554 research outputs found
QED can explain the non-thermal emission from SGRs and AXPs : Variability
Owing to effects arising from quantum electrodynamics (QED),
magnetohydrodynamical fast modes of sufficient strength will break down to form
electron-positron pairs while traversing the magnetospheres of strongly
magnetised neutron stars. The bulk of the energy of the fast mode fuels the
development of an electron-positron fireball. However, a small, but potentially
observable, fraction of the energy ( ergs) can generate a
non-thermal distribution of electrons and positrons far from the star. This
paper examines the cooling and radiative output of these particles. Small-scale
waves may produce only the non-thermal emission. The properties of this
non-thermal emission in the absence of a fireball match those of the quiescent,
non-thermal radiation recently observed non-thermal emission from several
anomalous X-ray pulsars and soft-gamma repeaters. Initial estimates of the
emission as a function of angle indicate that the non-thermal emission should
be beamed and therefore one would expect this emission to be pulsed as well.
According to this model the pulsation of the non-thermal emission should be
between 90 and 180 degrees out of phase from the thermal emission from the
stellar surface.Comment: 7 pages, 5 figures, to appear in the proceedings of the conference
"Isolated Neutron Stars: from the Interior to the Surface" (April 2006,
London), eds. D. Page, R. Turolla, & S. Zane, Astrophysics & Space Scienc
R-Modes on Rapidly Rotating, Relativistic Stars: I. Do Type-I Bursts Excite Modes in the Neutron-Star Ocean?
During a Type-I burst, the turbulent deflagation front may excite waves in
the neutron star ocean and upper atmosphere with frequencies,
Hz. These waves may be observed as highly coherent flux oscillations during the
burst. The frequencies of these waves changes as the upper layers of the
neutron star cool which accounts for the small variation in the observed QPO
frequencies. In principle several modes could be excited but the fundamental
buoyant mode exhibits significantly larger variability for a given
excitation than all of the other modes. An analysis of modes in the burning
layers themselves and the underlying ocean shows that it is unlikely these
modes can account for the observed burst oscillations. On the other hand,
photospheric modes which reside in a cooler portion of the neutron star
atmosphere may provide an excellent explanation for the observed oscillations.Comment: 18 pages, 1 figure, substantial changes and additions to reflect
version to appear in Ap
Analytic Gravitational-Force Calculations for Models of the Kuiper Belt, with Application to the Pioneer Anomaly
We use analytic techniques to study the gravitational force that would be
produced by different Kuiper-Belt mass distributions. In particular, we study
the 3-dimensional rings (and wedge) whose densities vary as the inverse of the
distance, as a constant, as the inverse-squared of the distance, as well as
that which varies according to the Boss-Peale model. These analytic
calculations yield physical insight into the physics of the problem. They also
verify that physically viable models of this type can produce neither the
magnitude nor the constancy of the Pioneer anomaly.Comment: 11 pages, 8 figures, final corrections for publicatio
Skin disease in the Western Cape Province: a survey the incidence of skin diseases in 2,500 White and 1,500 Coloured patients
Population Synthesis of Pulsars: Magnetic Field Effects
New results based on methods of population synthesis, concerning magnetic
field effects on the evolution of pulsars are reported. The present study
confirms that models with timescales for the magnetic field decay longer than
the pulsar lifetime are in better agreement with data. These new simulations
indicate that the diagram log() - log() alone cannot be used to
test field decay models. The dispersion of the values of the initial period and
magnetic field can explain the observed behaviour of the data points in such a
diagram. The simulations also indicate that the statistical properties of
anomalous X-ray pulsars and soft-gamma repeaters (magnetar candidates) are
compatible with those derived for objects born in the high side tail of the
magnetic field distribution. The predicted birth rate of neutron stars having
field strengths in excess of 10 G is one object born each 750 yr.Comment: accepted for publication in Astronomy & Astrophysic
Real-time dynamics of lattice gauge theories with a few-qubit quantum computer
Gauge theories are fundamental to our understanding of interactions between
the elementary constituents of matter as mediated by gauge bosons. However,
computing the real-time dynamics in gauge theories is a notorious challenge for
classical computational methods. In the spirit of Feynman's vision of a quantum
simulator, this has recently stimulated theoretical effort to devise schemes
for simulating such theories on engineered quantum-mechanical devices, with the
difficulty that gauge invariance and the associated local conservation laws
(Gauss laws) need to be implemented. Here we report the first experimental
demonstration of a digital quantum simulation of a lattice gauge theory, by
realising 1+1-dimensional quantum electrodynamics (Schwinger model) on a
few-qubit trapped-ion quantum computer. We are interested in the real-time
evolution of the Schwinger mechanism, describing the instability of the bare
vacuum due to quantum fluctuations, which manifests itself in the spontaneous
creation of electron-positron pairs. To make efficient use of our quantum
resources, we map the original problem to a spin model by eliminating the gauge
fields in favour of exotic long-range interactions, which have a direct and
efficient implementation on an ion trap architecture. We explore the Schwinger
mechanism of particle-antiparticle generation by monitoring the mass production
and the vacuum persistence amplitude. Moreover, we track the real-time
evolution of entanglement in the system, which illustrates how particle
creation and entanglement generation are directly related. Our work represents
a first step towards quantum simulating high-energy theories with atomic
physics experiments, the long-term vision being the extension to real-time
quantum simulations of non-Abelian lattice gauge theories
Quantifying and Controlling Prethermal Nonergodicity in Interacting Floquet Matter
The use of periodic driving for synthesizing many-body quantum states depends crucially on the existence of a prethermal regime, which exhibits drive-tunable properties while forestalling the effects of heating. This dependence motivates the search for direct experimental probes of the underlying localized nonergodic nature of the wave function in this metastable regime. We report experiments on a many-body Floquet system consisting of atoms in an optical lattice subjected to ultrastrong sign-changing amplitude modulation. Using a double-quench protocol, we measure an inverse participation ratio quantifying the degree of prethermal localization as a function of tunable drive parameters and interactions. We obtain a complete prethermal map of the drive-dependent properties of Floquet matter spanning four square decades of parameter space. Following the full time evolution, we observe sequential formation of two prethermal plateaux, interaction-driven ergodicity, and strongly frequency-dependent dynamics of long-time thermalization. The quantitative characterization of the prethermal Floquet matter realized in these experiments, along with the demonstration of control of its properties by variation of drive parameters and interactions, opens a new frontier for probing far-from-equilibrium quantum statistical mechanics and new possibilities for dynamical quantum engineering
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