Langevin diffusions
|
In this note, we establish that the stationary distribution of a possibly
non-equilibrium Langevin diffusion converges, as the damping parameter goes to
infinity (or equivalently in the Smoluchowski-Kramers vanishing mass limit),
toward a tensor product of the stationary distribution of the corresponding
overdamped process and of a Gaussian distribution.
|
2110.01238v2
|
2021-10-22
|
p-Laplacian wave equations in non-cylindrical domains
|
This paper is devoted to studying the stability of p-Laplacian wave equations
with strong damping in non-cylindrical domains. The method of proof based on
some estimates for time-varying coefficients rising from moving boundary and a
modified Kormonik inequality. Meanwhile, by selecting appropriate auxiliary
functions, finally we obtain the polynomial stability (p > 2) and exponential
stability (p = 2) for such systems in some unbounded development domains.
|
2110.11547v1
|
2021-11-17
|
Transverse kink oscillations of inhomogeneous prominence threads: numerical analysis and H$α$ forward modelling
|
Prominence threads are very long and thin flux tubes which are partially
filled with cold plasma. Observations have shown that transverse oscillations
are frequent in these solar structures. The observations are usually
interpreted as the fundamental kink mode, while the detection of the first
harmonic remains elusive. Here, we aim to study how the density inhomogeneity
in the longitudinal and radial directions modify the periods and damping times
of kink oscillations, and how this effect would be reflected in observations.
We solve the ideal magnetohydrodynamics equations through two different
methods: a) performing 3D numerical simulations, and b) solving a 2D
generalised eigenvalue problem. We study the dependence of the periods, damping
times and amplitudes of transverse kink oscillations on the ratio between the
densities at the centre and at the ends of the tube, and on the average
density. We apply forward modelling on our 3D simulations to compute synthetic
H$\alpha$ profiles. We confirm that the ratio of the period of the fundamental
oscillation mode to the period of the first harmonic increases as the ratio of
the central density to the footpoint density is increased or as the averaged
density of the tube is decreased. We find that the damping times due to
resonant absorption decrease as the central to footpoint density ratio
increases. Contrary to the case of longitudinally homogeneous tubes, we find
that the damping time to period ratio also increases as the density ratio is
increased or the average density is reduced. We present snapshots and
time-distance diagrams of the emission in the H$\alpha$ line. The results
presented here have implications for the field of prominence seismology. While
the H$\alpha$ emission can be used to detect the fundamental mode, the first
harmonic is barely detectable in H$\alpha$. This may explain the lack of
detections of the first harmonic.
|
2111.09036v1
|
2021-11-26
|
A novel measurement of marginal Alfvén Eigenmode stability during high power auxiliary heating in JET
|
The interaction of Alfv\'{e}n Eigenmodes (AEs) and energetic particles is one
of many important factors determining the success of future tokamaks. In JET,
eight in-vessel antennas were installed to actively probe stable AEs with
frequencies ranging 25-250 kHz and toroidal mode numbers $\vert n \vert < 20$.
During the 2019-2020 deuterium campaign, almost 7500 resonances and their
frequencies $f_0$, net damping rates $\gamma < 0$, and toroidal mode numbers
were measured in almost 800 plasma discharges. From a statistical analysis of
this database, continuum and radiative damping are inferred to increase with
edge safety factor, edge magnetic shear, and when including non-ideal effects.
Both stable AE observations and their associated damping rates are found to
decrease with $\vert n \vert$. Active antenna excitation is also found to be
ineffective in H-mode as opposed to L-mode; this is likely due to the increased
edge density gradient's effect on accessibility and ELM-related noise's impact
on mode identification. A novel measurement is reported of a marginally stable,
edge-localized Ellipticity-induced AE probed by the antennas during high-power
auxiliary heating (ICRH and NBI) up to 25 MW. NOVA-K kinetic-MHD simulations
show good agreement with experimental measurements of $f_0$, $\gamma$, and $n$,
indicating the dominance of continuum and electron Landau damping in this case.
Similar experimental and computational studies are planned for the recent
hydrogen and ongoing tritium campaigns, in preparation for the upcoming DT
campaign.
|
2111.13569v1
|
2021-12-08
|
IGM damping wing constraints on reionisation from covariance reconstruction of two $z\gtrsim7$ QSOs
|
Bright, high redshift ($z>6$) QSOs are powerful probes of the ionisation
state of the intervening intergalactic medium (IGM). The detection of
Ly$\alpha$ damping wing absorption imprinted in the spectrum of high-z QSOs can
provide strong constraints on the epoch of reionisation (EoR). In this work, we
perform an independent Ly$\alpha$ damping wing analysis of two known $z>7$
QSOs; DESJ0252-0503 at $z=7.00$ (Wang et al.) and J1007+2115 at $z=7.51$ (Yang
et al.). For this, we utilise our existing Bayesian framework which
simultaneously accounts for uncertainties in: (i) the intrinsic Ly$\alpha$
emission profile (reconstructed from a covariance matrix of measured emission
lines; extended in this work to include NV) and (ii) the distribution of
ionised (H\,{\scriptsize II}) regions within the IGM using a $1.6^3$ Gpc$^3$
reionisation simulation. This approach is complementary to that used in the
aforementioned works as it focuses solely redward of Ly$\alpha$ ($1218 <
\lambda < 1230$\AA) making it more robust to modelling uncertainties while also
using a different methodology for (i) and (ii). We find, for a fiducial EoR
morphology, $\bar{x}_{\rm HI} = 0.64\substack{+0.19 \\ -0.23}$ (68 per cent) at
$z=7$ and $\bar{x}_{\rm HI} = 0.27\substack{+0.21 \\ -0.17}$ at $z=7.51$
consistent within $1\sigma$ to the previous works above, though both are
slightly lower in amplitude. Following the inclusion of NV into our
reconstruction pipeline, we perform a reanalysis of ULASJ1120+0641 at $z=7.09$
(Mortlock et al.) and ULASJ1342+0928 at $z=7.54$ (Ba\~nados et al.) finding
$\bar{x}_{\rm HI} = 0.44\substack{+0.23 \\ -0.24}$ at $z=7.09$ and
$\bar{x}_{\rm HI} = 0.31\substack{+0.18 \\ -0.19}$ at $z=7.54$. Finally, we
combine the QSO damping wing constraints for all four $z\gtrsim7$ QSOs to
obtain a single, unified constraint of $\bar{x}_{\rm HI} = 0.49\substack{+0.11
\\ -0.11}$ at $z=7.29$.
|
2112.04091v1
|
2022-01-24
|
A blow-up result for a Nakao-type weakly coupled system with nonlinearities of derivative-type
|
In this paper, we consider a weakly coupled system of a wave and damped
Klein-Gordon equation with nonlinearities of derivative type. We prove a
blow-up result for the Cauchy problem associated with this system for
nonnegative and compactly supported data by means of an iteration argument.
|
2201.09462v1
|
2022-03-11
|
On the small noise limit in the Smoluchowski-Kramers approximation of nonlinear wave equations with variable friction
|
We study the validity of a large deviation principle for a class of
stochastic nonlinear damped wave equations, of Klein-Gordon type, in the joint
small mass and small noise limit. The friction term is assumed to be state
dependent.
|
2203.05923v2
|
2022-03-28
|
Composite Anderson acceleration method with dynamic window-sizes and optimized damping
|
In this paper, we propose and analyze a set of fully non-stationary Anderson
acceleration algorithms with dynamic window sizes and optimized damping.
Although Anderson acceleration (AA) has been used for decades to speed up
nonlinear solvers in many applications, most authors are simply using and
analyzing the stationary version of Anderson acceleration (sAA) with fixed
window size and a constant damping factor. The behavior and potential of the
non-stationary version of Anderson acceleration methods remain an open
question. Since most efficient linear solvers use composable algorithmic
components. Similar ideas can be used for AA to solve nonlinear systems. Thus
in the present work, to develop non-stationary Anderson acceleration
algorithms, we first propose two systematic ways to dynamically alternate the
window size $m$ by composition. One simple way to package sAA(m) with sAA(n) in
each iteration is applying sAA(m) and sAA(n) separately and then average their
results. It is an additive composite combination. The other more important way
is the multiplicative composite combination, which means we apply sAA(m) in the
outer loop and apply sAA(n) in the inner loop. By doing this, significant gains
can be achieved. Secondly, to make AA to be a fully non-stationary algorithm,
we need to combine these strategies with our recent work on the non-stationary
Anderson acceleration algorithm with optimized damping (AAoptD), which is
another important direction of producing non-stationary AA and nice performance
gains have been observed. Moreover, we also investigate the rate of convergence
of these non-stationary AA methods under suitable assumptions. Finally, our
numerical results show that some of these proposed non-stationary Anderson
acceleration algorithms converge faster than the stationary sAA method and they
may significantly reduce the storage and time to find the solution in many
cases.
|
2203.14627v1
|
2022-03-28
|
The higher order nonlinear Schrödinger equation with quadratic nonlinearity on the real axis
|
The initial value problem is considered for a higher order nonlinear
Schr\"odinger equation with quadratic nonlinearity. Results on existence and
uniqueness of weak solutions are obtained. In the case of an effective at
infinity additional damping large-time decay of solutions without any smallness
assumptions is also established. The main difficulty of the study is the
non-smooth character of the nonlinearity.
|
2203.14830v1
|
2022-04-03
|
Strong Solution of Modified Anistropic 3D-Navier-Stokes Equations
|
In this paper we study the anisotropic incompressible Navier-Stokes equations
with a logarithm damping $\alpha \log(e+|u|^2)|u|^2u$ in $H^{0.1}$, where we
used new methods, new tools and Fourier analysis.
|
2204.01717v2
|
2022-04-28
|
Coupling between turbulence and solar-like oscillations: A combined Lagrangian PDF/SPH approach. II - Mode driving, damping and modal surface effect
|
The first paper of this series established a linear stochastic wave equation
for solar-like p-modes, correctly taking the effect of turbulence thereon into
account. In this second paper, we aim at deriving simultaneous expressions for
the excitation rate, damping rate, and modal surface effect associated with any
given p-mode, as an explicit function of the statistical properties of the
turbulent velocity field. We reduce the stochastic wave equation to complex
amplitude equations for the normal oscillating modes of the system. We then
derive the equivalent Fokker-Planck equation for the real amplitudes and phases
of all the oscillating modes of the system simultaneously. The effect of the
finite-memory time of the turbulent fluctuations (comparable to the period of
the modes) on the modes themselves is consistently and rigorously accounted
for, by means of the simplified amplitude equation formalism. This formalism
accounts for mutual linear mode coupling in full, and we then turn to the
special single-mode case. This allows us to derive evolution equations for the
mean energy and mean phase of each mode, from which the excitation rate, the
damping rate, and the modal surface effect naturally arise.
We show that the expression for the excitation rate of the modes is identical
to previous results obtained through a different modelling approach, thus
supporting the validity of the formalism presented here. We also recover the
fact that the damping rate and modal surface effect correspond to the real and
imaginary part of the same single complex quantity. We explicitly separate the
different physical contributions to these observables, in particular the
turbulent pressure contribution and the joint effect of the
pressure-rate-of-strain correlation and the turbulent dissipation. We show that
the former dominates for high-frequency modes and the latter for low-frequency
modes.
|
2204.13367v1
|
2022-05-05
|
Blow-up solutions of damped Klein-Gordon equation on the Heisenberg group
|
Inthisnote,weprovetheblow-upofsolutionsofthesemilineardamped Klein-Gordon
equation in a finite time for arbitrary positive initial energy on the
Heisenberg group. This work complements the paper [21] by the first author and
Tokmagambetov, where the global in time well-posedness was proved for the small
energy solutions.
|
2205.02705v1
|
2022-05-06
|
Quaternion-based attitude stabilization via discrete-time IDA-PBC
|
In this paper, we propose a new sampled-data controller for stabilization of
the attitude dynamics at a desired constant configuration. The design is based
on discrete-time interconnection and damping assignment (IDA) passivity-based
control (PBC) and the recently proposed Hamiltonian representation of
discrete-time nonlinear dynamics. Approximate solutions are provided with
simulations illustrating performances.
|
2205.03086v1
|
2022-05-23
|
Extended random-phase-approximation study of fragmentation of giant quadrupole resonance in $^{16}$O
|
The damping of isoscalar giant quadrupole resonance in $^{16}$O is studied
using extended random-phase-approximation approaches derived from the
time-dependent density-matrix theory. It is pointed out that the effects of
ground-state correlations bring strong fragmentation of quadrupole strength
even if the number of two particle--two hole configurations is strongly
limited.
|
2205.11654v2
|
2022-06-21
|
Nonlinear Compton scattering and nonlinear Breit-Wheeler pair production including the damping of particle states
|
In the presence of an electromagnetic background plane-wave field, electron,
positron, and photon states are not stable, because electrons and positrons
emit photons and photons decay into electron-positron pairs. This decay of the
particle states leads to an exponential damping term in the probabilities of
single nonlinear Compton scattering and nonlinear Breit-Wheeler pair
production. In this paper we investigate analytically and numerically the
probabilities of nonlinear Compton scattering and nonlinear Breit-Wheeler pair
production including the particle states' decay. For this we first compute
spin- and polarization-resolved expressions of the probabilities, provide some
of their asymptotic behaviors and show that the results of the total
probabilities are independent of the spin and polarization bases. Then, we
present several plots of the total and differential probabilities for different
pulse lengths and for different spin and polarization quantum numbers. We
observe that it is crucial to take into account the damping of the states in
order for the probabilities to stay always below unity and we show that the
damping factors also scale with the intensity and pulse duration of the
background field. In the case of nonlinear Compton scattering we show
numerically that the total probability behaves like a Poissonian distribution
in the regime where the photon recoil is negligible. In all considered cases,
the kinematic conditions are such that the final particles momenta transverse
to the propagation direction of the plane wave are always much smaller than the
particles longitudinal momenta and the main spread of the momentum distribution
on the transverse plane is along the direction of the plane-wave electric
field.
|
2206.10345v2
|
2022-06-23
|
Nonlinear Landau damping for the 2d Vlasov-Poisson system with massless electrons around Penrose-stable equilibria
|
In this paper, we prove the nonlinear asymptotic stability of the
Penrose-stable equilibria among solutions of the $2d$ Vlasov-Poisson system
with massless electrons.
|
2206.11744v2
|
2022-07-25
|
Inviscid limit for the compressible Navier-Stokes equations with density dependent viscosity
|
We consider the compressible Navier-Stokes system describing the motion of a
barotropic fluid with density dependent viscosity confined in a
three-dimensional bounded domain $\Omega$. We show the convergence of the weak
solution to the compressible Navier-Stokes system to the strong solution to the
compressible Euler system when the viscosity and the damping coefficients tend
to zero.
|
2207.12222v1
|
2022-08-25
|
Polynomial energy decay rate of a 2D Piezoelectric beam with magnetic effect on a rectangular domain without geometric conditions
|
In this paper, we investigate the stability of coupled equations modelling a
2D piezoelectric beam with magnetic effect with only one local viscous damping
on a rectangular domain without geometric conditions. We prove that the energy
of the system decays polynomially with the rate 1/t .
|
2208.12012v1
|
2022-10-12
|
Backward problem for the 1D ionic Vlasov-Poisson equation
|
In this paper, we study the backward problem for the one-dimensional
Vlasov-Poisson system with massless electrons, and we show the Landau damping
by fixing the asymptotic behaviour of our solution.
|
2210.06123v2
|
2022-10-28
|
Oblique Quasi-Kink Modes in Solar Coronal Slabs Embedded in an Asymmetric Magnetic Environment: Resonant Damping, Phase and Group Diagrams
|
There has been considerable interest in magnetoacoustic waves in static,
straight, field-aligned, one-dimensional equilibria where the exteriors of a
magnetic slab are different between the two sides. We focus on trapped,
transverse fundamental, oblique quasi-kink modes in pressureless setups where
the density varies continuously from a uniform interior (with density
$\rho_{\rm i}$) to a uniform exterior on either side (with density $\rho_{\rm
L}$ or $\rho_{\rm R}$), assuming $\rho_{\rm L}\le\rho_{\rm R}\le\rho_{\rm i}$.
The continuous structuring and oblique propagation make our study new relative
to pertinent studies, and lead to wave damping via the Alfv$\acute{\rm e}$n
resonance. We compute resonantly damped quasi-kink modes as resistive
eigenmodes, and isolate the effects of system asymmetry by varying $\rho_{\rm
i}/\rho_{\rm R}$ from the ``Fully Symmetric'' ($\rho_{\rm i}/\rho_{\rm
R}=\rho_{\rm i}/\rho_{\rm L}$) to the ``Fully Asymmetric'' limit ($\rho_{\rm
i}/\rho_{\rm R}=1$). We find that the damping rates possess a nonmonotonic
$\rho_{\rm i}/\rho_{\rm R}$-dependence as a result of the difference between
the two Alfv$\acute{\rm e}$n continua, and resonant absorption occurs only in
one continuum when $\rho_{\rm i}/\rho_{\rm R}$ is below some threshold. We also
find that the system asymmetry results in two qualitatively different regimes
for the phase and group diagrams. The phase and group trajectories lie
essentially on the same side (different sides) relative to the equilibrium
magnetic field when the configuration is not far from a ``Fully Asymmetric''
(``Fully Symmetric'') one. Our numerical results are understood by making
analytical progress in the thin-boundary limit, and discussed for imaging
observations of axial standing modes and impulsively excited wavetrains.
|
2210.16091v1
|
2022-11-02
|
Data-driven modeling of Landau damping by physics-informed neural networks
|
Kinetic approaches are generally accurate in dealing with microscale plasma
physics problems but are computationally expensive for large-scale or
multiscale systems. One of the long-standing problems in plasma physics is the
integration of kinetic physics into fluid models, which is often achieved
through sophisticated analytical closure terms. In this paper, we successfully
construct a multi-moment fluid model with an implicit fluid closure included in
the neural network using machine learning. The multi-moment fluid model is
trained with a small fraction of sparsely sampled data from kinetic simulations
of Landau damping, using the physics-informed neural network (PINN) and the
gradient-enhanced physics-informed neural network (gPINN). The multi-moment
fluid model constructed using either PINN or gPINN reproduces the time
evolution of the electric field energy, including its damping rate, and the
plasma dynamics from the kinetic simulations. In addition, we introduce a
variant of the gPINN architecture, namely, gPINN$p$ to capture the Landau
damping process. Instead of including the gradients of all the equation
residuals, gPINN$p$ only adds the gradient of the pressure equation residual as
one additional constraint. Among the three approaches, the gPINN$p$-constructed
multi-moment fluid model offers the most accurate results. This work sheds
light on the accurate and efficient modeling of large-scale systems, which can
be extended to complex multiscale laboratory, space, and astrophysical plasma
physics problems.
|
2211.01021v3
|
2022-11-04
|
New Clues About Light Sterile Neutrinos: Preference for Models with Damping Effects in Global Fits
|
This article reports global fits of short-baseline neutrino data to
oscillation models involving light sterile neutrinos. In the commonly-used 3+1
plane wave model, there is a well-known 4.9$\sigma$ tension between data sets
sensitive to appearance versus disappearance of neutrinos. We find that models
that damp the oscillation prediction for the reactor data sets, especially at
low energy, substantially improve the fits and reduce the tension. We consider
two such scenarios. The first scenario introduces the quantum mechanical
wavepacket effect that accounts for the source size in reactor experiments into
the 3+1 model. We find that inclusion of the wavepacket effect greatly improves
the overall fit compared to a 3$\nu$ model by $\Delta \chi^2/$DOF$=61.1/4$
($7.1\sigma$ improvement) with best-fit $\Delta m^2=1.4$ eV$^2$ and wavepacket
length of 67fm. The internal tension is reduced to 3.4$\sigma$. If reactor-data
only is fit, then the wavepacket preferred length is 91 fm ($>20$ fm at 99\%
CL). The second model introduces oscillations involving sterile flavor and
allows the decay of the heaviest, mostly sterile mass state, $\nu_4$. This
model introduces a damping term similar to the wavepacket effect, but across
all experiments. Compared to a three-neutrino fit, this has a $\Delta
\chi^2/$DOF$=60.6/4$ ($7\sigma$ improvement) with preferred $\Delta m^2=1.4$
eV$^2$ and decay $\Gamma = 0.35$ eV$^2$. The internal tension is reduced to
3.7$\sigma$.
For many years, the reactor event rates have been observed to have structure
that deviates from prediction. Community discussion has focused on an excess
compared to prediction observed at 5 MeV; however, other deviations are
apparent. This structure has $L$ dependence that is well-fit by the damped
models. Before assuming this points to new physics, we urge closer examination
of systematic effects that could lead to this $L$ dependence.
|
2211.02610v5
|
2022-12-07
|
A recipe for orbital eccentricity damping in the type-I regime for low viscosity 2D-discs
|
It is known that gap opening depends on the disc's viscosity; however,
eccentricity damping formulas have only been derived at high viscosities,
ignoring partial gap opening. We aim at obtaining a simple formula to model
$e$-damping of the type-I regime in low viscosity discs, where even small
planets may start opening partial. We perform high resolution 2D locally
isothermal hydrodynamical simulations of planets with varying masses on fixed
orbits in discs with varying aspect ratios and viscosities. We determine the
torque and power felt by the planet to derive migration and eccentricity
damping timescales. We first find a lower limit to the gap depths below which
vortices appear; this happens roughly at the transition between type-I and
type-II regimes. For the simulations that remain stable, we obtain a fit to the
observed gap depth in the limit of vanishing eccentricities that is similar to
the one currently used in the literature but is accurate down to
$\alpha=3.16\times 10^{-5}$. We record the $e$-damping efficiency as a function
of the observed gap depth and $e$: when the planet has opened a deep enough
gap, a linear trend is observed independently of $e$; at shallower gaps this
linear trend is preserved at low $e$, while it deviates to more efficient
damping when $e$ is comparable to the disc's scale height. Both trends can be
understood on theoretical grounds and are reproduced by a simple fitting
formula. Our combined fits yield a simple recipe to implement type-I
$e$-damping in $N$-body for partial gap opening planets that is consistent with
high-resolution 2D hydro-simulations. The typical error of the fit is of the
order of a few percent, and lower than the error of type-I torque formulas
widely used in the literature. This will allow a more self-consistent treatment
of planet-disc interactions of the type-I regime for population synthesis
models at low viscosities.
|
2212.03608v1
|
2022-12-10
|
Linear stabilization for a degenerate wave equation in non divergence form with drift
|
We consider a degenerate wave equation in one dimension, with drift and in
presence of a leading operator which is not in divergence form. We impose a
homogeneous Dirichlet boundary condition where the degeneracy occurs and a
boundary damping at the other endpoint. We provide some conditions for the
uniform exponential decay of solutions for the associated Cauchy problem.
|
2212.05264v1
|
2022-12-31
|
On the stability of shear flows in bounded channels, II: non-monotonic shear flows
|
We give a proof of linear inviscid damping and vorticity depletion for
non-monotonic shear flows with one critical point in a bounded periodic
channel. In particular, we obtain quantitative depletion rates for the
vorticity function without any symmetry assumptions.
|
2301.00288v2
|
2023-03-18
|
Spin waves in a superconductor
|
Spin waves that can propagate in normal and superconducting metals are
investigated. Unlike normal metals, the velocity of spin waves becomes
temperature-dependent in a superconductor. The low frequency spin waves survive
within the narrow region below the superconducting transition temperature. At
low temperatures the high frequency waves alone can propagate with an
additional damping due to pair-breaking.
|
2303.10468v1
|
2023-04-07
|
Echo disappears: momentum term structure and cyclic information in turnover
|
We extract cyclic information in turnover and find it can explain the
momentum echo. The reversal in recent month momentum is the key factor that
cancels out the recent month momentum and excluding it makes the echo regress
to a damped shape. Both rational and behavioral theories can explain the
reversal. This study is the first explanation of the momentum echo in U.S.
stock markets.
|
2304.03437v1
|
2023-04-26
|
Plasma echoes in graphene
|
Plasma echo is a dramatic manifestation of plasma damping process
reversibility. In this paper we calculate temporal and spatial plasma echoes in
graphene in the acoustic plasmon regime when echoes dominate over plasmon
emission. We show an extremely strong spatial echo response and discuss how
electron collisions reduce the echo. We also discuss differences between
various electron dispersions, and differences between semiclassical and quantum
model of echoes.
|
2304.13440v1
|
2023-06-01
|
JWST Measurements of Neutral Hydrogen Fractions and Ionized Bubble Sizes at $z=7-12$ Obtained with Ly$α$ Damping Wing Absorptions in 26 Bright Continuum Galaxies
|
We present volume-averaged neutral hydrogen fractions $x_{\rm \HI}$ and
ionized bubble radii $R_{\rm b}$ measured with Ly$\alpha$ damping wing
absorption of galaxies at the epoch of reionization. We combine JWST/NIRSpec
spectra taken by CEERS, GO-1433, DDT-2750, and JADES programs, and obtain a
sample containing 26 bright UV-continuum ($M_{\rm UV}<-18.5~{\rm mag}$)
galaxies at $7<z<12$. We construct 4 composite spectra binned by redshift, and
find the clear evolution of softening break towards high redshift at the
rest-frame $1216$ {\AA}, suggesting the increase of Ly$\alpha$ damping wing
absorption. We estimate Ly$\alpha$ damping wing absorption in the galaxy
spectra with realistic templates including Ly$\alpha$ emission and
circum-galactic medium absorptions. Assuming the standard inside-out
reionization picture having an ionized bubble with radius $R_b$ around a galaxy
embedded in the intergalactic medium with $x_{\rm \HI}$, we obtain $x_{\rm
\HI}$ ($R_{\rm b}$) values generally increasing (decreasing) from $x_{\rm
\HI}={0.54}^{+0.13}_{-0.54}$ to ${0.94}^{+0.06}_{-0.41}$ ($\log R_{\rm
b}={1.89}^{+0.49}_{-1.54}$ to ${-0.72}^{+1.57}_{-0.28}$ comoving Mpc) at
redshift $7.12^{+0.06}_{-0.08}$ to $10.28^{+1.12}_{-1.40}$. The redshift
evolution of $x_{\rm \HI}$ indicates a moderately late reionization history
consistent with the one previously suggested from the electron scattering of
cosmic microwave background and the evolution of UV luminosity function with an
escape fraction $f_{\rm esc}\sim 0.2$. Our ${R_{\rm b}}$ measurements suggest
that bubble sizes could be up to a few dex larger than the cosmic average
values estimated by analytic calculations for a given $x_{\rm \HI}$, while our
$R_{\rm b}$ measurements are roughly comparable with the values for merged
ionized bubbles around bright galaxies predicted by recent numerical
simulations.
|
2306.00487v2
|
2023-06-20
|
New results on controllability and stability for degenerate Euler-Bernoulli type equations
|
In this paper we study the controllability and the stability for a degenerate
beam equation in divergence form via the energy method. The equation is clamped
at the left end and controlled by applying a shearing force or a damping at the
right end.
|
2306.11851v3
|
2023-07-18
|
Nonlinear feedback, double bracket dissipation and port control of Lie-Poisson systems
|
Methods from controlled Lagrangians, double bracket dissipation and
interconnection and damping assignment -- passivity based control (IDA-PBC) are
used to construct nonlinear feedback controls which (asymptotically) stabilize
previously unstable equilibria of Lie-Poisson Hamiltonian systems. The results
are applied to find an asymptotically stabilizing control for the rotor driven
satellite, and a stabilizing control for Hall magnetohydrodynamic flow.
|
2307.09235v1
|
2023-08-01
|
Aerodynamics of the square-back Ahmed body under rainfall conditions
|
We report an experimental investigation about the aerodynamics of a
simplified road vehicle, the so-called square-back Ahmed body, under rainfall
conditions. A particular emphasis is put on the evolution of the body base
pressure distribution with respect to the operating conditions. It is found
that rainfall significantly damps both mean base pressure drag and wake
dynamics in comparison to dry conditions.
|
2308.00276v1
|
2023-09-11
|
Study of damped oscillating structures from charged and neutral K-meson electromagnetic form factors data
|
The damped oscillating structures (OS) were recently revealed in the proton
"effective" form factor (FF) data. For the time being they can be neither
confirmed nor disproved by investigations of timelike data on the individual
proton electric and proton magnetic FFs because their precision and reliability
(especially of the proton electric FF data) has not achieved required level for
this aim. On the other hand, conjectures that the OS are direct manifestations
of the quark-gluon structure of the proton indicate that they must not be
specific only for the proton and neutron, but that they should be present also
for other hadrons. This opens a plausibility to find damped oscillatory
structures also from the EM FFs data of such hadrons, for which adequate EM FFs
data exist, by using the same procedure as for the proton. Consequently in this
paper damped oscillatory structures are investigated in the EM FFs data of the
charged and neutral $K$-mesons to be extracted from the corresponding
production cross sections, $\sigma^{bare}_{tot}(e^+e^-\to K^+ K^-)$ measured
from the threshold up to 64 GeV$^2$ and $\sigma^{bare}_{tot}(e^+e^-\to K_s
K_L)$ measured from the threshold up to 9.5 GeV$^2$ of the total c.m. energy
squared. The following results have been obtained. If the charged and neutral
K-meson EM FFs timelike data are described by the three parametric formula by
means of which OS have been revealed from the "effective" proton FF data then
OS appear. If physically well founded Unitary and Analytic model of the K-meson
EM structure is used for a description of the charged K-meson EM FFs data, no
OS are visible. However, in the case of the neutral K-meson EM FF data one
cannot make a definite decision. The overall results indicate that OS obtained
from the "effective" proton FF data are likely an artefact of the three
parametric formula which does not describe these data well.
|
2309.05354v1
|
2023-10-31
|
Variational principle for a damped, quadratically interacting particle chain with nonconservative forcing
|
A method for designing variational principles for the dynamics of a possibly
dissipative and non-conservatively forced chain of particles is demonstrated.
Some qualitative features of the formulation are discussed.
|
2311.00106v2
|
2023-12-25
|
IMEX-RK methods for Landau-Lifshitz equation with arbitrary damping
|
Magnetization dynamics in ferromagnetic materials is modeled by the
Landau-Lifshitz (LL) equation, a nonlinear system of partial differential
equations. Among the numerical approaches, semi-implicit schemes are widely
used in the micromagnetics simulation, due to a nice compromise between
accuracy and efficiency. At each time step, only a linear system needs to be
solved and a projection is then applied to preserve the length of
magnetization. However, this linear system contains variable coefficients and a
non-symmetric structure, and thus an efficient linear solver is highly desired.
If the damping parameter becomes large, it has been realized that efficient
solvers are only available to a linear system with constant, symmetric, and
positive definite (SPD) structure. In this work, based on the implicit-explicit
Runge-Kutta (IMEX-RK) time discretization, we introduce an artificial damping
term, which is treated implicitly. The remaining terms are treated explicitly.
This strategy leads to a semi-implicit scheme with the following properties:
(1) only a few linear system with constant and SPD structure needs to be solved
at each time step; (2) it works for the LL equation with arbitrary damping
parameter; (3) high-order accuracy can be obtained with high-order IMEX-RK time
discretization. Numerically, second-order and third-order IMEX-RK methods are
designed in both the 1-D and 3-D domains. A comparison with the backward
differentiation formula scheme is undertaken, in terms of accuracy and
efficiency. The robustness of both numerical methods is tested on the first
benchmark problem from National Institute of Standards and Technology. The
linearized stability estimate and optimal rate convergence analysis are
provided for an alternate IMEX-RK2 numerical scheme as well.
|
2312.15654v1
|
2024-01-30
|
Linear stability analysis of the Couette flow for the 2D Euler-Poisson system
|
This paper is concerned with the linear stability analysis for the Couette
flow of the Euler-Poisson system for both ionic fluid and electronic fluid in
the domain $\bb{T}\times\bb{R}$. We establish the upper and lower bounds of the
linearized solutions of the Euler-Poisson system near Couette flow. In
particular, the inviscid damping for the solenoidal component of the velocity
is obtained.
|
2401.17102v1
|
2024-03-21
|
Non-resonant invariant foliations of quasi-periodically forced systems
|
We show the existence and uniqueness of invariant foliations about invariant
tori in analytic discrete-time dynamical systems. The parametrisation method is
used prove the result. Our theory is a foundational block of data-driven model
order reduction, that can only be carried out using invariant foliations. The
theory is illustrated by two mechanical examples, where instantaneous
frequencies and damping ratios are calculated about the invariant tori.
|
2403.14771v1
|
2024-04-03
|
Comment on "Machine learning conservation laws from differential equations"
|
In lieu of abstract, first paragraph reads: Six months after the author
derived a constant of motion for a 1D damped harmonic oscillator [1], a similar
result appeared by Liu, Madhavan, and Tegmark [2, 3], without citing the
author. However, their derivation contained six serious errors, causing both
their method and result to be incorrect. In this Comment, those errors are
reviewed.
|
2404.02896v1
|
2007-03-01
|
Stellar Kinematics in the Complicated Inner Spheroid of M31: Discovery of Substructure Along the Southeastern Minor Axis and its Relationship to the Giant Southern Stream
|
We present the discovery of a kinematically-cold stellar population along the
SE minor axis of the Andromeda galaxy (M31) that is likely the forward
continuation of M31's giant southern stream. This discovery was made in the
course of an on-going spectroscopic survey of red giant branch (RGB) stars in
M31 using the DEIMOS instrument on the Keck II 10-m telescope. Stellar
kinematics are investigated in eight fields located 9-30 kpc from M31's center
(in projection). A likelihood method based on photometric and spectroscopic
diagnostics is used to isolate confirmed M31 RGB stars from foreground Milky
Way dwarf stars: for the first time, this is done without using radial velocity
as a selection criterion, allowing an unbiased study of M31's stellar
kinematics. The radial velocity distribution of the 1013 M31 RGB stars shows
evidence for the presence of two components. The broad (hot) component has a
velocity dispersion of 129 km/s and presumably represents M31's virialized
spheroid. A significant fraction (19%) of the population is in a narrow (cold)
component centered near M31's systemic velocity with a velocity dispersion that
decreases with increasing radial distance, from 55.5 km/s at R_proj=12 kpc to
10.6 km/s at R_proj=18 kpc. The spatial and velocity distribution of the cold
component matches that of the "Southeast shelf" predicted by the Fardal et al.
(2007) orbital model of the progenitor of the giant southern stream. The
metallicity distribution of the cold component matches that of the giant
southern stream, but is about 0.2 dex more metal rich on average than that of
the hot spheroidal component. We discuss the implications of our discovery on
the interpretation of the intermediate-age spheroid population found in this
region in recent ultra-deep HST imaging studies.
|
0703029v3
|
2017-02-26
|
Limits on the ultra-bright Fast Radio Burst population from the CHIME Pathfinder
|
We present results from a new incoherent-beam Fast Radio Burst (FRB) search
on the Canadian Hydrogen Intensity Mapping Experiment (CHIME) Pathfinder. Its
large instantaneous field of view (FoV) and relative thermal insensitivity
allow us to probe the ultra-bright tail of the FRB distribution, and to test a
recent claim that this distribution's slope, $\alpha\equiv-\frac{\partial \log
N}{\partial \log S}$, is quite small. A 256-input incoherent beamformer was
deployed on the CHIME Pathfinder for this purpose. If the FRB distribution were
described by a single power-law with $\alpha=0.7$, we would expect an FRB
detection every few days, making this the fastest survey on sky at present. We
collected 1268 hours of data, amounting to one of the largest exposures of any
FRB survey, with over 2.4\,$\times$\,10$^5$\,deg$^2$\,hrs. Having seen no
bursts, we have constrained the rate of extremely bright events to
$<\!13$\,sky$^{-1}$\,day$^{-1}$ above $\sim$\,220$\sqrt{(\tau/\rm ms)}$ Jy\,ms
for $\tau$ between 1.3 and 100\,ms, at 400--800\,MHz. The non-detection also
allows us to rule out $\alpha\lesssim0.9$ with 95$\%$ confidence, after
marginalizing over uncertainties in the GBT rate at 700--900\,MHz, though we
show that for a cosmological population and a large dynamic range in flux
density, $\alpha$ is brightness-dependent. Since FRBs now extend to large
enough distances that non-Euclidean effects are significant, there is still
expected to be a dearth of faint events and relative excess of bright events.
Nevertheless we have constrained the allowed number of ultra-intense FRBs.
While this does not have significant implications for deeper, large-FoV surveys
like full CHIME and APERTIF, it does have important consequences for other
wide-field, small dish experiments.
|
1702.08040v2
|
2019-04-01
|
Astro2020 Science White Paper: Construction of an L* Galaxy: the Transformative Power of Wide Fields for Revealing the Past, Present and Future of the Great Andromeda System
|
The Great Andromeda Galaxy (M31) is the nexus of the near-far galaxy
evolution connection and a principal data point for near-field cosmology. Due
to its proximity (780 kpc), M31 can be resolved into individual stars like the
Milky Way (MW). Unlike the MW, we have the advantage of a global view of M31,
enabling M31 to be observed with techniques that also apply to more distant
galaxies. Moreover, recent evidence suggests that M31 may have survived a major
merger within the last several Gyr, shaping the morphology of its stellar halo
and triggering a starburst, while leaving the stellar disk largely intact. The
MW and M31 thus provide complementary opportunities for in-depth studies of the
disks, halos, and satellites of L* galaxies.
Our understanding of the M31 system will be transformed in the 2020s if they
include wide field facilities for both photometry (HST-like sensitivity and
resolution) and spectroscopy (10-m class telescope, >1 sq. deg. field, highly
multiplexed, R~ 3000 to 6000). We focus here on the power of these facilities
to constrain the past, present, and future merger history of M31, via
chemo-dynamical analyses and star formation histories of phase-mixed stars
accreted at early times, as well as stars in surviving tidal debris features,
M31's extended disk, and intact satellite galaxies that will eventually be
tidally incorporated into the halo. This will yield an unprecedented view of
the hierarchical formation of the M31 system and the subhalos that built it
into the L* galaxy we observe today.
|
1904.01074v1
|
2021-09-28
|
Diving Beneath the Sea of Stellar Activity: Chromatic Radial Velocities of the Young AU Mic Planetary System
|
We present updated radial-velocity (RV) analyses of the AU Mic system. AU Mic
is a young (22 Myr) early M dwarf known to host two transiting planets -
$P_{b}\sim8.46$ days, $R_{b}=4.38_{-0.18}^{+0.18}\ R_{\oplus}$,
$P_{c}\sim18.86$ days, $R_{c}=3.51_{-0.16}^{+0.16}\ R_{\oplus}$. With visible
RVs from CARMENES-VIS, CHIRON, HARPS, HIRES, {\sc
{\textsc{Minerva}}}-Australis, and TRES, as well as near-infrared (NIR) RVs
from CARMENES-NIR, CSHELL, IRD, iSHELL, NIRSPEC, and SPIRou, we provide a
$5\sigma$ upper limit to the mass of AU Mic c of $M_{c}\leq20.13\ M_{\oplus}$
and present a refined mass of AU Mic b of $M_{b}=20.12_{-1.57}^{+1.72}\
M_{\oplus}$. Used in our analyses is a new RV modeling toolkit to exploit the
wavelength dependence of stellar activity present in our RVs via
wavelength-dependent Gaussian processes. By obtaining near-simultaneous visible
and near-infrared RVs, we also compute the temporal evolution of RV-``color''
and introduce a regressional method to aid in isolating Keplerian from stellar
activity signals when modeling RVs in future works. Using a multi-wavelength
Gaussian process model, we demonstrate the ability to recover injected planets
at $5\sigma$ significance with semi-amplitudes down to $\approx$
10\,m\,s$^{-1}$ with a known ephemeris, more than an order of magnitude below
the stellar activity amplitude. However, we find that the accuracy of the
recovered semi-amplitudes is $\sim$50\% for such signals with our model.
|
2109.13996v1
|
2022-03-04
|
Scaling K2. V. Statistical Validation of 60 New Exoplanets From K2 Campaigns 2-18
|
The NASA K2 mission, salvaged from the hardware failures of the Kepler
telescope, has continued Kepler's planet-hunting success. It has revealed
nearly 500 transiting planets around the ecliptic plane, many of which are the
subject of further study, and over 1000 additional candidates. Here we present
the results of an ongoing project to follow-up and statistically validate new
K2 planets, in particular to identify promising new targets for further
characterization. By analyzing the reconnaissance spectra, high-resolution
imaging, centroid variations, and statistical likelihood of the signals of 91
candidates, we validate 60 new planets in 46 systems. These include: a number
of planets amenable to transmission spectroscopy (K2-384 f, K2-387 b, K2-390 b,
K2-403 b, and K2-398 c), emission spectroscopy (K2-371 b, K2-370 b, and K2-399
b), and both (K2-405 b and K2-406 b); several systems with planets in or close
to mean motion resonances (K2-381, K2-398) including a compact, TRAPPIST-1-like
system of five small planets orbiting a mid-M dwarf (K2-384); an ultra-short
period sub-Saturn in the hot Saturn desert (K2-399 b); and a super-Earth
orbiting a moderately bright (V=11.93), metal-poor ([Fe/H]=-0.579+/-0.080) host
star (K2-408 b). In total we validate planets around 4 F stars, 26 G stars, 13
K stars, and 3 M dwarfs. In addition, we provide a list of 37 vetted planet
candidates that should be prioritized for future follow-up observation in order
to be confirmed or validated.
|
2203.02087v2
|
1994-06-27
|
Reverberation mapping of active galactic nuclei : The SOLA method for time-series inversion
|
In this paper a new method is presented to find the transfer function of the
broad-line region in active galactic nuclei. The subtractive optimally
localized averages (SOLA) method is a modified version of the Backus-Gilbert
method and is presented as an alternative to the more often used
maximum-entropy method. The SOLA method has been developed for use in
helioseismology. It has been applied to the solar oscillation frequency
splitting data currently available to deduce the internal rotation rate of the
sun. The original SOLA method is reformulated in the present paper to cope with
the slightly different problem of inverting time series. We use simulations to
test the viability of the method and apply the SOLA method to the real data of
the Seyfert-1 galaxy NGC 5548. We investigate the effects of measurement errors
and how the resolution of the TF critically depends upon both the sampling rate
and the photometric accuracy of the data. A uuencoded compressed postscript
file of the paper which includes the figures is available by anonymous ftp at
ftp://solaris.astro.uu.se/pub/articles/atmos/frank/PijWan.uue
|
9406070v1
|
1997-06-20
|
Rejection of the Binary Broad-Line Region Interpretation of Double-Peaked Emission Lines in Three Active Galactic Nuclei
|
It has been suggested that the peculiar double-peaked Balmer lines of certain
broad-line radio galaxies come from individual broad-line regions associated
with the black holes of a supermassive binary. We continue to search for
evidence of the radial velocity variations characteristic of a double-lined
spectroscopic binary that are required in such a model. After spectroscopic
monitoring of three suitable candidates (Arp 102B, 3C 390.3, and 3C 332)
spanning two decades, we find no such long-term systematic changes in radial
velocity. A trend noticed by Gaskell in one of the Balmer-line peaks of 3C
390.3 before 1988 did not continue after that year, invalidating his inferred
orbital period and mass. Instead, we find lower limits on the plausible orbital
periods that would require the assumed supermassive binaries in all three
objects to have total masses in excess of 10^10 solar masses. In the case of 3C
390.3 the total binary mass must exceed 10^11 solar masses to satisfy
additional observational constraints on the inclination angle. Such large
binary black hole masses are difficult to reconcile with other observations and
with theory. In addition, there are peculiar properties of the line profiles
and flux ratios in these objects that are not explained by ordinary broad-line
region cloud models. We therefore doubt that the double-peaked line profiles of
the three objects arise in a pair of broad-line regions. Rather, they are much
more likely to be intimately associated with a single black hole.
|
9706222v2
|
1999-08-13
|
Infrared Spectroscopy of the High Redshift Radio Galaxy MRC~2025-218 and a Neighboring Extremely Red Galaxy
|
This paper presents infrared spectra taken with the newly commissioned
NIRSPEC spectrograph on the Keck Telescope of the High Redshift Radio Galaxy
MRC 2025-218 (z=2.630) and an extremely red galaxy (R-K > 6 mag) 9'' away.
These observations represent the deepest infrared spectra of a radio galaxy to
date and have allowed for the detection of Hbeta, OIII (4959/5007), OI (6300),
Halpha, NII (6548/6583) and SII (6716/6713). The Halpha emission is very broad
(FWHM~6000 km/s) and strongly supports AGN unification models linking radio
galaxies and quasars. The line ratios are most consistent with a partially
obscured nuclear region and very high excitation. The OIII (5007) line is
extended several arcseconds and shows high velocity clouds in the extended
emission. The nucleus also appears spectrally double and we argue that the
radio galaxy is undergoing a violent merger process. The red galaxy, by
comparison, is very featureless even though we have a good continuum detection
in the H and K bands. We suggest that this object is a foreground galaxy,
probably at a redshift less than 1.5.
|
9908153v1
|
2000-01-11
|
2 micron Spectroscopy within 0.3 arcseconds of SgrA*
|
We present moderate (R~$\approx$~2,700) and high resolution
(R~$\approx$~22,000) 2.0$-$2.4 \micron\ spectroscopy of the central 0.1 square
arcseconds of the Galaxy obtained with NIRSPEC, the facility near-infrared
spectrometer for the Keck II telescope. The composite spectra do not have any
features attributable to the brightest stars in the central cluster, i.e.\
after background subtraction, W$_{\rm ^{12}CO(2-0)}$~$<$~2~\AA. This stringent
limit leads us to conclude that the majority, if not all, of the stars are
hotter than typical red giants. Coupled with previously reported photometry, we
conclude that the sources are likely OB main sequence stars. In addition, the
continuum slope in the composite spectrum is bluer than that of a red giant and
is similar to that of the nearby hot star, IRS16NW. It is unlikely that they
are late-type giants stripped of their outer envelopes because such sources
would be much fainter than those observed. Given their inferred youth
($\tau_{\rm age}$~$<$~20~\Myr), we suggest the possibility that the stars have
formed within 0.1 pc of the supermassive black hole. We find a newly-identified
broad-line component (V$_{\rm FWHM}$ $\approx$ 1,000 \kms) to the 2.2178
\micron\ [\ion{Fe}{3}] line located within a few arcseconds of Sgr~A$^*$. A
similar component is not seen in the Br-$\gamma$ emission.
|
0001171v1
|
2000-02-17
|
Discovery of an Obscured Broad Line Region in the High Redshift Radio Galaxy MRC 2025-218
|
This paper presents infrared spectra taken with the newly commissioned
NIRSPEC spectrograph on the Keck II Telescope of the High Redshift Radio Galaxy
MRC 2025-218 (z=2.63) These observations represent the deepest infrared spectra
of a radio galaxy to date and have allowed for the detection of Hbeta, [OIII]
(4959/5007), [OI] (6300), Halpha, [NII] (6548/6583) and [SII] (6716/6713). The
Halpha emission is very broad (FWHM = 9300 km/s) and luminous (2.6x10^44
ergs/s) and it is very comparable to the line widths and strengths of radio
loud quasars at the same redshift. This strongly supports AGN unification
models linking radio galaxies and quasars, although we discuss some of the
outstanding differences. The [OIII] (5007) line is extremely strong and has
extended emission with large relative velocities to the nucleus. We also derive
that if the extended emission is due to star formation, each knot has a star
formation rate comparable to a Lyman Break Galaxy at the same redshift.
|
0002335v1
|
2000-02-26
|
Hot Stars and Cool Clouds: The Photodissociation Region M16
|
We present high-resolution spectroscopy and images of a photodissociation
region (PDR) in M16 obtained during commissioning of NIRSPEC on the Keck II
telescope. PDRs play a significant role in regulating star formation, and M16
offers the opportunity to examine the physical processes of a PDR in detail. We
simultaneously observe both the molecular and ionized phases of the PDR and
resolve the spatial and kinematic differences between them. The most prominent
regions of the PDR are viewed edge-on. Fluorescent emission from nearby stars
is the primary excitation source, although collisions also preferentially
populate the lowest vibrational levels of H2. Variations in density-sensitive
emission line ratios demonstrate that the molecular cloud is clumpy, with an
average density n = 3x10^5 cm^(-3). We measure the kinetic temperature of the
molecular region directly and find T_H2 = 930 K. The observed density,
temperature, and UV flux imply a photoelectric heating efficiency of 4%. In the
ionized region, n_i=5x10^3 cm^(-3) and T_HII = 9500 K. In the brightest regions
of the PDR, the recombination line widths include a non-thermal component,
which we attribute to viewing geometry.
|
0002491v1
|
2000-02-28
|
The Rest-Frame Optical Spectrum of MS 1512-cB58
|
Moderate resolution, near-IR spectroscopy of MS1512-cB58 is presented,
obtained during commissioning of the the Near IR Spectrometer (NIRSPEC) on the
Keck II telescope. The strong lensing of this z=2.72 galaxy by the foreground
cluster MS1512+36 makes it the best candidate for detailed study of the
rest-frame optical properties of Lyman Break Galaxies.
A redshift of z=2.7290+/-0.0007 is inferred from the emission lines, in
contrast to the z=2.7233 calculated from UV observations of interstellar
absorption lines. Using the Balmer line ratios, we find an extinction of
E(B-V)=0.27. Using the line strengths, we infer an SFR=620+/-18 Msun/yr
(H_0=75, q_0=0.1, Lambda =0), a factor of 2 higher than that measured from
narrow-band imaging observations of the galaxy, but a factor of almost 4 lower
than the SFR inferred from the UV continuum luminosity. The width of the Balmer
lines yields a mass of M_vir=1.2x10^10 Msun. We find that the oxygen abundance
is 1/3 solar, in good agreement with other estimates of the metallicity.
However, we infer a high nitrogen abundance, which may argue for the presence
of an older stellar population.
|
0002508v1
|
2000-03-06
|
The Sunyaev-Zel'dovich Effect in Abell 370
|
We present interferometric measurements of the Sunyaev-Zel'dovich (SZ) effect
towards the galaxy cluster Abell 370. These measurements, which directly probe
the pressure of the cluster's gas, show the gas distribution to be strongly
aspherical, as do the x-ray and gravitational lensing observations. We
calculate the cluster's gas mass fraction in two ways. We first compare the gas
mass derived from the SZ measurements to the lensing-derived gravitational mass
near the critical lensing radius. We also calculate the gas mass fraction from
the SZ data by deprojecting the three-dimensional gas density distribution and
deriving the total mass under the assumption that the gas is in hydrostatic
equilibrium (HSE). We test the assumptions in the HSE method by comparing the
total cluster mass implied by the two methods and find that they agree within
the errors of the measurement. We discuss the possible systematic errors in the
gas mass fraction measurement and the constraints it places on the matter
density parameter, OmegaM.
|
0003085v1
|
2000-12-04
|
Galaxy Cluster Gas Mass Fractions from Sunyaev-Zel'dovich Effect Measurements: Constraints on Omega_M
|
Using sensitive centimeter-wave receivers mounted on the Owens Valley Radio
Observatory and Berkeley-Illinois-Maryland-Association millimeter arrays, we
have obtained interferometric measurements of the Sunyaev-Zel'dovich (SZ)
effect toward massive galaxy clusters. We use the SZ data to determine the
pressure distribution of the cluster gas and, in combination with published
X-ray temperatures, to infer the gas mass and total gravitational mass of 18
clusters. The gas mass fraction, f_g, is calculated for each cluster, and is
extrapolated to the fiducial radius r_{500} using the results of numerical
simulations. The mean f_g within r_{500} is 0.081+0.009 -0.011/(h_{100}
(statistical uncertainty at 68% confidence level, assuming OmegaM=0.3,
OmegaL=0.7). We discuss possible sources of systematic errors in the mean f_g
measurement. We derive an upper limit for OmegaM from this sample under the
assumption that the mass composition of clusters within r_{500} reflects the
universal mass composition: Omega_M h < Omega_B/f_g. The gas mass fractions
depend on cosmology through the angular diameter distance and the r_{500}
correction factors.
For a flat universe (OmegaL = 1 - OmegaM) and h=0.7, we find the measured gas
mass fractions are consistent with Omegam less than 0.40, at 68% confidence.
Including estimates of the baryons contained in galaxies and the baryons which
failed to become bound during the cluster formation process, we find OmegaM
\~0.25.
|
0012067v1
|
2001-01-02
|
Science Prospects for SPI
|
After the recent beautiful results on gamma-ray lines obtained with CGRO, the
INTEGRAL mission with the imaging-spectrometer SPI will set the next milestone,
combining improved sensitivity and angular resolution with a considerable
increase in spectral resolution. SPI is expected to provide significant new
information on galactic nucleosynthesis processes and star formation activity,
as traced by the distributions of annihilation radiation and radioactive
isotopes such as 26Al and 60Fe. The unprecedented spectral resolution will
allow the study of dynamic processes in stellar mass ejections and will provide
access to kinematic distance estimates for gamma-ray line sources. The study of
supernovae and their remnants will be prime objectives for SPI observations.
Nearby type Ia SN, within 15 Mpc or so, are in reach of the instrument and a
few such events are expected during the lifetime of INTEGRAL. Young galactic
supernova remnants, possibly hidden by interstellar dust, may be unveiled by
their characteristic gamma-ray line signature from the radioactive decay of
44Ti, as has been demonstrated by COMPTEL for Cas A and possibly RX
J0852.0-4622. Classical novae are also among the SPI targets, which may observe
the gamma-ray lines from radioactive 7Be and 22Na. Such observations can
constrain the physics of the nova explosions and will allow to evaluate their
role as nucleosynthesis sites. The interaction of cosmic rays with the dense
matter in molecular clouds may be another source of gamma-ray lines that is
potentially accessible to SPI. Finally after the SIGMA results on Nova Muscae
and 1E1740.7-2942, and a possible 2.223 MeV line detection by COMPTEL, the
search for lines from X novae is another way to participate in the
understanding of the physical conditions in these close binary systems.
|
0101018v1
|
2002-08-07
|
Massive Stars in the Arches Cluster
|
We present and use new spectra and narrow-band images, along with previously
published broad-band images, of stars in the Arches cluster to extract
photometry, astrometry, equivalent width, and velocity information. The data
are interpreted with a wind/atmosphere code to determine stellar temperatures,
luminosities, mass-loss rates, and abundances. We have doubled the number of
known emission-line stars, and we have also made the first spectroscopic
identification of the main sequence for any population in the Galactic Center.
We conclude that the most massive stars are bona-fide Wolf-Rayet (WR) stars and
are some of the most massive stars known, having M_{initial} > 100 Msun, and
prodigious winds, Mdot > 10^{-5} Msun yr^{-1}, that are enriched with helium
and nitrogen; with these identifications, the Arches cluster contains about 5%
of all known WR stars in the Galaxy. We find an upper limit to the velocity
dispersion of 22 kms^{-1}, implying an upper limit to the cluster mass of
7(10^4) Msun within a radius of 0.23 pc; we also estimate the bulk heliocentric
velocity of the cluster to be v_{cluster,odot} approximately +95 kms^{-1}.
|
0208145v1
|
2003-01-31
|
Ultraviolet spectroscopy of narrow coronal mass ejections
|
We present Ultraviolet Coronagraph Spectrometer (UVCS) observations of 5
narrow coronal mass ejections (CMEs) that were among 15 narrow CMEs originally
selected by Gilbert et al. (2001). Two events (1999 March 27, April 15) were
"structured", i.e. in white light data they exhibited well defined interior
features, and three (1999 May 9, May 21, June 3) were "unstructured", i.e.
appeared featureless. In UVCS data the events were seen as 4-13 deg wide
enhancements of the strongest coronal lines HI Ly-alpha and OVI (1032,1037 A).
We derived electron densities for several of the events from the Large Angle
Spectrometric Coronagraph (LASCO) C2 white light observations. They are
comparable to or smaller than densities inferred for other CMEs. We modeled the
observable properties of examples of the structured (1999 April 15) and
unstructured (1999 May 9) narrow CMEs at different heights in the corona
between 1.5 and 2 R(Sun). The derived electron temperatures, densities and
outflow speeds are similar for those two types of ejections. They were compared
with properties of polar coronal jets and other CMEs. We discuss different
scenarios of narrow CME formation either as a jet formed by reconnection onto
open field lines or CME ejected by expansion of closed field structures.
Overall, we conclude that the existing observations do not definitively place
the narrow CMEs into the jet or the CME picture, but the acceleration of the
1999 April 15 event resembles acceleration seen in many CMEs, rather than
constant speeds or deceleration observed in jets.
|
0301649v1
|
2003-02-05
|
Cluster Sunyaev-Zeldovich Effect Scaling Relations
|
X-ray observations of an "entropy floor" in nearby groups and clusters of
galaxies offer evidence that important non-gravitational processes, such as
radiative cooling and/or "preheating", have strongly influenced the evolution
of the intracluster medium (ICM). We examine how the presence of an entropy
floor modifies the thermal Sunyaev-Zeldovich (SZ) effect. A detailed analysis
of scaling relations between X-ray and SZ effect observables and also between
the two primary SZ effect observables is presented. We find that relationships
between the central Compton parameter and the temperature or mass of a cluster
are extremely sensitive to the presence of an entropy floor. The same is true
for correlations between the integrated Compton parameter and the X-ray
luminosity or the central Compton parameter. In fact, if the entropy floor is
as high as inferred in recent analyses of X-ray data, a comparison of these
correlations with both current and future SZ effect observations should show a
clear signature of this excess entropy. Moreover, because the SZ effect is
redshift-independent, the relations can potentially be used to track the
evolution of the cluster gas and possibly discriminate between the possible
sources of the excess entropy. To facilitate comparisons with observations, we
provide analytic fits to these scaling relations.
|
0302087v1
|
2003-03-20
|
The SZ Effect Signature of Excess Entropy in Distant, Massive Clusters
|
Studies of cluster X-ray scaling relations have led to suggestions that
non-gravitational processes, e.g., radiative cooling and/or "preheating", have
significantly modified the entropy of the intracluster medium (ICM). For the
first time, we test this hypothesis through a comparison of predicted thermal
Sunyaev-Zeldovich (SZ) effect scaling relations with available data from the
literature. One of the relations that we explore, in principle, depends solely
on SZ effect observations, thus offering an X-ray independent probe of the ICM.
A detailed comparison of the theoretical relations with the largest compilation
of high redshift SZ effect data to date indicates that the presence of an
entropy floor is favored by the data. Furthermore, the inferred level of that
floor, 300 keV cm^2 or higher, is comparable to that found in studies of X-ray
scaling relations of nearby massive clusters. Thus, we find no evidence for
significant evolution of the entropy floor out to z = 0.7. We further
demonstrate that the high quality data to be obtained from the upcoming
Sunyaev-Zeldovich Array (SZA) and the (soon-to-be) upgraded Owens Valley Radio
Observatory (OVRO) array will open powerful new windows into the properties of
the ICM. Specifically, the new measurements will allow for accurate
measurements of the ICM entropy for even the most distant galaxy clusters.
|
0303451v2
|
2005-08-01
|
Transit Photometry of the Core-Dominated Planet HD 149026b
|
We report g, V, and r photometric time series of HD 149026 spanning predicted
times of transit of the Saturn-mass planetary companion, which was recently
discovered by Sato and collaborators. We present a joint analysis of our
observations and the previously reported photometry and radial velocities of
the central star. We refine the estimate of the transit ephemeris to Tc [HJD] =
2453527.87455^{+0.00085}_{-0.00091} + N * 2.87598^{+0.00012}_{-0.00017}.
Assuming that the star has a radius of 1.45 +/- 0.10 R_Sun and a mass of 1.30
+/- 0.10 M_Sun, we estimate the planet radius to be 0.726 +/- 0.064 R_Jup,
which implies a mean density of 1.07^{+0.42}_{-0.30} g/cm^3. This density is
significantly greater than that predicted for models which include the effects
of stellar insolation and for which the planet has only a small core of solid
material. Thus we confirm that this planet likely contains a large core, and
that the ratio of core mass to total planet mass is more akin to that of Uranus
and Neptune than that of either Jupiter or Saturn.
|
0508051v1
|
2005-10-28
|
Photoionized HBeta Emission in NGC 5548: It Breathes!
|
Emission-line regions in active galactic nuclei and other photoionized
nebulae should become larger in size when the ionizing luminosity increases.
This 'breathing' effect is observed for the Hbeta emission in NGC 5548 by using
Hbeta and optical continuum lightcurves from the 13-year 1989-2001 AGN Watch
monitoring campaign. To model the breathing, we use two methods to fit the
observed lightcurves in detail: (i) parameterized models and, (ii) the MEMECHO
reverberation mapping code. Our models assume that optical continuum variations
track the ionizing radiation, and that the Hbeta variations respond with time
delays due to light travel time. By fitting the data using a delay map that is
allowed to change with continuum flux, we find that the strength of the Hbeta
response decreases and the time delay increases with ionizing luminosity. The
parameterized breathing models allow the time delay and the Hbeta flux to
depend on the continuum flux so that, the time delay is proportional to the
continuum flux to the power beta, and the Hbeta flux is proportional to the
continuum flux to the power alpha. Our fits give 0.1 < beta < 0.46 and 0.57 <
alpha < 0.66. alpha is consistent with previous work by Gilbert and Peterson
(2003) and Goad, Korista and Knigge (2004). Although we find beta to be flatter
than previously determined by Peterson et al. (2002) using cross-correlation
methods, it is closer to the predicted values from recent theoretical work by
Korista and Goad (2004).
|
0510800v1
|
2006-07-25
|
The Transit Light Curve (TLC) Project. I. Four Consecutive Transits of the Exoplanet XO-1b
|
We present RIz photometry of four consecutive transits of the newly
discovered exoplanet XO-1b. We improve upon the estimates of the transit
parameters, finding the planetary radius to be R_P = 1.184 +0.028/-0.018
R_Jupiter and the stellar radius to be R_S = 0.928 +0.018/-0.013 R_Sun,
assuming a stellar mass of M_S = 1.00 +/- 0.03 M_Sun. The uncertainties in the
planetary and stellar radii are dominated by the uncertainty in the stellar
mass. These uncertainties increase by a factor of 2-3 if a more conservative
uncertainty of 0.10 M_Sun is assumed for the stellar mass. Our estimate of the
planetary radius is smaller than that reported by McCullough et al. (2006) and
yields a mean density that is comparable to that of TrES-1 and HD 189733b. The
timings of the transits have an accuracy ranging from 0.2 to 2.5 minutes, and
are marginally consistent with a uniform period.
|
0607571v1
|
2006-09-12
|
TrES-2: The First Transiting Planet in the Kepler Field
|
We announce the discovery of the second transiting hot Jupiter discovered by
the Trans-atlantic Exoplanet Survey. The planet, which we dub TrES-2, orbits
the nearby star GSC 03549-02811 every 2.47063 days. From high-resolution
spectra, we determine that the star has T_eff = 5960 +/- 100 K and log(g) = 4.4
+/- 0.2, implying a spectral type of G0V and a mass of 1.08 +0.11/-0.05 M_sun.
High-precision radial-velocity measurements confirm a sinusoidal variation with
the period and phase predicted by the photometry, and rule out the presence of
line-bisector variations that would indicate that the spectroscopic orbit is
spurious. We estimate a planetary mass of 1.28 +0.09/-0.04 M_Jup. We model B,
r, R, and I photometric timeseries of the 1.4%-deep transits and find a
planetary radius of 1.24 +0.09/-0.06 R_Jup. This planet lies within the field
of view of the NASA Kepler mission, ensuring that hundreds of upcoming transits
will be monitored with exquisite precision and permitting a host of
unprecedented investigations.
|
0609335v1
|
2006-10-19
|
Precise Radius Estimates for the Exoplanets WASP-1b and WASP-2b
|
We present precise z-band photometric time series spanning times of transit
of the two exoplanets recently discovered by the SuperWASP collaboration. We
find planetary radii of 1.44 +/- 0.08 R_J and 1.04 +/- 0.06 R_J for WASP-1b and
WASP-2b, respectively. These error estimates include both random errors in the
photometry and also the uncertainty in the stellar masses. Our results are 5
times more precise than the values derived from the discovery data alone. Our
measurement of the radius of WASP-2b agrees with previously published models of
hot Jupiters that include both a 20-M_Earth core of solid material and the
effects of stellar insolation. In contrast, we find that the models cannot
account for the large size of WASP-1b, even if the planet has no core. Thus, we
add WASP-1b to the growing list of hot Jupiters that are larger than expected.
This suggests that ``inflated'' hot Jupiters are more common than previously
thought, and that any purported explanations involving highly unusual
circumstances are disfavored.
|
0610589v1
|
2007-02-16
|
The Extended Star Formation History of the Andromeda Spheroid at Twenty One Kiloparsecs on the Minor Axis
|
Using the HST ACS, we have obtained deep optical images of a southeast
minor-axis field in the Andromeda Galaxy, 21 kpc from the nucleus. In both star
counts and metallicity, this field represents a transition zone between the
metal-rich, highly-disturbed inner spheroid that dominates within 15 kpc and
the metal-poor, diffuse population that dominates beyond 30 kpc. The
color-magnitude diagram reaches well below the oldest main-sequence turnoff in
the population, allowing a reconstruction of the star formation history in this
field. Compared to the spheroid population at 11 kpc, the population at 21 kpc
is ~1.3 Gyr older and ~0.2 dex more metal-poor, on average. However, like the
population at 11 kpc, the population at 21 kpc exhibits an extended star
formation history; one third of the stars are younger than 10 Gyr, although
only a few percent are younger than 8 Gyr. The relatively wide range of
metallicity and age is inconsistent with a single, rapid star-formation
episode, and instead suggests that the spheroid even at 21 kpc is dominated by
the debris of earlier merging events likely occurring more than 8 Gyr ago.
|
0702448v1
|
2007-02-23
|
Discovery of Andromeda XIV: A Dwarf Spheroidal Dynamical Rogue in the Local Group?
|
In the course of our survey of the outer halo of the Andromeda Galaxy we have
discovered a remote, possible satellite of that system at a projected 162 kpc
(11.7 degrees) radius. The fairly elongated (0.31 +/- 0.09 ellipticity) dwarf
can be fit with a King profile of 1.07 kpc (d/784 kpc) limiting radius, where
the satellite distance, d, is estimated at ~630-850 kpc from the tip of the red
giant branch. The newfound galaxy, ``Andromeda XIV'' (``AndXIV''),
distinguishes itself from other Local Group galaxies by its extreme dynamics:
Keck/DEIMOS spectroscopy reveals it to have a large heliocentric radial
velocity (-481 km/s), or -206 km/s velocity relative to M31. Even at its
projected radius AndXIV already is at the M31 escape velocity based on the
latest M31 mass models. If AndXIV is bound to M31, then recent models with
reduced M31 virial masses need revision upward. If not bound to M31, then
AndXIV is just now falling into the Local Group for the first time and
represents a dwarf galaxy that formed and spent almost its entire life in
isolation.
|
0702635v2
|
1998-05-27
|
Survival Analysis, Master Equation, Efficient Simulation of Path-Related Quantities, and Hidden State Concept of Transitions
|
This paper presents and derives the interrelations between survival analysis
and master equation. Survival analysis deals with modeling the transitions
between succeeding states of a system in terms of hazard rates. Questions
related with this are the timing and sequencing of the states of a time series.
The frequency and characteristics of time series can be investigated by
Monte-Carlo simulations. If one is interested in cross-sectional data connected
with the stochastic process under consideration, one needs to know the temporal
evolution of the distribution of states. This can be obtained by simulation of
the associated master equation. Some new formulas allow the determination of
path-related (i.e. longitudinal) quantities like the occurence probability, the
occurence time distribution, or the effective cumulative life-time distribution
of a certain sequencing of states (path). These can be efficiently evaluated
with a recently developed simulation tool (EPIS). The effective cumulative
life-time distribution facilitates the formulation of a hidden state concept of
behavioral changes which allows an interpretation of the respective
time-dependence of hazard rates. Hidden states represent states which are
either not phenomenological distinguishable from other states, not externally
measurable, or simply not detected.
|
9805361v1
|
1999-09-09
|
Micromagnetic simulations of thermally activated magnetization reversal of nanoscale magnets
|
Numerical integration of a stochastic Landau-Lifshitz-Gilbert equation is
used to study dynamic processes in single-domain nanoscale magnets at nonzero
temperatures. Special attention is given to including thermal fluctuations as a
Langevin term, and the Fast Multipole Method is used to calculate dipole-dipole
interactions. It is feasible to simulate these dynamics on the nanosecond time
scale for spatial discretizations that involve on the order of 10000 nodes
using a desktop workstation. The nanoscale magnets considered here are single
pillars with large aspect ratio. Hysteresis-loop simulations are employed to
study the stable and metastable configurations of the magnetization. Each
pillar has magnetic end caps. In a time-dependent field the magnetization of
the pillars is observed to reverse via nucleation, propagation, and coalescence
of the end caps. In particular, the end caps propagate into the magnet and meet
near the middle. A relatively long-lived defect is formed when end caps with
opposite vorticity meet. Fluctuations are more important in the reversal of the
magnetization for fields weaker than the zero-temperature coercive field, where
the reversal is thermally activated. In this case, the process must be
described by its statistical properties, such as the distribution of switching
times, averaged over a large number of independent thermal histories.
|
9909136v2
|
2000-10-12
|
Thermal Magnetization Reversal in Arrays of Nanoparticles
|
The results of large-scale simulations investigating the dynamics of
magnetization reversal in arrays of single-domain nanomagnets after a rapid
reversal of the applied field at nonzero temperature are presented. The
numerical micromagnetic approach uses the Landau-Lifshitz-Gilbert equation
including contributions from thermal fluctuations and long-range dipole-dipole
demagnetizing effects implemented using a fast-multipole expansion. The
individual model nanomagnets are 9 nm x 9 nm x 150 nm iron pillars similar to
those fabricated on a surface with STM-assisted chemical vapor deposition [S.
Wirth, et al., J. Appl. Phys {\bf 85}, 5249 (1999)]. Nanomagnets oriented
perpendicular to the surface and spaced 300 nm apart in linear arrays are
considered. The applied field is always oriented perpendicular to the surface.
When the magnitude of the applied field is less than the coercive value, about
2000 Oe for an individual nanomagnet, magnetization reversal in the nanomagnets
can only occur by thermally activated processes. Even though the interaction
from the dipole moment of neighboring magnets in this geometry is only about 1
Oe, less than 1% of the coercive field, it can have a large impact on the
switching dynamics. What determines the height of the free-energy barrier is
the difference between the coercive and applied fields, and 1 Oe can be a
significant fraction of that. The magnetic orientations of the neighbors are
seen to change the behavior of the nanomagnets in the array significantly.
|
0010184v2
|
2001-12-12
|
High Magnetic Field NMR Studies of LiVGe$_2$O$_6$, a quasi 1-D Spin $S = 1$ System
|
We report $^{7}$Li pulsed NMR measurements in polycrystalline and single
crystal samples of the quasi one-dimensional S=1 antiferromagnet
LiVGe$_2$O$_6$, whose AF transition temperature is $T_{\text{N}}\simeq 24.5$ K.
The field ($B_0$) and temperature ($T$) ranges covered were 9-44.5 T and
1.7-300 K respectively. The measurements included NMR spectra, the spin-lattice
relaxation rate ($T_1^{-1}$), and the spin-phase relaxation rate ($T_2^{-1}$),
often as a function of the orientation of the field relative to the crystal
axes. The spectra indicate an AF magnetic structure consistent with that
obtained from neutron diffraction measurements, but with the moments aligned
parallel to the c-axis. The spectra also provide the $T$-dependence of the AF
order parameter and show that the transition is either second order or weakly
first order. Both the spectra and the $T_1^{-1}$ data show that $B_0$ has at
most a small effect on the alignment of the AF moment. There is no spin-flop
transition up to 44.5 T. These features indicate a very large magnetic
anisotropy energy in LiVGe$_2$O$_6$ with orbital degrees of freedom playing an
important role. Below 8 K, $T_1^{-1}$ varies substantially with the orientation
of $B_0$ in the plane perpendicular to the c-axis, suggesting a small energy
gap for magnetic fluctuations that is very anisotropic.
|
0112203v1
|
2003-10-07
|
Endogenous Versus Exogenous Shocks in Complex Networks: an Empirical Test Using Book Sale Ranking
|
Are large biological extinctions such as the Cretaceous/Tertiary KT boundary
due to a meteorite, extreme volcanic activity or self-organized critical
extinction cascades? Are commercial successes due to a progressive reputation
cascade or the result of a well orchestrated advertisement? Determining the
chain of causality for extreme events in complex systems requires disentangling
interwoven exogenous and endogenous contributions with either no clear or too
many signatures. Here, we study the precursory and recovery signatures
accompanying shocks, that we test on a unique database of the Amazon sales
ranking of books. We find clear distinguishing signatures classifying two types
of sales peaks. Exogenous peaks occur abruptly and are followed by a power law
relaxation, while endogenous sale peaks occur after a progressively
accelerating power law growth followed by an approximately symmetrical power
law relaxation which is slower than for exogenous peaks. These results are
rationalized quantitatively by a simple model of epidemic propagation of
interactions with long memory within a network of acquaintances. The slow
relaxation of sales implies that the sales dynamics is dominated by cascades
rather than by the direct effects of news or advertisements, indicating that
the social network is close to critical.
|
0310135v2
|
2004-08-18
|
Irreversible spin-transfer and magnetization reversal under spin-injection
|
In the context of spin electronics, the two spin-channel model assumes that
the spin carriers are composed of two distinct populations: the conduction
electrons of spin up, and the conduction electrons of spin down. In order to
distinguish the paramagnetic and ferromagnetic contributions in spin injection,
we describe the current injection with four channels : the two spin populations
of the conduction bands ($s$ or paramagnetic) and the two spin populations of
the more correlated electrons ($d$ or ferromagnetic). The redistribution of the
conduction electrons at the interface is described by relaxation mechanisms
between the channels. Providing that the $d$ majority-spin band is frozen,
$s-d$ relaxation essentially concerns the minority-spin channels. Accordingly,
even in the abscence of spin-flip scattering (i.e. without standard
spin-accumulation or giant magnetoresistance), the $s-d$ relaxation leads to a
$d$ spin accumulation effect. The coupled diffusion equations for the two
relaxation processes ($s-d$ and spin-flip) are derived. The link with the
ferromagnetic order parameter $\vec{M}$ is performed by assuming that only the
$d$ channel contributes to the Landau-Lifshitz-Gilbert equation. The effect of
magnetization reversal induced by spin injection is explained by these
relaxations under the assumption that the spins of the conduction electrons act
as environmental degrees of freedom on the magnetization.
|
0408410v1
|
2004-12-02
|
Dielectric resonances in disordered media
|
Binary disordered systems are usually obtained by mixing two ingredients in
variable proportions: conductor and insulator, or conductor and
super-conductor. and are naturally modeled by regular bi-dimensional or
tri-dimensional lattices, on which sites or bonds are chosen randomly with
given probabilities. In this article, we calculate the impedance of the
composite by two independent methods: the so-called spectral method, which
diagonalises Kirchhoff's Laws via a Green function formalism, and the Exact
Numerical Renormalization method (ENR). These methods are applied to mixtures
of resistors and capacitors (R-C systems), simulating e.g. ionic
conductor-insulator systems, and to composites consituted of resistive
inductances and capacitors (LR-C systems), representing metal inclusions in a
dielectric bulk. The frequency dependent impedances of the latter composites
present very intricate structures in the vicinity of the percolation threshold.
We analyse the LR-C behavior of compounds formed by the inclusion of small
conducting clusters (``$n$-legged animals'') in a dielectric medium. We
investigate in particular their absorption spectra who present a pattern of
sharp lines at very specific frequencies of the incident electromagnetic field,
the goal being to identify the signature of each animal. This enables us to
make suggestions of how to build compounds with specific absorption or
transmission properties in a given frequency domain.
|
0412061v1
|
2005-01-12
|
Current induced magnetization switching in exchange biased spin-valves for CPP-GMR heads
|
In contrast to earlier studies performed on simple Co/Cu/Co sandwiches, we
have investigated spin transfer effects in complex spin-valve pillars with a
diameter of 130nm developed for current-perpendicular to the plane (CPP)
magneto-resistive heads. The structure of the samples included an exchange
biased synthetic pinned layer and a free layer both laminated by insertion of
several ultrathin Cu layers. Despite the small thickness of the polarizing
layer, our results show that the free layer can be switched between the
parallel (P) and the antiparallel (AP) states by applying current densities of
the order of 10^7 A/cm^2. A strong asymmetry is observed between the two
critical currents IcAP-P and IcP-AP, as predicted by the model of Slonczewski
model. Thanks to the use of exchange biased structures, the stability phase
diagrams could be obtained in the four quadrants of the (H, I) plan. The
critical lines derived from the magnetoresistance curves measured with
different sense currents, and from the resistance versus current curves
measured for different applied fields, match each other very well. The main
features of the phase diagrams can be reproduced by investigating the stability
of the solutions of the Landau Lifshitz Gilbert equation including spin torque
term within a macrospin model. A spin-transfer saturation effect was observed
in the positive currents range. We attribute it to a de-depolarization effect
which appears as a consequence of the asymmetric heating of the pillars, whose
top and the bottom leads are made of different materials.
|
0501281v1
|
2005-12-20
|
Theory of Spin Torque in a nanomagnet
|
We present a complete theory of the spin torque phenomena in a ultrasmall
nanomagnet coupled to non-collinear ferromagnetic electrodes through tunnelling
junctions. This model system can be described by a simple microscopic model
which captures many physical effects characteristic of spintronics: tunneling
magneto resistance, intrinsic and transport induced magnetic relaxation,
current induced magnetization reversal and spin accumulation. Treating on the
same footing the magnetic and transport degrees of freedom, we arrive at a
closed equation for the time evolution of the magnetization. This equation is
very close to the Landau-Lifshitz-Gilbert equation used in spin valves
structures. We discuss how the presence of the Coulomb blockade phenomena and
the discretization of the one-body spectrum gives some additional features to
the current induced spin torque. Depending on the regime, the dynamic induced
by the coupling to electrode can be viewed either as a spin torque or as a
relaxation process. In addition to the possibility of stabilizing uniform spin
precession states, we find that the system is highly hysteretic: up to three
different magnetic states can be simultaneously stable in one region of the
parameter space (magnetic field and bias voltage).We also discuss how the
magneto-resistance can be used to provide additional information on the
non-equilibrium peaks present in the nanomagnet spectroscopy experiments.
|
0512508v1
|
2005-09-19
|
Capacity-Achieving Codes with Bounded Graphical Complexity on Noisy Channels
|
We introduce a new family of concatenated codes with an outer low-density
parity-check (LDPC) code and an inner low-density generator matrix (LDGM) code,
and prove that these codes can achieve capacity under any memoryless
binary-input output-symmetric (MBIOS) channel using maximum-likelihood (ML)
decoding with bounded graphical complexity, i.e., the number of edges per
information bit in their graphical representation is bounded. In particular, we
also show that these codes can achieve capacity on the binary erasure channel
(BEC) under belief propagation (BP) decoding with bounded decoding complexity
per information bit per iteration for all erasure probabilities in (0, 1). By
deriving and analyzing the average weight distribution (AWD) and the
corresponding asymptotic growth rate of these codes with a rate-1 inner LDGM
code, we also show that these codes achieve the Gilbert-Varshamov bound with
asymptotically high probability. This result can be attributed to the presence
of the inner rate-1 LDGM code, which is demonstrated to help eliminate high
weight codewords in the LDPC code while maintaining a vanishingly small amount
of low weight codewords.
|
0509062v3
|
2006-07-20
|
List decoding of noisy Reed-Muller-like codes
|
First- and second-order Reed-Muller (RM(1) and RM(2), respectively) codes are
two fundamental error-correcting codes which arise in communication as well as
in probabilistically-checkable proofs and learning. In this paper, we take the
first steps toward extending the quick randomized decoding tools of RM(1) into
the realm of quadratic binary and, equivalently, Z_4 codes. Our main
algorithmic result is an extension of the RM(1) techniques from Goldreich-Levin
and Kushilevitz-Mansour algorithms to the Hankel code, a code between RM(1) and
RM(2). That is, given signal s of length N, we find a list that is a superset
of all Hankel codewords phi with dot product to s at least (1/sqrt(k)) times
the norm of s, in time polynomial in k and log(N). We also give a new and
simple formulation of a known Kerdock code as a subcode of the Hankel code. As
a corollary, we can list-decode Kerdock, too. Also, we get a quick algorithm
for finding a sparse Kerdock approximation. That is, for k small compared with
1/sqrt{N} and for epsilon > 0, we find, in time polynomial in (k
log(N)/epsilon), a k-Kerdock-term approximation s~ to s with Euclidean error at
most the factor (1+epsilon+O(k^2/sqrt{N})) times that of the best such
approximation.
|
0607098v2
|
1994-05-31
|
The Behavior of a Spherical Hole in an Infinite Uniform Universe
|
In this paper, the behavior of a spherical hole in an otherwise infinite and
uniform universe is investigated. First, the Newtonian theory is developed. The
concept of negative gravity, an outward gravitational force acting away from
the center of the spherical hole, is presented, and the resulting expansion of
the hole is investigated. Then, the same result is derived using the techniques
of Einstein's theory of general relativity. The field equations are solved for
an infinite uniform universe and then for an infinite universe in which matter
is uniformly distributed except for a spherical hole. Negative pressure caused
by negative gravity is utilized. The physical significance of the cosmological
constant is explained, and a new physical concept, that of the gravitational
potential of a hole, is discussed. The relationship between the Newtonian
potential for a hole and the Schwarzschild solution of the field equations is
explored. Finally, the geodesic equations are considered. It is shown that
photons and particles are deflected away from the hole. An application of this
idea is pursued, in which a new cosmology based upon expanding holes in a
uniform universe is developed. The microwave background radiation and Hubble's
Law, among others, are explained. Finally, current astronomical data are used
to compute a remarkably accurate value of Hubble's constant, as well as
estimates of the average mass density of the universe and the cosmological
constant.
|
9405075v1
|
2002-11-21
|
SuSpect: a Fortran Code for the Supersymmetric and Higgs Particle Spectrum in the MSSM
|
We present the Fortran code SuSpect version 2.3, which calculates the
Supersymmetric and Higgs particle spectrum in the Minimal Supersymmetric
Standard Model (MSSM). The calculation can be performed in constrained models
with universal boundary conditions at high scales such as the gravity (mSUGRA),
anomaly (AMSB) or gauge (GMSB) mediated breaking models, but also in the
non-universal MSSM case with R-parity and CP conservation. Care has been taken
to treat important features such as the renormalization group evolution of
parameters between low and high energy scales, the consistent implementation of
radiative electroweak symmetry breaking and the calculation of the physical
masses of the Higgs bosons and supersymmetric particles taking into account the
dominant radiative corrections. Some checks of important theoretical and
experimental features, such as the absence of non desired minima, large
fine-tuning in the electroweak symmetry breaking condition, as well as
agreement with precision measurements can be performed. The program is user
friendly, simple to use, self-contained and can easily be linked with other
codes; it is rather fast and flexible, thus allowing scans of the parameter
space with several possible options and choices for model assumptions and
approximations.
|
0211331v2
|
2004-11-04
|
Theoretical and Experimental Analysis of a Randomized Algorithm for Sparse Fourier Transform Analysis
|
We analyze a sublinear RAlSFA (Randomized Algorithm for Sparse Fourier
Analysis) that finds a near-optimal B-term Sparse Representation R for a given
discrete signal S of length N, in time and space poly(B,log(N)), following the
approach given in \cite{GGIMS}. Its time cost poly(log(N)) should be compared
with the superlinear O(N log N) time requirement of the Fast Fourier Transform
(FFT). A straightforward implementation of the RAlSFA, as presented in the
theoretical paper \cite{GGIMS}, turns out to be very slow in practice. Our main
result is a greatly improved and practical RAlSFA. We introduce several new
ideas and techniques that speed up the algorithm. Both rigorous and heuristic
arguments for parameter choices are presented. Our RAlSFA constructs, with
probability at least 1-delta, a near-optimal B-term representation R in time
poly(B)log(N)log(1/delta)/ epsilon^{2} log(M) such that
||S-R||^{2}<=(1+epsilon)||S-R_{opt}||^{2}. Furthermore, this RAlSFA
implementation already beats the FFTW for not unreasonably large N. We extend
the algorithm to higher dimensional cases both theoretically and numerically.
The crossover point lies at N=70000 in one dimension, and at N=900 for data on
a N*N grid in two dimensions for small B signals where there is noise.
|
0411102v2
|
2006-12-04
|
Krull dimension and deviation in certain parafree groups
|
Hanna Neumann asked whether it was possible for two non-isomorphic residually
nilpotent finitely generated (fg) groups, one of them free, to share the lower
central sequence. Gilbert Baumslag answered the question in the affirmative and
thus gave rise to parafree groups. A group G is termed parafree of rank n if it
is residually nilpotent and shares the lower central sequence with a free group
of rank n. The deviation of a finitely generated (fg) parafree group G is the
difference between the minimum possible number of generators of G and the rank
of G.
Let G be a fg group, then Hom(G,SL(2, C)) inherits the structure of an
algebraic variety, denoted by R(G), and known as its "representation variety".
If G is an n generated parafree group, then the deviation of G is 0 iff
Dim(R(G))=3n. It is known that for n \ge 2 there exist infinitely many parafree
groups of rank n and deviation 1 with non-isomorphic representation varieties
of dimension 3n. In this paper it is shown that given integers n \ge 2, and k
\ge 1, there exist infinitely many parafree groups of rank n and deviation k
with non-isomorphic representation varieties of dimension different from 3n; in
particular, it is shown that there exist infinitely many parafree groups G of
rank n with Dim(R(G))> q, where q \ge 3n is an arbitrary integer.
|
0612102v2
|
2004-07-27
|
Domain wall dynamics driven by adiabatic spin transfer torques
|
In a first approximation, known as the adiabatic process, the direction of
the spin polarization of currents is parallel to the local magnetization vector
in a domain wall. Thus the spatial variation of the direction of the spin
current inside the domain wall results in an adiabatical spin transfer torque
on the magnetization. We show that domain wall motion driven by this spin
torque has many unique features that do not exist in the conventional wall
motion driven by a magnetic field. By analytically and numerically solving the
Landau-Lifshitz-Gilbert equation along with the adiabatic spin torque in
magnetic nanowires, we find the domain wall has its maximum velocity at the
initial application of the current but the velocity decreases to zero as the
domain wall begins to deform during its motion. We have computed domain wall
displacement and domain wall deformation of nanowires, and concluded that the
spin torque based on the adiabatic propagation of the spin current in the
domain wall is unable to maintain wall movement. We also introduce a novel
concept of domain wall inductance to characterize the capacity of the
spin-torque induced magnetic energy stored in a domain wall. In the presence of
domain wall pinning centers, we construct a phase diagram for the domain wall
depinning by the combined action of the magnetic field and the spin current.
|
0407064v1
|
1997-02-07
|
Two-pion correlations in Au+Au collisions at 10.8 GeV/c per nucleon
|
Two-particle correlation functions for positive and negative pions have been
measured in Au+Au collisions at 10.8~GeV/c per nucleon. The data were analyzed
using one- and three-dimensional correlation functions. From the results of the
three-dimensional fit the phase space density of pions was calculated. It is
consistent with local thermal equilibrium.
|
9702008v1
|
2007-01-30
|
Huddling behavior in emperor penguins : dynamics of huddling
|
Although huddling was shown to be the key by which emperor penguins
(Aptenodytes forsteri) save energy and sustain their breeding fast during the
Antarctic winter, the intricacies of this social behavior have been poorly
studied. We recorded abiotic variables with data loggers glued to the feathers
of eight individually marked emperor penguins to investigate their
thermoregulatory behavior and to estimate their "huddling time budget"
throughout the breeding season (pairing and incubation period). Contrary to the
classic view, huddling episodes were discontinuous and of short and variable
duration, lasting 1.6+/-1.7 (S.D.) h on average. Despite heterogeneous huddling
groups, birds had equal access to the warmth of the huddles. Throughout the
breeding season, males huddled for 38+/-18% (S.D.) of their time, which raised
the ambient temperature that birds were exposed to above 0 degrees C (at
average external temperatures of -17 degrees C). As a consequence of tight
huddles, ambient temperatures were above 20 degrees C during 13+/-12% (S.D.) of
their huddling time. Ambient temperatures increased up to 37.5 degrees C, close
to birds' body temperature. This complex social behavior therefore enables all
breeders to get a regular and equal access to an environment which allows them
to save energy and successfully incubate their eggs during the Antarctic
winter.
|
0701051v1
|
2003-10-14
|
The Minimum Distance Problem for Two-Way Entanglement Purification
|
Entanglement purification takes a number of noisy EPR pairs and processes
them to produce a smaller number of more reliable pairs. If this is done with
only a forward classical side channel, the procedure is equivalent to using a
quantum error-correcting code (QECC). We instead investigate entanglement
purification protocols with two-way classical side channels (2-EPPs) for finite
block sizes. In particular, we consider the analog of the minimum distance
problem for QECCs, and show that 2-EPPs can exceed the quantum Hamming bound
and the quantum Singleton bound. We also show that 2-EPPs can achieve the rate
k/n = 1 - (t/n) \log_2 3 - h(t/n) - O(1/n) (asymptotically reaching the quantum
Hamming bound), where the EPP produces at least k good pairs out of n total
pairs with up to t arbitrary errors, and h(x) = -x \log_2 x - (1-x) \log_2
(1-x) is the usual binary entropy. In contrast, the best known lower bound on
the rate of QECCs is the quantum Gilbert-Varshamov bound k/n \geq 1 - (2t/n)
\log_2 3 - h(2t/n). Indeed, in some regimes, the known upper bound on the
asymptotic rate of good QECCs is strictly below our lower bound on the
achievable rate of 2-EPPs.
|
0310097v4
|
2005-06-02
|
Enhanced algorithms for Local Search
|
Let G=(V,E) be a finite graph, and f:V->N be any function. The Local Search
problem consists in finding a local minimum of the function f on G, that is a
vertex v such that f(v) is not larger than the value of f on the neighbors of v
in G. In this note, we first prove a separation theorem slightly stronger than
the one of Gilbert, Hutchinson and Tarjan for graphs of constant genus. This
result allows us to enhance a previously known deterministic algorithm for
Local Search with query complexity O(\log n)\cdot d+O(\sqrt{g})\cdot\sqrt{n},
so that we obtain a deterministic query complexity of
d+O(\sqrt{g})\cdot\sqrt{n}, where n is the size of G, d is its maximum degree,
and $g$ is its genus. We also give a quantum version of our algorithm, whose
query complexity is of O(\sqrt{d})+O(\sqrt[4]{g})\cdot\sqrt[4]{n}\log\log n.
Our deterministic and quantum algorithms have query complexities respectively
smaller than the generic algorithms of Aldous and of Aaronson for large classes
of graphs, including graphs of bounded genus and planar graphs. Independently
from this work, Zhang has recently given a quantum algorithm which finds a
local minimum on the planar grid over \{1,...,\sqrt{n}\}^2 using
O(\sqrt[4]{n}(\log\log n)^2) queries. Our quantum algorithm can be viewed as a
strongly generalized, and slightly enhanced version of this algorithm.
|
0506019v1
|
2007-09-27
|
Predictions of the causal entropic principle for environmental conditions of the universe
|
The causal entropic principle has been proposed as a superior alternative to
the anthropic principle for understanding the magnitude of the cosmological
constant. In this approach, the probability to create observers is assumed to
be proportional to the entropy production \Delta S in a maximal causally
connected region -- the causal diamond. We improve on the original treatment by
better quantifying the entropy production due to stars, using an analytic model
for the star formation history which accurately accounts for changes in
cosmological parameters. We calculate the dependence of \Delta S on the density
contrast Q=\delta\rho/\rho, and find that our universe is much closer to the
most probable value of Q than in the usual anthropic approach and that
probabilities are relatively weakly dependent on this amplitude. In addition,
we make first estimates of the dependence of \Delta S on the baryon fraction
and overall matter abundance. Finally, we also explore the possibility that
decays of dark matter, suggested by various observed gamma ray excesses, might
produce a comparable amount of entropy to stars.
|
0709.4443v2
|
2007-10-24
|
The Impact of Halo Properties, Energy Feedback and Projection Effects on the Mass-SZ Flux Relation
|
We present a detailed analysis of the intrinsic scatter in the integrated SZ
effect - cluster mass (Y-M) relation, using semi-analytic and simulated cluster
samples. Specifically, we investigate the impact on the Y-M relation of energy
feedback, variations in the host halo concentration and substructure
populations, and projection effects due to unresolved clusters along the line
of sight (the SZ background). Furthermore, we investigate at what radius (or
overdensity) one should measure the integrated SZE and define cluster mass so
as to achieve the tightest possible scaling. We find that the measure of Y with
the least scatter is always obtained within a smaller radius than that at which
the mass is defined; e.g. for M_{200} (M_{500}) the scatter is least for
Y_{500} (Y_{1100}). The inclusion of energy feedback in the gas model
significantly increases the intrinsic scatter in the Y-M relation due to larger
variations in the gas mass fraction compared to models without feedback. We
also find that variations in halo concentration for clusters of a given mass
may partly explain why the integrated SZE provides a better mass proxy than the
central decrement. Substructure is found to account for approximately 20% of
the observed scatter in the Y-M relation. Above M_{200} = 2x10^{14} h^{-1}
msun, the SZ background does not significantly effect cluster mass
measurements; below this mass, variations in the background signal reduce the
optimal angular radius within which one should measure Y to achieve the
tightest scaling with M_{200}.
|
0710.4555v1
|
2007-10-31
|
Spin-Torque Driven Magnetization Dynamics: Micromagnetic Modelling
|
In this paper we present an overview of recent progress made in the
understanding of the spin-torque induced magnetization dynamics in nanodevices
using mesoscopic micromagnetic simulations. We first specify how a spin-torque
term may be added to the usual Landau-Lifshitz-Gilbert equation of
magnetization motion and detail its physical meaning. After a brief description
of spin-torque driven dynamics in the macrospin approximation, we discuss the
validity of this approximation for various experimentally relevant geometries.
Next, we perform a detailed comparison between accurate experimental data
obtained from nanopillar devices and corresponding numerical modelling. We show
that, on the one hand, many qualitatively important features of the observed
magnetization dynamics (e.g., non-linear frequency shift and frequency jumps
with increasing current) can be satisfactory explained by sophisticated
micromagnetic models, but on the other hand, understanding of these experiments
is still far from being complete. We proceed with the numerical analysis of
point-contact experiments, where an even more complicated magnetization
dynamics is observed. Simulations reveal that such a rich behaviour is due to
the formation of several strongly non-linear oscillation modes. In the last
part of the paper we emphasize the importance of sample characterization and
conclude with some important remarks concerning the relation between
micromagnetic modelling and real experiments.
|
0710.5924v1
|
2007-11-19
|
The Kohn-Sham system in one-matrix functional theory
|
A system of electrons in a local or nonlocal external potential can be
studied with 1-matrix functional theory (1MFT), which is similar to density
functional theory (DFT) but takes the one-particle reduced density matrix
(1-matrix) instead of the density as its basic variable. Within 1MFT, Gilbert
derived [PRB 12, 2111 (1975)] effective single-particle equations analogous to
the Kohn-Sham (KS) equations in DFT. The self-consistent solution of these
1MFT-KS equations reproduces not only the density of the original electron
system but also its 1-matrix. While in DFT it is usually possible to reproduce
the density using KS orbitals with integer (0 or 1) occupancy, in 1MFT
reproducing the 1-matrix requires in general fractional occupancies. The
variational principle implies that the KS eigenvalues of all fractionally
occupied orbitals must collapse at self-consistency to a single level, equal to
the chemical potential. We show that as a consequence of the degeneracy the
iteration of the KS equations is intrinsically divergent. Fortunately, the
level shifting method, commonly introduced in Hartree-Fock calculations, is
always able to force convergence. We introduce an alternative derivation of the
1MFT-KS equations that allows control of the eigenvalue collapse by
constraining the occupancies. As an explicit example, we apply the 1MFT-KS
scheme to calculate the ground state 1-matrix of an exactly solvable two-site
Hubbard model.
|
0711.2996v1
|
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