21,921 research outputs found
Radio-wave propagation in the non-Gaussian interstellar medium
Radio waves propagating from distant pulsars in the interstellar medium
(ISM), are refracted by electron density inhomogeneities, so that the intensity
of observed pulses fluctuates with time. The theory relating the observed pulse
time-shapes to the electron-density correlation function has developed for 30
years, however, two puzzles have remained. First, observational scaling of
pulse broadening with the pulsar distance is anomalously strong; it is
consistent with the standard model only when non-uniform statistics of electron
fluctuations along the line of sight are assumed. Second, the observed pulse
shapes are consistent with the standard model only when the scattering material
is concentrated in a narrow slab between the pulsar and the Earth.
We propose that both paradoxes are resolved at once if one assumes stationary
and uniform, but non-Gaussian statistics of the electron-density distribution.
Such statistics must be of Levy type, and the propagating ray should exhibit a
Levy flight. We propose that a natural realization of such statistics may be
provided by the interstellar medium with random electron-density
discontinuities. We develop a theory of wave propagation in such a non-Gaussian
random medium, and demonstrate its good agreement with observations. The
qualitative introduction of the approach and the resolution of the
anomalous-scaling paradox was presented earlier in [PRL 91, 131101 (2003); ApJ
584, 791 (2003)].Comment: 27 pages, changes to match published versio
Correlation induced non-Abelian quantum holonomies
In the context of two-particle interferometry, we construct a parallel
transport condition that is based on the maximization of coincidence intensity
with respect to local unitary operations on one of the subsystems. The
dependence on correlation is investigated and it is found that the holonomy
group is generally non-Abelian, but Abelian for uncorrelated systems. It is
found that our framework contains the L\'{e}vay geometric phase [2004 {\it J.
Phys. A: Math. Gen.} {\bf 37} 1821] in the case of two-qubit systems undergoing
local SU(2) evolutions.Comment: Minor corrections; journal reference adde
Anomalous Radio-Wave Scattering from Interstellar Plasma Structures
This paper considers scattering screens that have arbitrary spatial
variations of scattering strength transverse to the line of sight, including
screens that are spatially well confined, such as disks and filaments. We
calculate the scattered image of a point source and the observed pulse shape of
a scattered impulse. The consequences of screen confinement include: (1) Source
image shapes that are determined by the physical extent of the screen rather
than by the shapes of much-smaller diffracting microirregularities. These
include image elongations and orientations that are frequency dependent. (2)
Variation with frequency of angular broadening that is much weaker than the
trademark \nu^{-2} scaling law (for a cold, unmagnetized plasma), including
frequency-independent cases; and (3) Similar departure of the pulse broadening
time from the usually expected \nu^{-4} scaling law. We briefly discuss
applications that include scattering of pulses from the Crab pulsar by
filaments in the Crab Nebula; image asymmetries from Galactic scattering of the
sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background active
galactic nuclei by intervening galaxies. We also address the consequences for
inferences about the shape of the wavenumber spectrum of electron density
irregularities, which depend on scaling laws for the image size and the pulse
broadening. Future low-frequency (< 100 MHz) array observations will also be
strongly affected by the Galactic structure of scattering material. Our
formalism is derived in the context of radio scattering by plasma density
fluctuations. It is also applicable to optical, UV and X-ray scattering by
grains in the interstellar medium.Comment: 21 pages, LaTeX2e with AASTeX-4.0, 6 PostScript figures, accepted by
ApJ, revised version has minor changes to respond to referee comments and
suggestion
Time dependent spectral modeling of Markarian 421 during a violent outburst in 2010
We present the results of extensive modeling of the spectral energy
distributions (SEDs) of the closest blazar (z=0.031) Markarian 421 (Mrk 421)
during a giant outburst in February 2010. The source underwent rapid flux
variations in both X-rays and very high energy (VHE) gamma-rays as it evolved
from a low-flux state on 2010 February 13-15 to a high-flux state on 2010
February 17. During this period, the source exhibited significant spectral
hardening from X-rays to VHE gamma-rays while exhibiting a "harder when
brighter" behavior in these energy bands. We reproduce the broadband SED using
a time-dependent multi-zone leptonic jet model with radiation feedback. We find
that an injection of the leptonic particle population with a single power-law
energy distribution at shock fronts followed by energy losses in an
inhomogeneous emission region is suitable for explaining the evolution of Mrk
421 from low- to high-flux state in February 2010. The spectral states are
successfully reproduced by a combination of a few key physical parameters, such
as the maximum minimum cutoffs and power-law slope of the electron
injection energies, magnetic field strength, and bulk Lorentz factor of the
emission region. The simulated light curves and spectral evolution of Mrk 421
during this period imply an almost linear correlation between X-ray flux at
1-10 keV energies and VHE gamma-ray flux above 200 GeV, as has been previously
exhibited by this source. Through this study, a general trend that has emerged
for the role of physical parameters is that, as the flare evolves from a low-
to a high-flux state, higher bulk kinetic energy is injected into the system
with a harder particle population and a lower magnetic field strength.Comment: 13 pages, 5 figures, accepted for publication in MNRA
Phosphate availability regulates root system architecture in Arabidopsis
Plant root systems are highly plastic in their development and can adapt their architecture in response to the prevailing environmental conditions. One important parameter is the availability of phosphate, which is highly immobile in soil such that the arrangement of roots within the soil will profoundly affect the ability of the plant to acquire this essential nutrient. Consistent with this, the availability of phosphate was found to have a marked effect on the root system architecture of Arabidopsis. Low phosphate availability favored lateral root growth over primary root growth, through increased lateral root density and length, and reduced primary root growth mediated by reduced cell elongation. The ability of the root system to respond to phosphate availability was found to be independent of sucrose supply and auxin signaling. In contrast, shoot phosphate status was found to influence the root system architecture response to phosphate availability
What can GLAST say about the origin of cosmic rays in other galaxies ?
Gamma rays in the band from 20 MeV to 300 GeV, used in combination with data
from radio and X-ray bands, provide a powerful tool for studying the origin of
cosmic rays in our sister galaxies Andromeda and the Magellanic Clouds.
Gamma-ray Large Area Space Telescope (GLAST) will spatially resolve these
galaxies and measure the spectrum and intensity of diffuse gamma radiation from
the collisions of cosmic rays with gas and dust in them. Observations of
Andromeda will give an external perspective on a spiral galaxy like the Milky
Way. Observations of the Magellanic Clouds will permit a study of cosmic rays
in dwarf irregular galaxies, where the confinement is certainly different and
the massive star formation rate is much greater.Comment: 4 pages including 6 figures; to appear in Proc. ACE-2000 Symp. "The
Acceleration and Transport of Energetic Particles Observed in the
Heliosphere" (Jan. 5-8, 2000, Indian Wells, CA), AIP Conf. Proc. More details
can be found at the LHEA GLAST page at
http://lhea-glast.gsfc.nasa.gov/pub/science/index.htm
Adjustment for treatment changes in epilepsy trials: A comparison of causal methods for time-to-event outcomes
BACKGROUND:
When trials are subject to departures from randomised treatment, simple statistical methods that aim to estimate treatment efficacy, such as per protocol or as treated analyses, typically introduce selection bias. More appropriate methods to adjust for departure from randomised treatment are rarely employed, primarily due to their complexity and unfamiliarity. We demonstrate the use of causal methodologies for the production of estimands with valid causal interpretation for time-to-event outcomes in the analysis of a complex epilepsy trial, as an example to guide non-specialist analysts undertaking similar analyses.
METHODS:
Two causal methods, the structural failure time model and inverse probability of censoring weighting, are adapted to allow for skewed time-varying confounders, competing reasons for treatment changes and a complicated time to remission outcome. We demonstrate the impact of various factors: choice of method (structural failure time model versus inverse probability of censoring weighting), model for inverse probability of censoring weighting (pooled logistic regression versus Cox models), time interval (for creating panel data for time-varying confounders and outcome), choice of confounders and (in pooled logistic regression) use of splines to estimate underlying risk.
RESULTS:
The structural failure time model could adjust for switches between trial treatments but had limited ability to adjust for the other treatment changes that occurred in this epilepsy trial. Inverse probability of censoring weighting was able to adjust for all treatment changes and demonstrated very similar results with Cox and pooled logistic regression models. Accounting for increasing numbers of time-varying confounders and reasons for treatment change suggested a more pronounced advantage of the control treatment than that obtained using intention to treat.
CONCLUSIONS:
In a complex trial featuring a remission outcome, underlying assumptions of the structural failure time model are likely to be violated, and inverse probability of censoring weighting may provide the most useful option, assuming availability of appropriate data and sufficient sample sizes. Recommendations are provided for analysts when considering which of these methods should be applied in a given trial setting
DNET: A communications facility for distributed heterogeneous computing
This document describes DNET, a heterogeneous data communications networking facility. DNET allows programs operating on hosts on dissimilar networks to communicate with one another without concern for computer hardware, network protocol, or operating system differences. The overall DNET network is defined as the collection of host machines/networks on which the DNET software is operating. Each underlying network is considered a DNET 'domain'. Data communications service is provided between any two processes on any two hosts on any of the networks (domains) that may be reached via DNET. DNET provides protocol transparent, reliable, streaming data transmission between hosts (restricted, initially to DECnet and TCP/IP networks). DNET also provides variable length datagram service with optional return receipts
Non-Gaussian Radio-Wave Scattering in the Interstellar Medium
It was recently suggested by Boldyrev & Gwinn that the characteristics of
radio scintillations from distant pulsars are best understood if the
interstellar electron-density fluctuations that cause the time broadening of
the radio pulses obey non-Gaussian statistics. In this picture the density
fluctuations are inferred to be strong on very small scales (). We argue that such density structures could correspond to the ionized
boundaries of molecular regions (clouds) and demonstrate that the power-law
distribution of scattering angles that is required to match the observations
arises naturally from the expected intersections of our line of sight with
randomly distributed, thin, approximately spherical ionized shells of this
type. We show that the observed change in the time-broadening behavior for
pulsar dispersion measures is consistent
with the expected effect of the general ISM turbulence, which should dominate
the scattering for nearby pulsars. We also point out that if the clouds are
ionized by nearby stars, then their boundaries may become turbulent on account
of an ionization front instability. This turbulence could be an alternative
cause of the inferred density structures. An additional effect that might
contribute to the strength of the small-scale fluctuations in this case is the
expected flattening of the turbulent density spectrum when the eddy sizes
approach the proton gyroscale.Comment: 15 pages, 3 figures, accepted to Ap
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