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تسجيل مستخدم جديدLimits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves
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We present an analysis of high-precision pulsar timing data taken as part of the North American Nanohertz Observatory for Gravitational waves (NANOGrav) project. We have observed 17 pulsars for a span of roughly five years using the Green Bank and Arecibo radio telescopes. We analyze these data using standard pulsar timing models, with the addition of time-variable dispersion measure and frequency-variable pulse shape terms. Sub-microsecond timing residuals are obtained in nearly all cases, and the best root-mean-square timing residuals in this set are ~30-50 ns. We present methods for analyzing post-fit timing residuals for the presence of a gravitational wave signal with a specified spectral shape. These optimally take into account the timing fluctuation power removed by the model fit, and can be applied to either data from a single pulsar, or to a set of pulsars to detect a correlated signal. We apply these methods to our dataset to set an upper limit on the strength of the nHz-frequency stochastic supermassive black hole gravitational wave background of h_c (1 yr^-1) < 7x10^-15 (95%). This result is dominated by the timing of the two best pulsars in the set, PSRs J1713+0747 and J1909-3744.
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The North American Nanohertz Observatory for Gravitational Waves (NANOGrav) is a consortium of astronomers whose goal is the creation of a galactic scale gravitational wave observatory sensitive to gravitational waves in the nHz-microHz band. It is j
ust one component of an international collaboration involving similar organizations of European and Australian astronomers who share the same goal. Gravitational waves, a prediction of Einsteins general theory of relativity, are a phenomenon of dynamical space-time generated by the bulk motion of matter, and the dynamics of space-time itself. They are detectable by the small disturbance they cause in the light travel time between some light source and an observer. NANOGrav exploits radio pulsars as both the light (radio) source and the clock against which the light travel time is measured. In an array of radio pulsars gravitational waves manifest themselves as correlated disturbances in the pulse arrival times. The timing precision of todays best measured pulsars is less than 100 ns. With improved instrumentation and signal-to-noise it is widely believed that the next decade could see a pulsar timing network of 100 pulsars each with better than 100 ns timing precision. Such a pulsar timing array (PTA), observed with a regular cadence of days to weeks, would be capable of observing supermassive black hole binaries following galactic mergers, relic radiation from early universe phenomena such as cosmic strings, cosmic superstrings, or inflation, and more generally providing a vantage on the universe whose revolutionary potential has not been seen in the 400 years since Galileo first turned a telescope to the heavens.
The paucity of observed supermassive black hole binaries (SMBHBs) may imply that the gravitational wave background (GWB) from this population is anisotropic, rendering existing analyses sub-optimal. We present the first constraints on the angular dis
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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 simultaneou
sly 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 $A<3.0times 10^{-15}$ at a reference frequency of $1mathrm{yr^{-1}}$ and a spectral index of $13/3$, corresponding to a background from inspiralling super-massive black hole binaries, constraining the GW energy density to $Omega_mathrm{gw}(f)h^2 < 1.1times10^{-9}$ 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 $sim 5times10^{-9}$~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, $Gmu/c^2$, 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 $Gmu/c^2<1.3times10^{-7}$, 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.
Within the next several years pulsar timing arrays (PTAs) are positioned to detect the stochastic gravitational-wave background (GWB) likely produced by the collection of inspiralling super-massive black holes binaries, and potentially constrain some
exotic physics. So far most of the pulsar timing data analysis has focused on the monopole of the GWB, assuming it is perfectly isotropic. The natural next step is to search for anisotropies in the GWB. In this paper, we use the recently developed PTA Fisher matrix to gain insights into optimal search strategies for GWB anisotropies. For concreteness, we apply our results to EPTA data, using realistic noise characteristics of its pulsars. We project the detectability of a GWB whose angular dependence is assumed to be a linear combination of predetermined maps, such as spherical harmonics or coarse pixels. We find that the GWB monopole is always statistically correlated with these maps, implying a loss of sensitivity to the monopole when searching simultaneously for anisotropies. We then derive the angular distributions of the GWB intensity to which a PTA is most sensitive, and illustrate how one may use these principal maps to approximately reconstruct the angular dependence of the GWB. Since the principal maps are neither perfectly anisotropic nor uncorrelated with the monopole, we also develop a frequentist criterion to specifically search for anisotropies in the GWB without any prior knowledge about their angular distribution. Lastly, we show how to recover existing EPTA results with our Fisher formalism, and clarify their meaning. The tools presented here will be valuable in guiding and optimizing the computationally demanding analyses of pulsar timing data.
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