44,177 research outputs found
Long-term Observations of Three Nulling Pulsars
We present an analysis of approximately 200 hours of observations of the
pulsars J16345107, J17174054 and J18530505, taken over the course of
14.7 yr. We show that all of these objects exhibit long term nulls and
radio-emitting phases (i.e. minutes to many hours), as well as considerable
nulling fractions (NFs) in the range . PSR J17174054 is
also found to exhibit short timescale nulls () and burst phases
() during its radio-emitting phases. This behaviour acts to
modulate the NF, and therefore the detection rate of the source, over
timescales of minutes. Furthermore, PSR J18530505 is shown to exhibit a weak
emission state, in addition to its strong and null states, after sufficient
pulse integration. This further indicates that nulls may often only represent
transitions to weaker emission states which are below the sensitivity
thresholds of particular observing systems. In addition, we detected a
peak-to-peak variation of in the spin-down rate of PSR
J17174054, over timescales of hundreds of days. However, no long-term
correlation with emission variation was found.Comment: 10 pages, 8 figures, accepted for publication in MNRA
On the Apparent Nulls and Extreme Variability of PSR J1107-5907
We present an analysis of the emission behaviour of PSR J1107-5907, a source
known to exhibit separate modes of emission, using observations obtained over
approximately 10 yr. We find that the object exhibits two distinct modes of
emission; a strong mode with a broad profile and a weak mode with a narrow
profile. During the strong mode of emission, the pulsar typically radiates very
energetic emission over sequences of ~200-6000 pulses (~60 s-24 min), with
apparent nulls over time-scales of up to a few pulses at a time. Emission
during the weak mode is observed outside of these strong-mode sequences and
manifests as occasional bursts of up to a few clearly detectable pulses at a
time, as well as low-level underlying emission which is only detected through
profile integration. This implies that the previously described null mode may
in fact be representative of the bottom-end of the pulse intensity distribution
for the source. This is supported by the dramatic pulse-to-pulse intensity
modulation and rarity of exceptionally bright pulses observed during both modes
of emission. Coupled with the fact that the source could be interpreted as a
rotating radio transient (RRAT)-like object for the vast majority of the time,
if placed at a further distance, we advance that this object likely represents
a bridge between RRATs and extreme moding pulsars. Further to these emission
properties, we also show that the source is consistent with being a
near-aligned rotator and that it does not exhibit any measurable spin-down rate
variation. These results suggest that nulls observed in other intermittent
objects may in fact be representative of very weak emission without the need
for complete cessation. As such, we argue that longer (> 1 h) observations of
pulsars are required to discern their true modulation properties.Comment: 15 pages, 10 figures, accepted for publication in MNRA
Devroye Inequality for a Class of Non-Uniformly Hyperbolic Dynamical Systems
In this paper, we prove an inequality, which we call "Devroye inequality",
for a large class of non-uniformly hyperbolic dynamical systems (M,f). This
class, introduced by L.-S. Young, includes families of piece-wise hyperbolic
maps (Lozi-like maps), scattering billiards (e.g., planar Lorentz gas),
unimodal and H{\'e}non-like maps. Devroye inequality provides an upper bound
for the variance of observables of the form K(x,f(x),...,f^{n-1}(x)), where K
is any separately Holder continuous function of n variables. In particular, we
can deal with observables which are not Birkhoff averages. We will show in
\cite{CCS} some applications of Devroye inequality to statistical properties of
this class of dynamical systems.Comment: Corrected version; To appear in Nonlinearit
Dynamic van der Waals Theory of two-phase fluids in heat flow
We present a dynamic van der Waals theory. It is useful to study phase
separation when the temperature varies in space. We show that if heat flow is
applied to liquid suspending a gas droplet at zero gravity, a convective flow
occurs such that the temperature gradient within the droplet nearly vanishes.
As the heat flux is increased, the droplet becomes attached to the heated wall
that is wetted by liquid in equilibrium. In one case corresponding to partial
wetting by gas, an apparent contact angle can be defined. In the ther case with
larger heat flux, the droplet completely wets the heated wall expelling liquid.Comment: 6pages, 8figure
Ultimate strength and design of lipped channel columns experiencing local/distortional mode interaction - Part I: Experimental investigation
This paper reports the results of an experimental investigation aimed at assessing the post-buckling behaviour and ultimate strength of fixed-ended cold-formed steel lipped channel columns experiencing local/distortional mode interaction. A total of 26 columns were tested and the specimens were carefully selected to ensure various levels of local/distortional interaction effects (more or less close local and distortional critical stresses). The experimental results presented consist of the specimen geometries, material properties, initial imperfections, non-linear equilibrium paths and ultimate strength values. Since the collapse of most columns combines local and distortional deformations, these experimental results may be used to (i) assess the relevance of local/distortional interaction, (ii) calibrate and validate numerical simulations and (iii) provide experimental data aimed at developing a Direct Strength Method (DSM) approach to design cold-formed steel lipped channel columns against local/distortional interaction - such a DSM approach is addressed in Part II of this paper.published_or_final_versionThe 6th International Conference on Advances in Steel Structures in conjunction with IJSSD Symposium on Progress in Structural Stability and Dynamics & IStructE Asia-Pacific Forum (ICASS '09 / IJSSD / IStructE Asia-Pacific Forum), Hong Kong, China, 16-18 December 2009. In Proceedings of the 6th ICASS, 2009, v. 1, p. 460-46
Long-term Radio Observations of the Intermittent Pulsar B1931+24
We present an analysis of approximately 13-yr of observations of the
intermittent pulsar B1931+24 to further elucidate its behaviour. We find that
while the source exhibits a wide range of nulling (~4-39 d) and radio-emitting
(~1-19 d) timescales, it cycles between its different emission phases over an
average timescale of approximately 38 d, which is remarkably stable over many
years. On average, the neutron star is found to be radio emitting for 26 +- 6 %
of the time. No evidence is obtained to suggest that the pulsar undergoes any
systematic, intrinsic variations in pulse intensity during the radio-emitting
phases. In addition, we find no evidence for any correlation between the length
of consecutive emission phases. An analysis of the rotational behaviour of the
source shows that it consistently assumes the same spin-down rates, i.e. nudot
= -16 +- 1 x 10^-15 s^-2 when emitting and nudot = -10.8 +- 0.4 x 10^-15 s^-2
when not emitting, over the entire observation span. Coupled with the stable
switching timescale, this implies that the pulsar retains a high degree of
magnetospheric memory, and stability, in spite of comparatively rapid (~ms)
dynamical plasma timescales. While this provides further evidence to suggest
that the behaviour of the neutron star is governed by magnetospheric-state
switching, the underlying trigger mechanism remains illusive. This should be
elucidated by future surveys with next generation telescopes such as LOFAR,
MeerKAT and the SKA, which should detect similar sources and provide more clues
to how their radio emission is regulated.Comment: 12 pages, 12 figures, accepted for publication in MNRA
Far-infrared imaging of tokamak plasma
A 20-channel interferometer has been developed which utilizes a linear, one-dimensional microbolometer array to obtain single-shot density profiles from the UCLA Microtor tokamak plasma. The interferometer has been used to study time-dependent phenomena in the plasma density profile. Observations of the sawtooth instability clearly show the growth of the m=0 mode from a localized oscillation (r=1 cm) on axis to an oscillation of the entire plasma. Also, measurements during the initial startup phase of the discharge show evidence of hollow density profiles. In addition, a simultaneous measurement of the poloidal magnetic field has been developed which provides 20 channels of polarimetry. Interferometry and polarimetry both use the same imaging system and the spatial resolution of both measurements has been tested using plastic and crystal-quartz test objects. The signal-to-noise ratio for the polarimeter has also proved adequate for the expected Faraday rotation angle (alphamax=7°, Ip=70 kA, n=5Ă10^13 cm^â3)
Chiral extrapolation beyond the power-counting regime
Chiral effective field theory can provide valuable insight into the chiral
physics of hadrons when used in conjunction with non-perturbative schemes such
as lattice QCD. In this discourse, the attention is focused on extrapolating
the mass of the rho meson to the physical pion mass in quenched QCD (QQCD).
With the absence of a known experimental value, this serves to demonstrate the
ability of the extrapolation scheme to make predictions without prior bias. By
using extended effective field theory developed previously, an extrapolation is
performed using quenched lattice QCD data that extends outside the chiral
power-counting regime (PCR). The method involves an analysis of the
renormalization flow curves of the low energy coefficients in a finite-range
regularized effective field theory. The analysis identifies an optimal
regulator, which is embedded in the lattice QCD data themselves. This optimal
regulator is the regulator value at which the renormalization of the low energy
coefficients is approximately independent of the range of quark masses
considered. By using recent precision, quenched lattice results, the
extrapolation is tested directly by truncating the analysis to a set of points
above 380 MeV, while being blinded of the results probing deeply into the
chiral regime. The result is a successful extrapolation to the chiral regime.Comment: 8 pages, 18 figure
Deformation-based design of aluminium alloy beams
Two series of simply supported bending tests on aluminium alloy square and rectangular hollow sections have been performed. The test program comprised 14 three-point bending tests and 15 four-point bending tests. The test specimens were fabricated by extrusion from grades 6061-T6 and 6063-T5 heat-treated aluminium alloys, with width-to-thickness ratios ranging from 2.8 to 20.5. Measured geometric and material properties, together with the full loadâdeflection histories from the test specimens, were reported. Observed failure modes included local buckling, material yielding and tensile fracture. Further experimental data were gathered from the literature. Finite element (FE) models were developed and validated against the test results, and then used to perform parametric studies, in which a total of 132 numerical results were generated. The experimental and numerical results were used to evaluate the bending resistance provisions of the American [1], Australian/New Zealand [2] and European [3] Specifications, as well as the continuous strength method (CSM). The moment capacities predicted by the three design specifications were found to be generally conservative, while the CSM provided more accurate and more consistent predictions due to the recognition and systematic exploitation of strain hardening
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