Binary system delays and timing noise in searches for gravitational waves from known pulsars

The majority of fast millisecond pulsars are in binary systems, so that any periodic signal they emit is modulated by both Doppler and relativistic effects. Here we show how well-established binary models can be used to account for these effects in searches for gravitational waves from known pulsars within binary systems. A seperate issue affecting certain pulsar signals is that of timing noise and we show how this, with particular reference to the Crab pulsar, can be compensated for by using regularly updated timing ephemerides.Comment: 10 pages, 11 figures, accepted by Phys. Rev.

Detecting Beyond-Einstein Polarizations of Continuous Gravitational Waves

The direct detection of gravitational waves with the next generation detectors, like Advanced LIGO, provides the opportunity to measure deviations from the predictions of General Relativity. One such departure would be the existence of alternative polarizations. To measure these, we study a single detector measurement of a continuous gravitational wave from a triaxial pulsar source. We develop methods to detect signals of any polarization content and distinguish between them in a model independent way. We present LIGO S5 sensitivity estimates for 115 pulsars.Comment: submitted to PR

Searches for continuous and transient gravitational waves from known neutron stars and their astrophysical implications

We have used data from the third and fourth science runs of the laser interferometric gravitational wave detectors LIGO and GEO600 to produce upper limits on the emission of gravitational waves from a selection of known neutron stars. Two different emission mechanisms are looked into; i) the emission of continuous gravitational waves from triaxial neutron stars; and ii) emission of quasi-normal mode ring-downs from glitching neutron stars. We have produced upper limits on the gravitational wave amplitude and ellipticity for 93 known pulsars assuming continuous emission via triaxiality. This selection of pulsars includes the majority of currently known pulsars with frequencies > 25 Hz, with many within binary systems and globular clusters. New algorithms to take into account the motions within binary systems and possible effects of pulsar timing noise are presented. Also shown is the first analysis to combine the data sets from two distinct science runs as a method of lowering the upper limits. The results are starting to push into the range of plausible neutron star ellipticities, with the Crab pulsar closely approaching the limit that can be set through spin-down arguments. For the 32 of these pulsars in globular clusters the results provide upper limits independent of the cluster dynamics. The astrophysical significance of these results is discussed. Along with results from true pulsars we also present the extraction of simulated signals injected into the interferometers during the science runs. These provide validation checks of both the extraction software and the coherence of the detectors. Two techniques are discussed in relation to searching for quasi-normal mode ring-down signals from excited neutron stars, for example during a glitch; one based on matched filtering and the other based on Bayesian evidence. These are both applied to a search for such a signal from SGR1806 20 during a GRB on 27th December 2004, using the LIGO H1 detector and GEO600 data. This search provided upper limits on the energy released in gravitational waves via quasi-normal modes over the range of frequencies from 1-4 kHz. These are compared with results from a previous search using the bar detector AURIGA (Baggio et al, 2005) and theoretical arguments. The limitations of the search and search techniques, and possible extensions to these, are discussed. The future of these searches is discussed with regard to extensions to the analysis techniques and number of potential sources. Particular emphasis is placed on searches using data from the current LSC S5 science run

An Evidence Based Search Method For Gravitational Waves From Neutron Star Ring-downs

The excitation of quadrupolar quasi-normal modes in a neutron star leads to the emission of a short, distinctive, burst of gravitational radiation in the form of a decaying sinusoid or `ring-down'. We present a Bayesian analysis method which incorporates relevant prior information about the source and known instrumental artifacts to conduct a robust search for the gravitational wave emission associated with pulsar glitches and soft $\gamma$-ray repeater flares. Instrumental transients are modelled as sine-Gaussian and their evidence, or marginal likelihood, is compared with that of Gaussian white noise and ring-downs via the `odds-ratio'. Tests using simulated data with a noise spectral density similar to the LIGO interferometer around 1 kHz yield 50% detection efficiency and 1% false alarm probability for ring-down signals with signal-to-noise ratio $\rho=5.2$. For a source at 15 kpc this requires an energy of 1.3\times 10^{-5}M_{\astrosun}c^2 to be emitted as gravitational waves.Comment: 14 pages, 12 figure

Population synthesis and parameter estimation of neutron stars with continuous gravitational waves and third-generation detectors

Precise measurement of stellar properties through the observation of continuous gravitational waves from spinning non-axisymmetric neutron stars can shed light onto new physics beyond terrestrial laboratories. Although hitherto undetected, prospects for detecting continuous gravitational waves improve with longer observation periods and more sensitive gravitational wave detectors. We study the capability of the Advanced Laser Interferometer Gravitational-Wave Observatory, and the Einstein Telescope to measure the physical properties of neutron stars through continuous gravitational wave observations. We simulate a population of Galactic neutron stars, assume continuous gravitational waves from the stars have been detected, and perform parameter estimation of the detected signals. Using the estimated parameters, we infer the stars' moments of inertia, ellipticities, and the components of the magnetic dipole moment perpendicular to the rotation axis. The estimation of the braking index proved challenging and is responsible for the majority of the uncertainties in the inferred parameters. Using the Einstein Telescope with an observation period of 5 yrs, point estimates using median can be made with errors of ~ 10 - 100% and ~ 5 - 50% respectively, subject to the inference of the braking index. The perpendicular magnetic dipole moment could not be accurately inferred for neutron stars that emit mainly gravitational waves.Comment: 11 pages, 7 figure

Searching for gravitational waves from the Crab pulsar - the problem of timing noise

Of the current known pulsars, the Crab pulsar (B0531+21) is one of the most promising sources of gravitational waves. The relatively large timing noise of the Crab causes its phase evolution to depart from a simple spin-down model. This effect needs to be taken in to account when performing time domain searches for the Crab pulsar in order to avoid severely degrading the search efficiency. The Jodrell Bank Crab pulsar ephemeris is examined to see if it can be used for tracking the phase evolution of any gravitational wave signal from the pulsar, and we present a method of heterodyning the data that takes account of the phase wander. The possibility of obtaining physical information about the pulsar from comparisons of the electromagnetically and a gravitationally observed timing noise is discussed. Finally, additional problems caused by pulsar glitches are discussed.Comment: 5 pages, 1 figure, Proceedings of the 5th Amaldi Conference on Gravitational Waves, Pisa, Italy, 6-11 July 200

Advanced technologies for future ground-based, laser-interferometric gravitational wave detectors

We present a review of modern optical techniques being used and developed for the field of gravitational wave detection. We describe the current state-of-the-art of gravitational waves detector technologies with regard to optical layouts, suspensions and test masses. We discuss the dominant sources and noise in each of these subsystems and the developments that will help mitigate them for future generations of detectors. We very briefly summarise some of the novel astrophysics that will be possible with these upgraded detectors

Probing dynamical gravity with the polarization of continuous gravitational waves

The direct detection of gravitational waves provides the opportunity to measure fundamental aspects of gravity which have never been directly probed before, including the polarization of gravitational waves. In the context of searches for continuous waves from known pulsars, we present novel methods to detect signals of any polarization content, measure the modes present and place upper limits on the amplitude of nontensorial components. This will allow us to obtain new model-independent, dynamical constraints on deviations from general relativity. We test this framework on multiple potential sources using simulated data from three advanced-era detectors at design sensitivity. We find that signals of any polarization will become detectable and distinguishable for characteristic strains h ≳3 ×10-27√{1 yr /T }, for an observation time T . We also find that our ability to detect nontensorial components depends only on the power present in those modes, irrespective of the strength of the tensorial strain

Establishing the significance of continuous gravitational-wave detections from known pulsars

We present a method for assigning a statistical significance to detection candidates in targeted searches for continuous gravitational waves from known pulsars, without assuming the detector noise is Gaussian and stationary. We take advantage of the expected Doppler phase modulation of the signal induced by Earth’s orbital motion, as well as the amplitude modulation induced by Earth’s spin, to effectively blind the search to real astrophysical signals from a given location in the sky. We use this “sky shifting” to produce a large number of noise-only data realizations to empirically estimate the background of a search and assign detection significances, in a similar fashion to the use of time slides in searches for compact binaries. We demonstrate the potential of this approach by means of simulated signals, as well as hardware injections into real detector data. In a study of simulated signals in non-Gaussian noise, we find that our method outperforms another common strategy for evaluating detection significance. We thus demonstrate that this and similar techniques have the potential to enable a first confident detection of continuous gravitational waves

Population synthesis and parameter estimation of neutron stars with continuous gravitational waves and third-generation detectors

Precise measurement of stellar properties through the observation of continuous gravitational waves from spinning non-axisymmetric neutron stars can shed light onto new physics beyond terrestrial laboratories. Although hitherto undetected, prospects for detecting continuous gravitational waves improve with longer observation periods and more sensitive gravitational wave detectors. We study the capability of the Advanced Laser Interferometer Gravitational-Wave Observatory, and the Einstein Telescope to measure the physical properties of neutron stars through continuous gravitational wave observations. We simulate a population of Galactic neutron stars, assume continuous gravitational waves from the stars have been detected, and perform parameter estimation of the detected signals. Using the estimated parameters, we infer the stars’ moments of inertia, ellipticities, and the components of the magnetic dipole moment perpendicular to the rotation axis. The estimation of the braking index proved challenging and is responsible for the majority of the uncertainties in the inferred parameters. Using the Einstein Telescope with an observation period of $5\, {\rm {yr}}$, point estimates using median can be made on the moments of inertia with error of $\sim 10\!-\!100~{{\ \rm per\ cent}}$ and on the ellipticities with error of $\sim 5\!-\!50~{{\ \rm per\ cent}}$, subject to the inference of the braking index. The perpendicular magnetic dipole moment could not be accurately inferred for neutron stars that emit mainly gravitational waves