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Register a new userLimits on the speed of gravitational waves from pulsar timing
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Added by
Deepak Baskaran Dr.
Publication date
2008
fields
Physics
and research's language is
English
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In this work, analyzing the propagation of electromagnetic waves in the field of gravitational waves, we show the presence and significance of the so called surfing effect for pulsar timing measurements. It is shown that, due to the transverse nature of gravitational waves, the surfing effect leads to enormous pulsar timing residuals if the speed of gravitational waves is smaller than speed of light. This fact allows to place significant constraints on parameter $epsilon$, which characterizes the relative deviation of the speed of gravitational waves from the speed of light. We show that the existing constraints from pulsar timing measurements already place stringent limits on $epsilon$ and consequently on the mass of graviton $m_g$. These limits on $m_g$ are three orders of magnitude stronger than the current constraints from Solar System tests. The current constraints also allow to rule out massive gravitons as possible candidates for cold dark matter in galactic halo. In the near future, the gravitational wave background from extragalactic super massive black hole binaries, along with the expected sub-microsecond pulsar timing accuracy, will allow to achieve constrains of $epsilonlesssim 0.4%$ and possibly stronger.
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We search for isotropic stochastic gravitational-wave background including non-tensorial polarizations allowed in general metric theories of gravity in the Parkes Pulsar Timing Array (PPTA) second data release (DR2). We find no statistically significant evidence that the common process reported by the PPTA collaboration has the tensor transverse (TT), scalar transverse (ST), vector longitudinal (VL), or scalar longitudinal (SL) correlations in PPTA DR2. Therefore, we place $95%$ upper limit on the amplitude of each polarization mode as $mathcal{A}_{mathrm{TT}} lesssim 3.2times 10^{-15}$, $mathcal{A}_{mathrm{ST}} lesssim 1.8times 10^{-15}$, $mathcal{A}_{mathrm{VL}}lesssim 3.5times 10^{-16}$ and $mathcal{A}_{mathrm{SL}}lesssim 4.2times 10^{-17}$; or equivalently, the $95%$ upper limit on the energy density parameter per logarithm frequency as $Omega_{mathrm{GW}}^{mathrm{TT}} lesssim 1.4times 10^{-8}$, $Omega_{mathrm{GW}}^{mathrm{ST}} lesssim 4.5times 10^{-9}$, $Omega_{mathrm{GW}}^{mathrm{VL}} lesssim 1.7times 10^{-10}$ and $Omega_{mathrm{GW}}^{mathrm{SL}} lesssim 2.4times 10^{-12}$ at frequency of 1/year.
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 $6times 10^{-15}<A<1.5times10^{-14}$ 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 $Aapprox10^{-14}$. 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 ($zapprox0.2$). 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.
Massive black holes are key components of the assembly and evolution of cosmic structures and a number of surveys are currently on-going or planned to probe the demographics of these objects and to gain insight into the relevant physical processes. Pulsar Timing Arrays (PTAs) currently provide the only means to observe gravitational radiation from massive black hole binary systems with masses >10^7 solar masses. The whole cosmic population produces a stochastic background that could be detectable with upcoming Pulsar Timing Arrays. Sources sufficiently close and/or massive generate gravitational radiation that significantly exceeds the level of the background and could be individually resolved. We consider a wide range of massive black hole binary assembly scenarios, we investigate the distribution of the main physical parameters of the sources, such as masses and redshift, and explore the consequences for Pulsar Timing Arrays observations. Depending on the specific massive black hole population model, we estimate that on average at least one resolvable source produces timing residuals in the range ~5-50 ns. Pulsar Timing Arrays, and in particular the future Square Kilometre Array (SKA), can plausibly detect these unique systems, although the events are likely to be rare. These observations would naturally complement on the high-mass end of the massive black hole distribution function future surveys carried out by the Laser Interferometer Space Antenna (LISA)
Merging supermassive black hole binaries produce low-frequency gravitational waves, which pulsar timing experiments are searching for. Much of the current theory is developed within the plane-wave formalism, and here we develop the more general Fresnel formalism. We show that Fresnel corrections to gravitational wave timing residual models allow novel measurements to be made, such as direct measurements of the source distance from the timing residual phase and frequency, as well as direct measurements of chirp mass from a monochromatic source. Probing the Fresnel corrections in these models will require future pulsar timing arrays with more distant pulsars across our Galaxy (ideally at the distance of the Magellanic Clouds), timed with precisions less than $100$ ns, with distance uncertainties reduced to the order of the gravitational wavelength. We find that sources with chirp mass of order $10^9 mathrm{M}_odot$ and orbital frequency $omega_0 > 10$ nHz are good candidates for probing Fresnel corrections. With these conditions met, the measured source distance uncertainty can be made less than 10 per cent of the distance to the source for sources out to $sim 100$ Mpc, source sky localization can be reduced to sub-arcminute precision, and source volume localization can be made to less than $1 text{Mpc}^3$ for sources out to 1-Gpc distances.
Pulsar timing arrays act to detect gravitational waves by observing the small, correlated effect the waves have on pulse arrival times at Earth. This effect has conventionally been evaluated assuming the gravitational wave phasefronts are planar across the array, an assumption that is valid only for sources at distances $Rgg2pi{}L^2/lambda$, where $L$ is physical extent of the array and $lambda$ the radiation wavelength. In the case of pulsar timing arrays (PTAs) the array size is of order the pulsar-Earth distance (kpc) and $lambda$ is of order pc. Correspondingly, for point gravitational wave sources closer than $sim100$~Mpc the PTA response is sensitive to the source parallax across the pulsar-Earth baseline. Here we evaluate the PTA response to gravitational wave point sources including the important wavefront curvature effects. Taking the wavefront curvature into account the relative amplitude and phase of the timing residuals associated with a collection of pulsars allows us to measure the distance to, and sky position of, the source.
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