4,305 research outputs found
A Study of Single Pulses in the Parkes Multibeam Pulsar Survey
We reprocessed the Parkes Multibeam Pulsar Survey, searching for single
pulses out to a DM of 5000 pc cm with widths of up to one second. We
recorded single pulses from 264 known pulsars and 14 Rotating Radio Transients.
We produced amplitude distributions for each pulsar which we fit with
log-normal distributions, power-law tails, and a power-law function divided by
an exponential function, finding that some pulsars show a deviation from a
log-normal distribution in the form of an excess of high-energy pulses. We
found that a function consisting of a power-law divided by an exponential fit
the distributions of most pulsars better than either log-normal or power-law
functions. For pulsars that were detected in a periodicity search, we computed
the ratio of their single-pulse signal-to-noise ratios to their signal-to-noise
ratios from a Fourier transform and looked for correlations between this ratio
and physical parameters of the pulsars. The only correlation found is the
expected relationship between this ratio and the spin period. Fitting
log-normal distributions to the amplitudes of pulses from RRATs showed similar
behaviour for most RRATs. Here, however, there seem to be two distinct
distributions of pulses, with the lower-energy distribution being consistent
with noise. Pulse-energy distributions for two of the RRATS processed were
consistent with those found for normal pulsars, suggesting that pulsars and
RRATs have a common emission mechanism, but other factors influence the
specific emission properties of each source class.Comment: 11 pages, 6 figures, 3 tables, accepted for publication in MNRA
Detection of the ISW effect and corresponding dark energy constraints made with directional spherical wavelets
Using a directional spherical wavelet analysis we detect the integrated
Sachs-Wolfe (ISW) effect, indicated by a positive correlation between the
first-year Wilkinson Microwave Anisotropy Probe (WMAP) and NRAO VLA Sky Survey
(NVSS) data. Detections are made using both a directional extension of the
spherical Mexican hat wavelet and the spherical butterfly wavelet. We examine
the possibility of foreground contamination and systematics in the WMAP data
and conclude that these factors are not responsible for the signal that we
detect. The wavelet analysis inherently enables us to localise on the sky those
regions that contribute most strongly to the correlation. On removing these
localised regions the correlation that we detect is reduced in significance, as
expected, but it is not eliminated, suggesting that these regions are not the
sole source of correlation between the data. This finding is consistent with
predictions made using the ISW effect, where one would expect weak correlations
over the entire sky. In a flat universe the detection of the ISW effect
provides direct and independent evidence for dark energy. We use our detection
to constrain dark energy parameters by deriving a theoretical prediction for
the directional wavelet covariance statistic for a given cosmological model.
Comparing these predictions with the data we place constraints on the
equation-of-state parameter and the vacuum energy density .
We also consider the case of a pure cosmological constant, i.e. . For
this case we rule out a zero cosmological constant at greater than the 99.9%
significance level. All parameter estimates that we obtain are consistent with
the standand cosmological concordance model values.Comment: 16 pages, 13 figures; replaced to match version accepted by MNRA
Magnetic and electrical properties of dhcp NpPd3 and U(1-x)Np(x)Pd3
We have made an extensive study of the magnetic and electrical properties of
double-hexagonal closepacked NpPd3 and a range of U(1-x)Np(x)Pd3 compounds with
x=0.01, 0.02, 0.05, and 0.50 using magnetization, magnetic susceptibility,
electrical resistivity, and heat capacity measurements on polycrystalline
samples, performed in the temperature range 2-300 K and in magnetic fields up
to 9 T. Two transitions are observed in NpPd3 at T=10 and 30 K. Dilute Np
samples (x<0.05) exhibit quadrupolar transitions, with the transition
temperatures reduced from those of pure UPd3.Comment: 10 pages, 18 figure
Magnetic and electrical properties of (Pu,Lu)Pd3
We present measurements of the magnetic susceptibility, heat capacity and
electrical resistivity of PuLuPd, with =0, 0.1, 0.2, 0.5,
0.8 and 1. PuPd is an antiferromagnetic heavy fermion compound with
~K. With increasing Lu doping, both the Kondo and RKKY interaction
strengths fall, as judged by the Sommerfeld coefficient and N\'eel
temperature . Fits to a crystal field model of the resistivity also
support these conclusions. The paramagnetic effective moment
increases with Lu dilution, indicating a decrease in the
Kondo screening. In the highly dilute limit, approaches
the value predicted by intermediate coupling calculations. In conjunction with
an observed Schottky peak at 60~K in the magnetic heat capacity,
corresponding to a crystal field splitting of 12~meV, a mean-field
intermediate coupling model with nearest neighbour interactions has been
developed.Comment: 13 pages, 13 figure
Wilkinson Microwave Anisotropy Probe 7-yr constraints on fNL with a fast wavelet estimator
A new method to constrain the local non-linear coupling parameter fNL based
on a fast wavelet decomposition is presented. Using a multiresolution wavelet
adapted to the HEALPix pixelization, we have developed a method that is 10^2
times faster than previous estimators based on isotropic wavelets and 10^3
faster than the KSW bispectrum estimator, at the resolution of the Wilkinson
Microwave Anisotropy Probe (WMAP) data. The method has been applied to the WMAP
7-yr V+W combined map, imposing constraints on fNL of -69 < fNL < 65 at the 95
per cent CL. This result has been obtained after correcting for the
contribution of the residual point sources which has been estimated to be fNL =
7 +/- 6. In addition, a Gaussianity analysis of the data has been carried out
using the third order moments of the wavelet coefficients, finding consistency
with Gaussianity. Although the constrainsts imposed on fNL are less stringent
than those found with optimal estimators, we believe that a very fast method,
as the one proposed in this work, can be very useful, especially bearing in
mind the large amount of data that will be provided by future experiments, such
as the Planck satellite. Moreover, the localisation of wavelets allows one to
carry out analyses on different regions of the sky. As an application, we have
separately analysed the two hemispheres defined by the dipolar modulation
proposed by Hoftuft et al. (2009). We do not find any significant asymmetry
regarding the estimated value of fNL in those hemispheres.Comment: 8 pages, 5 figures. Submitted and Accepted for publication in MNRA
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