8,226 research outputs found
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
Emotional disorders: Cluster 4 of the proposed meta-structure for DSM-V and ICD-11
Background The extant major psychiatric classifications DSM-IV, and ICD-10, are atheoretical and largely descriptive. Although this achieves good reliability, the validity of a medical diagnosis would be greatly enhanced by an understanding of risk factors and clinical manifestations. In an effort to group mental disorders on the basis of aetiology, five clusters have been proposed. This paper considers the validity of the fourth cluster, emotional disorders, within that proposal.Method We reviewed the literature in relation to 11 validating criteria proposed by a Study Group of the DSM-V Task Force, as applied to the cluster of emotional disorders.Results An emotional cluster of disorders identified using the 11 validators is feasible. Negative affectivity is the defining feature of the emotional cluster. Although there are differences between disorders in the remaining validating criteria, there are similarities that support the feasibility of an emotional cluster. Strong intra-cluster co-morbidity may reflect the action of common risk factors and also shared higher-order symptom dimensions in these emotional disorders.Conclusion Emotional disorders meet many of the salient criteria proposed by the Study Group of the DSM-V Task Force to suggest a classification cluster.</p
Supernova Remnants in the Magellanic Clouds. V. The Complex Interior Structure of the N206 SNR
The N206 supernova remnant (SNR) in the Large Magellanic Cloud (LMC) has long
been considered a prototypical "mixed morphology" SNR. Recent observations,
however, have added a new twist to this familiar plot: an elongated,
radially-oriented radio feature seen in projection against the SNR face.
Utilizing the high resolution and sensitivity available with the Hubble Space
Telescope, Chandra, and XMM-Newton, we have obtained optical emission-line
images and spatially resolved X-ray spectral maps for this intriguing SNR. Our
findings present the SNR itself as a remnant in the mid to late stages of its
evolution. X-ray emission associated with the radio "linear feature" strongly
suggests it to be a pulsar-wind nebula (PWN). A small X-ray knot is discovered
at the outer tip of this feature. The feature's elongated morphology and the
surrounding wedge-shaped X-ray enhancement strongly suggest a bow-shock PWN
structure.Comment: 41 pages including 7 figures, accepted for publication by the
Astrophysical Journa
Agent-based homeostatic control for green energy in the smart grid
With dwindling non-renewable energy reserves and the adverse effects of climate change, the development of the smart electricity grid is seen as key to solving global energy security issues and to reducing carbon emissions. In this respect, there is a growing need to integrate renewable (or green) energy sources in the grid. However, the intermittency of these energy sources requires that demand must also be made more responsive to changes in supply, and a number of smart grid technologies are being developed, such as high-capacity batteries and smart meters for the home, to enable consumers to be more responsive to conditions on the grid in real-time. Traditional solutions based on these technologies, however, tend to ignore the fact that individual consumers will behave in such a way that best satisfies their own preferences to use or store energy (as opposed to that of the supplier or the grid operator). Hence, in practice, it is unclear how these solutions will cope with large numbers of consumers using their devices in this way. Against this background, in this paper, we develop novel control mechanisms based on the use of autonomous agents to better incorporate consumer preferences in managing demand. These agents, residing on consumers' smart meters, can both communicate with the grid and optimise their owner's energy consumption to satisfy their preferences. More specifically, we provide a novel control mechanism that models and controls a system comprising of a green energy supplier operating within the grid and a number of individual homes (each possibly owning a storage device). This control mechanism is based on the concept of homeostasis whereby control signals are sent to individual components of a system, based on their continuous feedback, in order to change their state so that the system may reach a stable equilibrium. Thus, we define a new carbon-based pricing mechanism for this green energy supplier that takes advantage of carbon-intensity signals available on the internet in order to provide real-time pricing. The pricing scheme is designed in such a way that it can be readily implemented using existing communication technologies and is easily understandable by consumers. Building upon this, we develop new control signals that the supplier can use to incentivise agents to shift demand (using their storage device) to times when green energy is available. Moreover, we show how these signals can be adapted according to changes in supply and to various degrees of penetration of storage in the system. We empirically evaluate our system and show that, when all homes are equipped with storage devices, the supplier can significantly reduce its reliance on other carbon-emitting power sources to cater for its own shortfalls. By so doing, the supplier reduces the carbon emission of the system by up to 25% while the consumer reduces its costs by up to 14.5%. Finally, we demonstrate that our homeostatic control mechanism is not sensitive to small prediction errors and the supplier is incentivised to accurately predict its green production to minimise costs
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
Constraining the coalescence rate of supermassive black-hole binaries using pulsar timing
Pulsar timing observations are used to place constraints on the rate of
coalescence of supermassive black-hole (SMBH) binaries as a function of mass
and redshift. In contrast to the indirect constraints obtained from other
techniques, pulsar timing observations provide a direct constraint on the
black-hole merger rate. This is possible since pulsar timing is sensitive to
the gravitational waves (GWs) emitted by these sources in the final stages of
their evolution. We find that upper bounds calculated from the recently
published Parkes Pulsar Timing Array data are just above theoretical
predictions for redshifts below 10. In the future, with improved timing
precision and longer data spans, we show that a non-detection of GWs will rule
out some of the available parameter space in a particular class of SMBH binary
merger models. We also show that if we can time a set of pulsars to 10ns timing
accuracy, for example, using the proposed Square Kilometre Array, it should be
possible to detect one or more individual SMBH binary systems
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
On detection of the stochastic gravitational-wave background using the Parkes pulsar timing array
We search for the signature of an isotropic stochastic gravitational-wave
background in pulsar timing observations using a frequency-domain correlation
technique. These observations, which span roughly 12 yr, were obtained with the
64-m Parkes radio telescope augmented by public domain observations from the
Arecibo Observatory. A wide range of signal processing issues unique to pulsar
timing and not previously presented in the literature are discussed. These
include the effects of quadratic removal, irregular sampling, and variable
errors which exacerbate the spectral leakage inherent in estimating the steep
red spectrum of the gravitational-wave background. These observations are found
to be consistent with the null hypothesis, that no gravitational-wave
background is present, with 76 percent confidence. We show that the detection
statistic is dominated by the contributions of only a few pulsars because of
the inhomogeneity of this data set. The issues of detecting the signature of a
gravitational-wave background with future observations are discussed.Comment: 12 pages, 8 figures, 7 tables, accepted for publication in MNRA
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
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