222 research outputs found
Constraints on cosmic string tension imposed by the limit on the stochastic gravitational wave background from the European Pulsar Timing Array
We investigate the constraints that can be placed on the cosmic string
tension by using the current Pulsar Timing Array limits on the stochastic
gravitational wave background (SGWB). We have developed a code to compute the
spectrum of gravitational waves (GWs) based on the widely accepted one-scale
model. In its simplest form the one-scale model allows one to vary: (i) the
string tension, G\mu/c^2; (ii) the size of cosmic string loops relative to the
horizon at birth, \alpha; (iii) the spectral index of the emission spectrum, q;
(iv) the cut-off in the emission spectrum, n_*; and (v) the intercommutation
probability, p. The amplitude and slope of the spectrum in the nHz frequency
range is very sensitive to these unknown parameters. We have also investigated
the impact of more complicated scenarios with multiple initial loop sizes, in
particular the 2-\alpha models proposed in the literature and a log-normal
distribution for \alpha. We have computed the constraint on G\mu/c^2 due to the
limit on a SGWB imposed by data from the European Pulsar Timing Array. Taking
into account all the possible uncertainties in the parameters we find a
conservative upper limit of G\mu/c^2<5.3x 10^{-7} which typically occurs when
the loop production scale is close to the gravitational backreaction scale,
\alpha\approx\Gamma G\mu/c^2. Stronger limits are possible for specific values
of the parameters which typically correspond to the extremal cases \alpha\ll
\Gamma G\mu/c^2 and \alpha\gg \Gamma G\mu/c^2. This limit is less stringent
than the previously published limits which are based on cusp emission, an
approach which does not necessarily model all the possible uncertainties. We
discuss the prospects for lowering this limit by two orders of magnitude, or
even a detection of the SGWB, in the very near future in the context of the
Large European Array for Pulsars and the Square Kilometre Array.Comment: 24 pages, 14 figures, accepted for publication in Physical Review D.
Minor corrections and additional comments - updated to match the published
versio
On Pulsar Distance Measurements and their Uncertainties
Accurate distances to pulsars can be used for a variety of studies of the
Galaxy and its electron content. However, most distance measures to pulsars
have been derived from the absorption (or lack thereof) of pulsar emission by
Galactic HI gas, which typically implies that only upper or lower limits on the
pulsar distance are available. We present a critical analysis of all measured
HI distance limits to pulsars and other neutron stars, and translate these
limits into actual distance estimates through a likelihood analysis that
simultaneously corrects for statistical biases. We also apply this analysis to
parallax measurements of pulsars in order to obtain accurate distance estimates
and find that the parallax and HI distance measurements are biased in different
ways, because of differences in the sampled populations. Parallax measurements
typically underestimate a pulsar's distance because of the limited distance to
which this technique works and the consequential strong effect of the Galactic
pulsar distribution (i.e. the original Lutz-Kelker bias), in HI distance
limits, however, the luminosity bias dominates the Lutz-Kelker effect, leading
to overestimated distances because the bright pulsars on which this technique
is applicable are more likely to be nearby given their brightness.Comment: 32 pages, 1 figure, 2 tables; Accepted for publication in the
Astrophysical Journa
Gravitational wave detection using pulsars: status of the Parkes Pulsar Timing Array project
The first direct detection of gravitational waves may be made through
observations of pulsars. The principal aim of pulsar timing array projects
being carried out worldwide is to detect ultra-low frequency gravitational
waves (f ~ 10^-9 to 10^-8 Hz). Such waves are expected to be caused by
coalescing supermassive binary black holes in the cores of merged galaxies. It
is also possible that a detectable signal could have been produced in the
inflationary era or by cosmic strings. In this paper we review the current
status of the Parkes Pulsar Timing Array project (the only such project in the
Southern hemisphere) and compare the pulsar timing technique with other forms
of gravitational-wave detection such as ground- and space-based interferometer
systems.Comment: Accepted for publication in PAS
The Sensitivity of the Parkes Pulsar Timing Array to Individual Sources of Gravitational Waves
We present the sensitivity of the Parkes Pulsar Timing Array to gravitational
waves emitted by individual super-massive black-hole binary systems in the
early phases of coalescing at the cores of merged galaxies. Our analysis
includes a detailed study of the effects of fitting a pulsar timing model to
non-white timing residuals. Pulsar timing is sensitive at nanoHertz frequencies
and hence complementary to LIGO and LISA. We place a sky-averaged constraint on
the merger rate of nearby () black-hole binaries in the early phases
of coalescence with a chirp mass of 10^{10}\,\rmn{M}_\odot of less than one
merger every seven years. The prospects for future gravitational-wave astronomy
of this type with the proposed Square Kilometre Array telescope are discussed.Comment: fixed error in equation (4). [13 pages, 6 figures, 1 table, published
in MNRAS
Status Update of the Parkes Pulsar Timing Array
The Parkes Pulsar Timing Array project aims to make a direct detection of a
gravitational-wave background through timing of millisecond pulsars. In this
article, the main requirements for that endeavour are described and recent and
ongoing progress is outlined. We demonstrate that the timing properties of
millisecond pulsars are adequate and that technological progress is timely to
expect a successful detection of gravitational waves within a decade, or
alternatively to rule out all current predictions for gravitational wave
backgrounds formed by supermassive black-hole mergers.Comment: 10 pages, 3 figures, Amaldi 8 conference proceedings, accepted by
Classical & Quantum Gravit
New Pulsars from an Arecibo Drift Scan Search
We report the discovery of pulsars J0030+0451, J0711+0931, and J1313+0931
that were found in a search of 470 square degrees at 430 MHz using the 305m
Arecibo telescope. The search has an estimated sensitivity for long period, low
dispersion measure, low zenith angle, and high Galactic latitude pulsars of ~1
mJy, comparable to previous Arecibo searches. Spin and astrometric parameters
for the three pulsars are presented along with polarimetry at 430 MHz. PSR
J0030+0451, a nearby pulsar with a period of 4.8 ms, belongs to the less common
category of isolated millisecond pulsars. We have measured significant
polarization in PSR J0030+0451 over more than 50% of the period, and use these
data for a detailed discussion of its magnetospheric geometry. Scintillation
observations of PSR J0030+0451 provide an estimate of the plasma turbulence
level along the line of sight through the local interstellar medium.Comment: 21 pages, 4 figures, Accepted for Publication in Ap
Better together?:A randomized controlled microtrial comparing different levels of therapist and parental involvement in exposure-based treatment of childhood specific phobia
INTRODUCTION: Exposure is often limited to homework assignments in routine clinical care. The current study compares minimally-guided (MGE) and parent-guided (PGE) out-session homework formats to the 'golden standard' of therapist-guided in-session exposure with minimally-guided exposure at home (TGE).METHODS: Children with specific phobia (N = 55, age 8-12, 56% girls) participated in a single-blind, randomized controlled microtrial with a four-week baseline-treatment period design. Clinical interviews, behavioral avoidance tests, and self-report measures were assessed at pre-treatment, post-treatment, and at one-month follow-up.RESULTS: TGE resulted in a larger decline of specific phobia severity from baseline to post-treatment compared to MGE but not compared to PGE. Parental anxiety was found to be a moderator of less treatment efficacy of PGE from baseline to post-treatment. Overall, there was no meaningful difference in efficacy of TGE versus MGE or PGE from baseline to follow-up.CONCLUSIONS: These findings suggest that for improving short-term treatment gains, exposure exercises can best be conducted with the help of a therapist within the therapy session before they are conducted as homework assignments outside the therapy session. However, for long-term treatment gains exposure exercises can be handled by the child itself or with help of its parents.</p
A Bayesian parameter estimation approach to pulsar time-of-arrival analysis
The increasing sensitivities of pulsar timing arrays to ultra-low frequency
(nHz) gravitational waves promises to achieve direct gravitational wave
detection within the next 5-10 years. While there are many parallel efforts
being made in the improvement of telescope sensitivity, the detection of stable
millisecond pulsars and the improvement of the timing software, there are
reasons to believe that the methods used to accurately determine the
time-of-arrival (TOA) of pulses from radio pulsars can be improved upon. More
specifically, the determination of the uncertainties on these TOAs, which
strongly affect the ability to detect GWs through pulsar timing, may be
unreliable. We propose two Bayesian methods for the generation of pulsar TOAs
starting from pulsar "search-mode" data and pre-folded data. These methods are
applied to simulated toy-model examples and in this initial work we focus on
the issue of uncertainties in the folding period. The final results of our
analysis are expressed in the form of posterior probability distributions on
the signal parameters (including the TOA) from a single observation.Comment: 16 pages, 4 figure
Placing limits on the stochastic gravitational-wave background using European Pulsar Timing Array data
Direct detection of low-frequency gravitational waves (
Hz) is the main goal of pulsar timing array (PTA) projects. One of the main
targets for the PTAs is to measure the stochastic background of gravitational
waves (GWB) whose characteristic strain is expected to approximately follow a
power-law of the form , where is the
gravitational-wave frequency. In this paper we use the current data from the
European PTA to determine an upper limit on the GWB amplitude as a function
of the unknown spectral slope with a Bayesian algorithm, by modelling
the GWB as a random Gaussian process. For the case , which is
expected if the GWB is produced by supermassive black-hole binaries, we obtain
a 95% confidence upper limit on of , which is 1.8 times
lower than the 95% confidence GWB limit obtained by the Parkes PTA in 2006. Our
approach to the data analysis incorporates the multi-telescope nature of the
European PTA and thus can serve as a useful template for future
intercontinental PTA collaborations.Comment: 14 pages, 8 figures, 3 tables, mnras accepte
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