3,999 research outputs found
Ionospheric E-region Irregularities Produced by Non-linear Coupling of Unstable Plasma Waves
Ionospheric E region irregularities produced by nonlinear coupling of unstable plasma wave
Non-thermal high-energy emission from colliding winds of massive stars
Colliding winds of massive star binary systems are considered as potential
sites of non-thermal high-energy photon production. This is motivated merely by
the detection of synchrotron radio emission from the expected colliding wind
location. Here we investigate the properties of high-energy photon production
in colliding winds of long-period WR+OB-systems. We found that in the
dominating leptonic radiation process anisotropy and Klein-Nishina effects may
yield spectral and variability signatures in the gamma-ray domain at or above
the sensitivity of current or upcoming gamma-ray telescopes. Analytical
formulae for the steady-state particle spectra are derived assuming diffusive
particle acceleration out of a pool of thermal wind particles, and taking into
account adiabatic and all relevant radiative losses. For the first time we
include their advection/convection in the wind collision zone, and distinguish
two regions within this extended region: the acceleration region where spatial
diffusion is superior to convective/advective motion, and the convection region
defined by the convection time shorter than the diffusion time scale. The
calculation of the Inverse Compton radiation uses the full Klein-Nishina cross
section, and takes into account the anisotropic nature of the scattering
process. This leads to orbital flux variations by up to several orders of
magnitude which may, however, be blurred by the geometry of the system. The
calculations are applied to the typical WR+OB-systems WR 140 and WR 147 to
yield predictions of their expected spectral and temporal characteristica and
to evaluate chances to detect high-energy emission with the current and
upcoming gamma-ray experiments. (abridged)Comment: 67 pages, 24 figures, submitted to Ap
A Very Large Array 3.6cm continuum survey of Galactic Wolf-Rayet stars
We report the results of a survey of radio continuum emission of Galactic
Wolf-Rayet stars north of declination -46 degrees. The observations were
obtained at 8.46 GHz (3.6cm) using the Very Large Array (VLA), with an angular
resolution of about 6 x 9 arcsec and typical rms noise of 0.04 mJy/beam. Our
survey of 34 WR stars resulted in 15 definite and 5 probable detections, 13 of
these for the first time at radio wavelengths. All detections are unresolved.
Time variations in flux are confirmed in the cases of WR98a, WR104, WR105 and
WR125. WR79a and WR89 are also variable in flux and we suspect they are also
non-thermal emitters. Thus, of our sample 20-30% of the detected stars are
non-thermal emiters. Average mass loss rates determinations obtained excluding
definite and suspected non-thermal cases give similar values for WN (all
subtypes) and WC5-7 stars, while a lower value was obtained for WC8-9 stars.
Uncertainties in stellar distances largely contribute to the observed scatter
in mass loss rates. Upper limits to the mass loss rates were obtained in cases
of undetected sources or for sources which probably show additional non-thermal
emission.Comment: 21 pages, Latex, 21 postscript figures, to be published in The
Astronomical Journal, May 200
Automated Certification of Authorisation Policy Resistance
Attribute-based Access Control (ABAC) extends traditional Access Control by
considering an access request as a set of pairs attribute name-value, making it
particularly useful in the context of open and distributed systems, where
security relevant information can be collected from different sources. However,
ABAC enables attribute hiding attacks, allowing an attacker to gain some access
by withholding information. In this paper, we first introduce the notion of
policy resistance to attribute hiding attacks. We then propose the tool ATRAP
(Automatic Term Rewriting for Authorisation Policies), based on the recent
formal ABAC language PTaCL, which first automatically searches for resistance
counter-examples using Maude, and then automatically searches for an Isabelle
proof of resistance. We illustrate our approach with two simple examples of
policies and propose an evaluation of ATRAP performances.Comment: 20 pages, 4 figures, version including proofs of the paper that will
be presented at ESORICS 201
V723 Cas (Nova Cassiopeiae 1995): MERLIN observations from 1996 to 2001
MERLIN observations of the unusually slow nova V723 Cas are presented. Nine
epochs of 6-cm data between 1996 and 2001 are mapped, showing the initial
expansion and brightening of the radio remnant, the development of structure
and the final decline. A radio light curve is presented and fitted by the
standard Hubble flow model for radio emission from novae in order to determine
the values of various physical parameters for the shell. The model is
consistent with the overall development of the radio emission. Assuming a
distance of 2.39 (+/-0.38) kpc and a shell temperature of 17000 K, the model
yields values for expansion velocity of 414 +/- 0.1 km s^-1 and shell mass of
1.13 +/- 0.04 * 10^-4 Msolar. These values are consistent with those derived
from other observations although the ejected masses are rather higher than
theoretical predictions. The structure of the shell is resolved by MERLIN and
shows that the assumption of spherical symmetry in the standard model is
unlikely to be correct.Comment: 6 pages, 6 figure
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Pilot randomized trial demonstrating reversal of obesity-related abnormalities in reward system responsivity to food cues with a behavioral intervention
Objectives: Obesity is associated with hyperactivation of the reward system for high-calorie (HC) versus low-calorie (LC) food cues, which encourages unhealthy food selection and overeating. However, the extent to which this hyperactivation can be reversed is uncertain, and to date there has been no demonstration of changes by behavioral intervention. Subjects and methods: We used functional magnetic resonance imaging to measure changes in activation of the striatum for food images at baseline and 6 months in a pilot study of 13 overweight or obese adults randomized to a control group or a novel weight-loss intervention. Results: Compared to controls, intervention participants achieved significant weight loss (−6.3±1.0 kg versus +2.1±1.1 kg, P<0.001) and had increased activation for LC food images with a composition consistent with that recommended in the behavioral intervention at 6 months versus baseline in the right ventral putamen (P=0.04), decreased activation for HC images of typically consumed foods in the left dorsal putamen (P=0.01). There was also a large significant shift in relative activation favoring LC versus HC foods in both regions (P<0.04). Conclusions: This study provides the first demonstration of a positive shift in activation of the reward system toward healthy versus unhealthy food cues in a behavioral intervention, suggesting new avenues to enhance behavioral treatments of obesity
Compilation of extended recursion in call-by-value functional languages
This paper formalizes and proves correct a compilation scheme for
mutually-recursive definitions in call-by-value functional languages. This
scheme supports a wider range of recursive definitions than previous methods.
We formalize our technique as a translation scheme to a lambda-calculus
featuring in-place update of memory blocks, and prove the translation to be
correct.Comment: 62 pages, uses pi
Information theory explanation of the fluctuation theorem, maximum entropy production and self-organized criticality in non-equilibrium stationary states
Jaynes' information theory formalism of statistical mechanics is applied to
the stationary states of open, non-equilibrium systems. The key result is the
construction of the probability distribution for the underlying microscopic
phase space trajectories. Three consequences of this result are then derived :
the fluctuation theorem, the principle of maximum entropy production, and the
emergence of self-organized criticality for flux-driven systems in the
slowly-driven limit. The accumulating empirical evidence for these results
lends support to Jaynes' formalism as a common predictive framework for
equilibrium and non-equilibrium statistical mechanics.Comment: 21 pages, 0 figures, minor modifications, version to appear in J.
Phys. A. (2003
Millimeter-wave emission during the 2003 low excitation phase of eta Carinae
In this paper we present observations of eta Carinae in the 1.3 mm and 7 mm
radio continuum, during the 2003.5 low excitation phase. The expected minimum
in the light curves was confirmed at both wavelengths and was probably due to a
decrease in the number of UV photons available to ionize the gas surrounding
the binary system. At 7 mm a very well defined peak was superimposed on the
declining flux density. It presented maximum amplitude in 29 June 2003 and
lasted for about 10 days. We show that its origin can be free-free emission
from the gas at the shock formed by wind-wind collision, which is also
responsible for the observed X-ray emission. Even though the shock strength is
strongly enhanced as the two stars in the binary system approach each other,
during periastron passage the X-ray emission is strongly absorbed and the 7 mm
observations represent the only direct evidence of this event
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