20,909 research outputs found
Relativistic Precessing Jets and Cosmological Gamma Ray Bursts
We discuss the possibility that gamma-ray bursts may result from cosmological
relativistic blob emitting neutron star jets that precess past the line of
sight. Beaming reduces the energy requirements, so that the jet emission can
last longer than the observed burst duration. One precession mode maintains a
short duration time scale, while a second keeps the beam from returning to the
line of sight, consistent with the paucity of repeaters. The long life of these
objects reduces the number required for production as compared to short lived
jets. Blobs can account for the time structure of the bursts. Here we focus
largely on kinematic and time scale considerations of beaming, precession, and
blobs--issues which are reasonably independent of the acceleration and jet
collimation mechanisms. We do suggest that large amplitude electro-magnetic
waves could be a source of blob acceleration.Comment: 15 pages, plain TeX, accepted to ApJ
Edgeworth expansions for slow-fast systems with finite time scale separation
We derive Edgeworth expansions that describe corrections to the Gaussian limiting behaviour of slow-fast systems. The Edgeworth expansion is achieved using a semi-group formalism for the transfer operator, where a Duhamel-Dyson series is used to asymptotically determine the corrections at any desired order of the time scale parameter ε. The corrections involve integrals over higher-order auto-correlation functions. We develop a diagrammatic representation of the series to control the combinatorial wealth of the asymptotic expansion in ε and provide explicit expressions for the first two orders. At a formal level, the expressions derived are valid in the case when the fast dynamics is stochastic as well as when the fast dynamics is entirely deterministic. We corroborate our analytical results with numerical simulations and show that our method provides an improvement on the classical homogenization limit which is restricted to the limit of infinite time scale separation
Childhood Hypertrophic Cardiomyopathy: A Disease of the Cardiac Sarcomere
Hypertrophic cardiomyopathy is the second most common cause of cardiomyopathy presenting during childhood and whilst its underlying aetiology is variable, the majority of disease is caused by sarcomeric protein gene variants. Sarcomeric disease can present at any age with highly variable disease phenotype, progression and outcomes. The majority have good childhood-outcomes with reported 5-year survival rates above 80%. However, childhood onset disease is associated with considerable life-long morbidity and mortality, including a higher SCD rate during childhood than seen in adults. Management is currently focused on relieving symptoms and preventing disease-related complications, but the possibility of future disease-modifying therapies offers an exciting opportunity to modulate disease expression and outcomes in these young patients
Simulation of a non-invasive charge detector for quantum cellular automata
Information in a Quantum Cellular Automata architecture is encoded in the
polarizazion state of a cell, i.e., in the occupation numbers of the quantum
dots of which the cell is made up. Non-invasive charge detectors of single
electrons in a quantum dot are therefore needed, and recent experiments have
shown that a quantum constriction electrostatically coupled to the quantum dot
may be a viable solution. We have performed a numerical simulation of a system
made of a quantum dot and a nearby quantum point contact defined, by means of
depleting metal gates, in a two-dimensional electron gas at a GaAs/AlGaAs
heterointerface. We have computed the occupancy of each dot and the resistance
of the quantum wire as a function of the voltage applied to the plunger gate,
and have derived design criteria for achieving optimal sensitivity.Comment: 8 pages, RevTeX, epsf, 5 figure
Area Decay Law Implementation for Quark String Fragmentation
We apply the Area Decay Law (ADL) straightforwardly to simulate a quark
string hadronization and compare the results with the explicit analytic
calculations. We show that the usual "inclusive" Monte--Carlo simulations do
not correspond to the ADL because of two mistakes: not proper simulation of
two--dimensional probability density and lack of an important combinatorial
factor in a binary tree simulation. We also show how to simulate area decay law
"inclusively" avoiding the above--mentioned mistakes.Comment: 5 pages (REVTEX) + 3 figures (available in ps format from
G.G.Leptoukh , IPGAS-HE/93-3, to be
published in Phys. Rev.
Local dependence of ion temperature gradient on magnetic configuration, rotational shear and turbulent heat flux in MAST
Experimental data from the Mega Amp Spherical Tokamak (MAST) is used to show
that the inverse gradient scale length of the ion temperature R/LTi (normalized
to the major radius R) has its strongest local correlation with the rotational
shear and the pitch angle of the magnetic field (or, equivalently, an inverse
correlation with q/{\epsilon}, the safety factor/the inverse aspect ratio).
Furthermore, R/LTi is found to be inversely correlated with the
gyro-Bohm-normalized local turbulent heat flux estimated from the density
fluctuation level measured using a 2D Beam Emission Spectroscopy (BES)
diagnostic. These results can be explained in terms of the conjecture that the
turbulent system adjusts to keep R/LTi close to a certain critical value
(marginal for the excitation of turbulence) determined by local equilibrium
parameters (although not necessarily by linear stability).Comment: 6 pages, 3 figures, submitted to PR
Cross-Correlation Studies between CMB Temperature Anisotropies and 21 cm Fluctuations
During the transition from a neutral to a fully reionized universe,
scattering of cosmic microwave background (CMB) photons via free-electrons
leads to a new anisotropy contribution to the temperature distribution. If the
reionization process is inhomogeneous and patchy, the era of reionization is
also visible via brightness temperature fluctuations in the redshifted 21 cm
line emission from neutral Hydrogen. Since regions containing electrons and
neutral Hydrogen are expected to trace the same underlying density field, the
two are (anti) correlated and this is expected to be reflected in the
anisotropy maps via a correlation between arcminute-scale CMB temperature and
the 21 cm background. In terms of the angular cross-power spectrum,
unfortunately, this correlation is insignificant due to a geometric
cancellation associated with second order CMB anisotropies. The same
cross-correlation between ionized and neutral regions, however, can be studied
using a bispectrum involving large scale velocity field of ionized regions from
the Doppler effect, arcminute scale CMB anisotropies during reionization, and
the 21 cm background. While the geometric cancellation is partly avoided, the
signal-to-noise ratio related to this bispectrum is reduced due to the large
cosmic variance related to velocity fluctuations traced by the Doppler effect.
Unless the velocity field during reionization can be independently established,
it is unlikely that the correlation information related to the relative
distribution of ionized electrons and regions containing neutral Hydrogen can
be obtained with a combined study involving CMB and 21 cm fluctuations.Comment: 10 pages, 3 figure
Transition to subcritical turbulence in a tokamak plasma
Tokamak turbulence, driven by the ion-temperature gradient and occurring in
the presence of flow shear, is investigated by means of local, ion-scale,
electrostatic gyrokinetic simulations (with both kinetic ions and electrons) of
the conditions in the outer core of the Mega-Ampere Spherical Tokamak (MAST). A
parameter scan in the local values of the ion-temperature gradient and flow
shear is performed. It is demonstrated that the experimentally observed state
is near the stability threshold and that this stability threshold is nonlinear:
sheared turbulence is subcritical, i.e. the system is formally stable to small
perturbations, but, given a large enough initial perturbation, it transitions
to a turbulent state. A scenario for such a transition is proposed and
supported by numerical results: close to threshold, the nonlinear saturated
state and the associated anomalous heat transport are dominated by long-lived
coherent structures, which drift across the domain, have finite amplitudes, but
are not volume filling; as the system is taken away from the threshold into the
more unstable regime, the number of these structures increases until they
overlap and a more conventional chaotic state emerges. Whereas this appears to
represent a new scenario for transition to turbulence in tokamak plasmas, it is
reminiscent of the behaviour of other subcritically turbulent systems, e.g.
pipe flows and Keplerian magnetorotational accretion flows.Comment: 16 pages, 5 figures, accepted to Journal of Plasma Physic
- …