20,522 research outputs found
Cosmic-ray acceleration at collisionless astrophysical shocks using Monte-Carlo simulations
Context. The diffusive shock acceleration mechanism has been widely accepted
as the acceleration mechanism for galactic cosmic rays. While self-consistent
hybrid simulations have shown how power-law spectra are produced, detailed
information on the interplay of diffusive particle motion and the turbulent
electromagnetic fields responsible for repeated shock crossings are still
elusive. Aims. The framework of test-particle theory is applied to investigate
the effect of diffusive shock acceleration by inspecting the obtained
cosmic-ray energy spectra. The resulting energy spectra can be obtained this
way from the particle motion and, depending on the prescribed turbulence model,
the influence of stochastic acceleration through plasma waves can be studied.
Methods. A numerical Monte-Carlo simulation code is extended to include
collisionless shock waves. This allows one to trace the trajectories of test
particle while they are being accelerated. In addition, the diffusion
coefficients can be obtained directly from the particle motion, which allows
for a detailed understanding of the acceleration process. Results. The classic
result of an energy spectrum with is only reproduced for parallel
shocks, while, for all other cases, the energy spectral index is reduced
depending on the shock obliqueness. Qualitatively, this can be explained in
terms of the diffusion coefficients in the directions that are parallel and
perpendicular to the shock front.Comment: 12 pages, 15 figures, accepted for publication in A&
Joint observations of 4U1223-62 by the SAS-3 satellite and Columbia University proportional counter experiment on NASA rocket 26.054 UH
The pulsating X-ray binary 4U1223-62 and Vela X-1 were observed by Aerobee rocket-borne proportional counters. Valid X-ray events were telemetered and analyzed for possible flaring, quasiperiodic, and periodic pulsations, and for other nonstatistical behavior in the source. Both fast Fourier transform and autocorrelation programs were used. For several hours four days before and after the rocket flight, the SAS-3 satellite scanned the galactic plane in order to identify X-ray sources in the vicinity of 4U1223-62 and their intensities, and to provide positional accuracy of 0.25 for sources with intensity greater than 10% of the target. Observations of the source near the main peak of its pulsating period as defined by SAS-3 are discussed. There is no evidence of a spectral feature although twice as many photons were received as than from Vela X-1
Solar Irradiance Observed from PVO and Inferred Solar Rotation
Solar irradiance in the extreme ultraviolet flux (EUV) has been monitored for 11 years by the Pioneer Venus Orbiter (PVO). Since the experiment moves around the Sun with the orbital rate of Venus rather than that of Earth, the measurement gives us a second viewing location from which to begin unravelling which irradiance variations are intrinsic to the Sun, and which are merely rotational modulations whose periods depend on the motion of the observer. Researchers confirm an earlier detection, made with only 8.6 years of data, that the EUV irradiance is modulated by rotation rates of two families of global oscillation modes. One family is assumed to be r-modes occupying the convective envelope and sharing its rotation, while the other family (g-modes) lies in the radiative interior which as a slower rotation. Measured power in r-modes of low angular harmonic number indicates that the Sun's envelope rotated about 0.7 percent faster near the last solar maximum (1979 thru 1982) than it did during the next rise to maximum (1986 to 1989). No change was seen in the g-mode family of lines, as would be expected from the much greater rotational inertia of the radiative interior
High speed binary to decimal conversion system Patent
Design and operation of high speed binary to decimal conversion syste
Green's Functions from Quantum Cluster Algorithms
We show that cluster algorithms for quantum models have a meaning independent
of the basis chosen to construct them. Using this idea, we propose a new method
for measuring with little effort a whole class of Green's functions, once a
cluster algorithm for the partition function has been constructed. To explain
the idea, we consider the quantum XY model and compute its two point Green's
function in various ways, showing that all of them are equivalent. We also
provide numerical evidence confirming the analytic arguments. Similar
techniques are applicable to other models. In particular, in the recently
constructed quantum link models, the new technique allows us to construct
improved estimators for Wilson loops and may lead to a very precise
determination of the glueball spectrum.Comment: 15 pages, LaTeX, with four figures. Added preprint numbe
Critical exponents of a three dimensional O(4) spin model
By Monte Carlo simulation we study the critical exponents governing the
transition of the three-dimensional classical O(4) Heisenberg model, which is
considered to be in the same universality class as the finite-temperature QCD
with massless two flavors. We use the single cluster algorithm and the
histogram reweighting technique to obtain observables at the critical
temperature. After estimating an accurate value of the inverse critical
temperature \Kc=0.9360(1), we make non-perturbative estimates for various
critical exponents by finite-size scaling analysis. They are in excellent
agreement with those obtained with the expansion method with
errors reduced to about halves of them.Comment: 25 pages with 8 PS figures, LaTeX, UTHEP-28
Improving a joint inversion of GRACE, GPS and modelled ocean bottom pressure by using in-situ data.
To investigate the changes in ocean bottom pressure (OBP) and ocean mass Rietbroek et al. (2009) performed a joint least square inversion of weekly GRACE solutions, patterns of large-scale deformation measured by a network of GPS stations and modelled OBP from the Finite Element Sea ice Ocean Model (FESOM). The correlation of this inversion with in-situ OBP ranges between 0.7 and 0.8 in some regions but for example in the tropical Atlantic the correlation is below 0.4. To improve the agreement of the inversion with in-situ data, a part of the in-situ data is included directly into the inversion. The in-situ OBP data was taken from the global OBP data base of Macrander et al. (2010) and averaged to weekly means. Depending on the weight put on the in-situ data, the correlation and regression increases significantly to a value larger than 0.9.
The variance of the system is locally reduced by almost 50% at the locations included into the inversion while the difference of the global ocean mean is on average below 10%. Furthermore the global ocean mean is used to compute a bias term for correcting the global ocean mean obtained by the FESOM model
Opiate Drug Seeking and Addiction: The Influence of Sucrose Consumption on the Acquisition and Expression of Morphine-induced Conditioned Place Preferences (CPP)
Sucrose intake may lead to changes in brain and behavior similar to the effects of abused drugs. For example, sucrose may agonize endogenous opiate systems and modulate opiate-seeking behavior. Previous research reported equivocal outcomes where sucrose may either enhance (i.e., cross-sensitization) or attenuate (i.e., cross-tolerance) drug seeking as measured by morphine-induced CPP. The present experiment extends from past work and evaluated the impact of sucrose administered prior to place conditioning. Additionally, unique groups received sucrose prior to tests for CPP to measure sucrose influences on CPP expression.
Sprague-Dawley rats (n=24) were assigned to sucrose (15% w/v) or water pre-exposure conditions. Subsequently, subjects received morphine place conditioning where morphine (10mg/kg) was administered on the initially non-preferred side of the apparatus. Three post-tests were conducted and, prior to each test, animals within each pre-exposure group (sucrose or water) received either sucrose or water. Factorial ANOVA was used to analyze data. Results showed robust morphine-induced CPP. Although animals in the sucrose pre-exposure condition displayed enhanced CPP, the outcomes were not statistically significant. The present findings support the value of CPP techniques to measure opiate drug-seeking behavior. Future work may discover the sufficient conditions for detecting sucrose cross-sensitization of morphine CPP
A Swendsen-Wang update algorithm for the Symanzik improved sigma model
We study a generalization of Swendsen-Wang algorithm suited for Potts models
with next-next-neighborhood interactions. Using the embedding technique
proposed by Wolff we test it on the Symanzik improved bidimensional non-linear
model. For some long range observables we find a little slowing down
exponent () that we interpret as an effect of the partial
frustration of the induced spin model.Comment: Self extracting archive fil
- …