2,000 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
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
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
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
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
Gravitational wave astronomy of single sources with a pulsar timing array
Abbreviated:
We investigate the potential of detecting the gravitational wave from
individual binary black hole systems using pulsar timing arrays (PTAs) and
calculate the accuracy for determining the GW properties. This is done in a
consistent analysis, which at the same time accounts for the measurement of the
pulsar distances via the timing parallax.
We find that, at low redshift, a PTA is able to detect the nano-Hertz GW from
super massive black hole binary systems with masses of \sim10^8 -
10^{10}\,M_{\sun} less than \,years before the final merger, and
those with less than years before merger may allow us to
detect the evolution of binaries.
We derive an analytical expression to describe the accuracy of a pulsar
distance measurement via timing parallax. We consider five years of bi-weekly
observations at a precision of 15\,ns for close-by (\,kpc)
pulsars. Timing twenty pulsars would allow us to detect a GW source with an
amplitude larger than . We calculate the corresponding GW and
binary orbital parameters and their measurement precision. The accuracy of
measuring the binary orbital inclination angle, the sky position, and the GW
frequency are calculated as functions of the GW amplitude. We note that the
"pulsar term", which is commonly regarded as noise, is essential for obtaining
an accurate measurement for the GW source location.
We also show that utilizing the information encoded in the GW signal passing
the Earth also increases the accuracy of pulsar distance measurements. If the
gravitational wave is strong enough, one can achieve sub-parsec distance
measurements for nearby pulsars with distance less than \,kpc.Comment: 16 pages, 5 figure,, accepted by MNRA
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 noise properties of 42 millisecond pulsars from the European Pulsar Timing Array and their impact on gravitational wave searches
The sensitivity of Pulsar Timing Arrays to gravitational waves depends on the
noise present in the individual pulsar timing data. Noise may be either
intrinsic or extrinsic to the pulsar. Intrinsic sources of noise will include
rotational instabilities, for example. Extrinsic sources of noise include
contributions from physical processes which are not sufficiently well modelled,
for example, dispersion and scattering effects, analysis errors and
instrumental instabilities. We present the results from a noise analysis for 42
millisecond pulsars (MSPs) observed with the European Pulsar Timing Array. For
characterising the low-frequency, stochastic and achromatic noise component, or
"timing noise", we employ two methods, based on Bayesian and frequentist
statistics. For 25 MSPs, we achieve statistically significant measurements of
their timing noise parameters and find that the two methods give consistent
results. For the remaining 17 MSPs, we place upper limits on the timing noise
amplitude at the 95% confidence level. We additionally place an upper limit on
the contribution to the pulsar noise budget from errors in the reference
terrestrial time standards (below 1%), and we find evidence for a noise
component which is present only in the data of one of the four used telescopes.
Finally, we estimate that the timing noise of individual pulsars reduces the
sensitivity of this data set to an isotropic, stochastic GW background by a
factor of >9.1 and by a factor of >2.3 for continuous GWs from resolvable,
inspiralling supermassive black-hole binaries with circular orbits.Comment: Accepted for publication by the Monthly Notices of the Royal
Astronomical Societ
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
Pulsars with the Australian Square Kilometre Array Pathfinder
The Australian Square Kilometre Array Pathfinder (ASKAP) is a 36-element
array with a 30-square-degree field of view being built at the proposed SKA
site in Western Australia. We are conducting a Design Study for pulsar
observations with ASKAP, planning both timing and search observations. We
provide an overview of the ASKAP telescope and an update on pulsar-related
progress.Comment: To appear in proceedings of "Radio Pulsars: An astrophysical key to
unlock the secrets of the Universe
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