3,232 research outputs found
The optimal schedule for pulsar timing array observations
In order to maximize the sensitivity of pulsar timing arrays to a stochastic
gravitational wave background, we present computational techniques to optimize
observing schedules. The techniques are applicable to both single and
multi-telescope experiments. The observing schedule is optimized for each
telescope by adjusting the observing time allocated to each pulsar while
keeping the total amount of observing time constant. The optimized schedule
depends on the timing noise characteristics of each individual pulsar as well
as the performance of instrumentation. Several examples are given to illustrate
the effects of different types of noise. A method to select the most suitable
pulsars to be included in a pulsar timing array project is also presented.Comment: 16 pages, 6 figures, accepted by MNRA
Supra-oscillatory critical temperature dependence of Nb-Ho bilayers
We investigate the critical temperature Tc of a thin s-wave superconductor
(Nb) proximity coupled to a helical rare earth ferromagnet (Ho). As a function
of the Ho layer thickness, we observe multiple oscillations of Tc superimposed
on a slow decay, that we attribute to the influence of the Ho on the Nb
proximity effect. Because of Ho inhomogeneous magnetization, singlet and
triplet pair correlations are present in the bilayers. We take both into
consideration when solving the self consistent Bogoliubov-de Gennes equations,
and we observe a reasonable agreement. We also observe non-trivial transitions
into the superconducting state, the zero resistance state being attained after
two successive transitions which appear to be associated with the magnetic
structure of Ho.Comment: Main article: 5 pages, 4 figures; Supplementary materials: 4 pages, 5
figure
On the origin of the extremely different solubilities of polyethers in water
The solubilities of polyethers are surprisingly counter-intuitive. The best-known example is the difference between polyethylene glycol ([âCH2âCH2âOâ]n) which is infinitely soluble, and polyoxymethylene ([âCH2âOâ]n) which is completely insoluble in water, exactly the opposite of what one expects from the C/O ratios of these molecules. Similar anomalies exist for oligomeric and cyclic polyethers. To solve this apparent mystery, we use femtosecond vibrational and GHz dielectric spectroscopy with complementary ab initio calculations and molecular dynamics simulations. We find that the dynamics of water molecules solvating polyethers is fundamentally different depending on their C/O composition. The ab initio calculations and simulations show that this is not because of steric effects (as is commonly believed), but because the partial charge on the O atoms depends on the number of C atoms by which they are separated. Our results thus show that inductive effects can have a major impact on aqueous solubilities
The beamformer and correlator for the Large European Array for Pulsars
The Large European Array for Pulsars combines Europe's largest radio
telescopes to form a tied-array telescope that provides high signal-to-noise
observations of millisecond pulsars (MSPs) with the objective to increase the
sensitivity of detecting low-frequency gravitational waves. As part of this
endeavor we have developed a software correlator and beamformer which enables
the formation of a tied-array beam from the raw voltages from each of
telescopes. We explain the concepts and techniques involved in the process of
adding the raw voltages coherently. We further present the software processing
pipeline that is specifically designed to deal with data from widely spaced,
inhomogeneous radio telescopes and describe the steps involved in preparing,
correlating and creating the tied-array beam. This includes polarization
calibration, bandpass correction, frequency dependent phase correction,
interference mitigation and pulsar gating. A link is provided where the
software can be obtained.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and
Computin
Improving Predictions for Helium Emission Lines
We have combined the detailed He I recombination model of Smits with the
collisional transitions of Sawey & Berrington in order to produce new accurate
helium emissivities that include the effects of collisional excitation from
both the 2 (3)S and 2 (1) S levels. We present a grid of emissivities for a
range of temperature and densities along with analytical fits and error
estimates.
Fits accurate to within 1% are given for the emissivities of the brightest
lines over a restricted range for estimates of primordial helium abundance. We
characterize the analysis uncertainties associated with uncertainties in
temperature, density, fitting functions, and input atomic data. We estimate
that atomic data uncertainties alone may limit abundance estimates to an
accuracy of 1.5%; systematic errors may be greater than this. This analysis
uncertainty must be incorporated when attempting to make high accuracy
estimates of the helium abundance. For example, in recent determinations of the
primordial helium abundance, uncertainties in the input atomic data have been
neglected.Comment: ApJ, accepte
Model-based asymptotically optimal dispersion measure correction for pulsar timing
In order to reach the sensitivity required to detect gravitational waves,
pulsar timing array experiments need to mitigate as much noise as possible in
timing data. A dominant amount of noise is likely due to variations in the
dispersion measure. To correct for such variations, we develop a statistical
method inspired by the maximum likelihood estimator and optimal filtering. Our
method consists of two major steps. First, the spectral index and amplitude of
dispersion measure variations are measured via a time-domain spectral analysis.
Second, the linear optimal filter is constructed based on the model parameters
found in the first step, and is used to extract the dispersion measure
variation waveforms. Compared to current existing methods, this method has
better time resolution for the study of short timescale dispersion variations,
and generally produces smaller errors in waveform estimations. This method can
process irregularly sampled data without any interpolation because of its
time-domain nature. Furthermore, it offers the possibility to interpolate or
extrapolate the waveform estimation to regions where no data is available.
Examples using simulated data sets are included for demonstration.Comment: 15 pages, 15 figures, submitted 15th Sept. 2013, accepted 2nd April
2014 by MNRAS. MNRAS, 201
Radio Emission from GRO J1655-40 during the 1994 Jet Ejection Episodes
We report multifrequency radio observations of GRO J1655-40 obtained with the
Australia Telescope Compact Array, the Molonglo Observatory Synthesis Telescope
and the Hartebeesthoek Radio Astronomy Observatory at the time of the major
hard X-ray and radio outbursts in 1994 August-September. The radio emission
reached levels of the order of a few Jy and was found to be linearly polarized
by up to 10%, indicating a synchrotron origin. The light curves are in good
agreement with those measured with the VLA, but our closer time sampling has
revealed two new short-lived events and significant deviations from a simple
exponential decay. The polarization data show that the magnetic field is well
ordered and aligned at right angles to the radio jets for most of the
monitoring period. The time evolution of the polarization cannot be explained
solely in terms of a simple synchrotron bubble model, and we invoke a hybrid
`core-lobe' model with a core which contributes both synchrotron and free-free
emission and `lobes' which are classical synchrotron emitters.Comment: 36 pages, 5 tables, 9 figures; accepted for publication in Ap
The extraordinary outburst in the massive protostellar system NGC6334I-MM1 : emergence of strong 6.7 GHz methanol masers
C. J. Cyganowski acknowledges support from the STFC (grant number ST/M001296/1).We report the first sub-arcsecond VLA imaging of 6 GHz continuum, methanol maser, and excited-state hydroxyl maser emission toward the massive protostellar cluster NGC6334I following the recent 2015 outburst in (sub)millimeter continuum toward MM1, the strongest (sub)millimeter source in the protocluster. In addition to detections toward the previously known 6.7 GHz Class II methanol maser sites in the hot core MM2 and the UCHII region MM3 (NGC6334F), we find new maser features toward several components of MM1, along with weaker features âŒ1â north, west, and southwest of MM1, and toward the non-thermal radio continuum source CM2. None of these areas have heretofore exhibited Class II methanol maser emission in three decades of observations. The strongest MM1 masers trace a dust cavity, while no masers are seen toward the strongest dust sources MM1A, 1B and 1D. The locations of the masers are consistent with a combination of increased radiative pumping due to elevated dust grain temperature following the outburst, the presence of infrared photon propagation cavities, and the presence of high methanol column densities as indicated by ALMA images of thermal transitions. The non-thermal radio emission source CM2 (2â north of MM1) also exhibits new maser emission from the excited 6.035 and 6.030 GHz OH lines. Using the Zeeman effect, we measure a line-of-sight magnetic field of +0.5 to +3.7 mG toward CM2. In agreement with previous studies, we also detect numerous methanol and excited OH maser spots toward the UCHII region MM3, with predominantly negative line-of-sight magnetic field strengths of -2 to -5 mG and an intriguing south-north field reversal.PostprintPeer reviewe
Roughness-induced critical phenomena in a turbulent flow
I present empirical evidence that turbulent flows are closely analogous to
critical phenomena, from a reanalysis of friction factor measurements in rough
pipes. The data collapse found here corresponds to Widom scaling near critical
points, and implies that a full understanding of turbulence requires explicit
accounting for boundary roughness
- âŠ