Single pulses preserve information about the pulsar radio emission and propagation in the pulsar magnetosphere, and understanding the behaviour of their variability is essential for estimating the ...fundamental limit on the achievable pulsar timing precision. Here we report the findings of our analysis of single pulses from PSR J1713+0747 with data collected by the Large European Array for Pulsars (LEAP). We present statistical studies of the pulse properties that include distributions of their energy, phase and width. Two modes of systematic sub-pulse drifting have been detected, with a periodicity of 7 and 3 pulse periods. The two modes appear at different ranges of pulse longitude but overlap under the main peak of the integrated profile. No evidence for pulse micro-structure is seen with a time resolution down to 140 ns. In addition, we show that the fractional polarisation of single pulses increases with their pulse peak flux density. By mapping the probability density of linear polarisation position angle with pulse longitude, we reveal the existence of two orthogonal polarisation modes. Finally, we find that the resulting phase jitter of integrated profiles caused by single pulse variability can be described by a Gaussian probability distribution only when at least 100 pulses are used for integration. Pulses of different flux densities and widths contribute approximately equally to the phase jitter, and no improvement on timing precision is achieved by using a sub-set of pulses with a specific range of flux density or width.
The CALO meeting assistant provides for distributed meeting capture, annotation, automatic transcription and semantic analysis of multiparty meetings, and is part of the larger CALO personal ...assistant system. This paper summarizes the CALO-MA architecture and its speech recognition and understanding components, which include real-time and offline speech transcription, dialog act segmentation and tagging, question-answer pair identification, action item recognition, decision extraction, and summarization.
We have searched for continuous gravitational wave (CGW) signals produced by individually resolvable, circular supermassive black hole binaries (SMBHBs) in the latest EPTA dataset, which consists of ...ultra-precise timing data on 41 millisecond pulsars. We develop frequentist and Bayesian detection algorithms to search both for monochromatic and frequency-evolving systems. None of the adopted algorithms show evidence for the presence of such a CGW signal, indicating that the data are best described by pulsar and radiometer noise only. Depending on the adopted detection algorithm, the 95\% upper limit on the sky-averaged strain amplitude lies in the range \(6\times 10^{-15}<A<1.5\times10^{-14}\) at \(5{\rm nHz}<f<7{\rm nHz}\). This limit varies by a factor of five, depending on the assumed source position, and the most constraining limit is achieved towards the positions of the most sensitive pulsars in the timing array. The most robust upper limit -- obtained via a full Bayesian analysis searching simultaneously over the signal and pulsar noise on the subset of ours six best pulsars -- is \(A\approx10^{-14}\). These limits, the most stringent to date at \(f<10{\rm nHz}\), exclude the presence of sub-centiparsec binaries with chirp mass \(\cal{M}_c>10^9\)M\(_\odot\) out to a distance of about 25Mpc, and with \(\cal{M}_c>10^{10}\)M\(_\odot\) out to a distance of about 1Gpc (\(z\approx0.2\)). We show that state-of-the-art SMBHB population models predict \(<1\%\) probability of detecting a CGW with the current EPTA dataset, consistent with the reported non-detection. We stress, however, that PTA limits on individual CGW have improved by almost an order of magnitude in the last five years. The continuing advances in pulsar timing data acquisition and analysis techniques will allow for strong astrophysical constraints on the population of nearby SMBHBs in the coming years.
We present new limits on an isotropic stochastic gravitational-wave background (GWB) using a six pulsar dataset spanning 18 yr of observations from the 2015 European Pulsar Timing Array data release. ...Performing a Bayesian analysis, we fit simultaneously for the intrinsic noise parameters for each pulsar, along with common correlated signals including clock, and Solar System ephemeris errors, obtaining a robust 95\(\%\) upper limit on the dimensionless strain amplitude \(A\) of the background of \(A<3.0\times 10^{-15}\) at a reference frequency of \(1\mathrm{yr^{-1}}\) and a spectral index of \(13/3\), corresponding to a background from inspiralling super-massive black hole binaries, constraining the GW energy density to \(\Omega_\mathrm{gw}(f)h^2 < 1.1\times10^{-9}\) at 2.8 nHz. We also present limits on the correlated power spectrum at a series of discrete frequencies, and show that our sensitivity to a fiducial isotropic GWB is highest at a frequency of \(\sim 5\times10^{-9}\)~Hz. Finally we discuss the implications of our analysis for the astrophysics of supermassive black hole binaries, and present 95\(\%\) upper limits on the string tension, \(G\mu/c^2\), characterising a background produced by a cosmic string network for a set of possible scenarios, and for a stochastic relic GWB. For a Nambu-Goto field theory cosmic string network, we set a limit \(G\mu/c^2<1.3\times10^{-7}\), identical to that set by the {\it Planck} Collaboration, when combining {\it Planck} and high-\(\ell\) Cosmic Microwave Background data from other experiments. For a stochastic relic background we set a limit of \(\Omega^\mathrm{relic}_\mathrm{gw}(f)h^2<1.2 \times10^{-9}\), a factor of 9 improvement over the most stringent limits previously set by a pulsar timing array.
The International Pulsar Timing Array project combines observations of pulsars from both Northern and Southern hemisphere observatories with the main aim of detecting ultra-low frequency (~10^-9 to ...10^-8 Hz) gravitational waves. Here we introduce the project, review the methods used to search for gravitational waves emitted from coalescing supermassive binary black-hole systems in the centres of merging galaxies and discuss the status of the project.
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