J. P. W. Verbiest

Upper Bounds on the Low-Frequency Stochastic Gravitational Wave Background from Pulsar Timing Observations: Current Limits and Future Prospects

Using a statistically rigorous analysis method, we place limits on the existence of an isotropic stochastic gravitational wave background using pulsar timing observations. We consider backgrounds whose characteristic strain spectra may be described as a power-law dependence with frequency. Such backgrounds include an astrophysical background produced by coalescing supermassive black-hole binary systems and cosmological backgrounds due to relic gravitational waves and cosmic strings. Using the best available data, we obtain an upper limit on the energy density per unit logarithmic frequency interval of Ω h2 ≤ 1.9 × 10-8 for an astrophysical background that is 5 times more stringent than the earlier limit of 1.1 × 10-7 found by Kaspi and colleagues. We also provide limits on a background due to relic gravitational waves and cosmic strings of Ω h2 ≤ 2.0 × 10-8 and Ω h2 ≤ 1.9 × 10-8, respectively. All of the quoted upper limits correspond to a 0.1% false alarm rate together with a 95% detection rate. We discuss the physical implications of these results and highlight the future possibilities of the Parkes Pulsar Timing Array project. We find that our current results can (1) constrain the merger rate of supermassive binary black hole systems at high redshift, (2) rule out some relationships between the black hole mass and the galactic halo mass, (3) constrain the rate of expansion in the inflationary era, and (4) provide an upper bound on the dimensionless tension of a cosmic string background.

Precision Timing of PSR J0437?4715: An Accurate Pulsar Distance, a High Pulsar Mass, and a Limit on the Variation of Newton's Gravitational Constant

Analysis of 10 years of high-precision timing data on the millisecond pulsar PSR J0437–4715 has resulted in a model-independent kinematic distance based on an apparent orbital period derivative, P_b, determined at the 1.5% level of precision (D_k = 157.0 ± 2.4 pc), making it one of the most accurate stellar distance estimates published to date. The discrepancy between this measurement and a previously published parallax distance estimate is attributed to errors in the DE200 solar system ephemerides. The precise measurement of P_b allows a limit on the variation of Newtons gravitational constant, |G/G| ≤ 23 × 10^−12 yr^−1. We also constrain any anomalous acceleration along the line of sight to the pulsar to |a⊙/c| ≤ 1.5 × 10^−18 s^−1 at 95% confidence, and derive a pulsar mass, m_(psr) = 1.76 ± 0.20 M⊙, one of the highest estimates so far obtained.

The parkes pulsar timing array project

A pulsar timing array (PTA), in which observations of a large sample of pulsars spread across the celestial sphere are combined, allows investigation of global phenomena such as a background of gravitational waves or instabilities in atomic timescales that produce correlated timing residuals in the pulsars of the array. The Parkes Pulsar Timing Array (PPTA) is an implementation of the PTA concept based on observations with the Parkes 64-m radio telescope. A sample of 20 ms pulsars is being observed at three radio-frequency bands, 50 cm (similar to 700MHz), 20 cm (similar to 1400 MHz), and 10 cm (similar to 3100 MHz), with observations at intervals of two to three weeks. Regular observations commenced in early 2005. This paper describes the systems used for the PPTA observations and data processing, including calibration and timing analysis. The strategy behind the choice of pulsars, observing parameters, and analysis methods is discussed. Results are presented for PPTA data in the three bands taken between 2005 March and 2011 March. For 10 of the 20 pulsars, rms timing residuals are less than 1 mu s for the best band after fitting for pulse frequency and its first time derivative. Significant red timing noise is detected in about half of the sample. We discuss the implications of these results on future projects including the International Pulsar Timing Array and a PTA based on the Square Kilometre Array. We also present an extended PPTA data set that combines PPTA data with earlier Parkes timing data for these pulsars.

Dispersion measure variations and their effect on precision pulsar timing

We present an analysis of the variations seen in the dispersion measures (DMs) of 20-ms pulsars nobserved as part of the Parkes Pulsar Timing Array project.We carry out a statistically rigorous nstructure function analysis for each pulsar and show that the variations seen for most pulsars nare consistent with those expected for an interstellar medium characterized by a Kolmogorov nturbulence spectrum. The structure functions for PSRs J1045−4509 and J1909−3744 provide nthe first clear evidence for a large inner scale, possibly due to ion–neutral damping. We also nshow the effect of the solar wind on the DMs and show that the simple models presently nimplemented into pulsar timing packages cannot reliably correct for this effect. For the first ntime we clearly show how DM variations affect pulsar timing residuals and how they can be ncorrected in order to obtain the highest possible timing precision. Even with our presently nlimited data span, the residuals (and all parameters derived from the timing) for six of our npulsars have been significantly improved by correcting for the DM variations.

Extremely High Precision VLBI Astrometry of PSR J0437?4715 and Implications for Theories of Gravity

Using the recently upgraded Long Baseline Array, we have measured the trigonometric parallax of PSR J0437–4715 to better than 1% precision, the most precise pulsar distance determination made to date. Comparing this VLBI distance measurement to the kinematic distance obtained from pulsar timing, which is calculated from the pulsars proper motion and apparent rate of change of orbital period, gives a precise limit on the unmodeled relative acceleration between the solar system and PSR J0437–4715, which can be used in a variety of applications. First, it shows that Newtons gravitational constant G is stable with time (Ġ/G = (−5 ± 26) × 10−13 yr−1, 95% confidence). Second, if a stochastic gravitational wave background existed at the currently quoted limit, this null result would fail ~50% of the time. Third, it excludes Jupiter-mass planets within 226 AU of the Sun in 50% of the sky (95% confidence). Finally, the ~1% agreement of the parallax and orbital period derivative distances provides a fundamental confirmation of the parallax distance method on which all astronomical distances are based.

Gravitational-radiation losses from the pulsar-white-dwarf binary PSR J1141-6545

Pulsars in close binary systems with white dwarfs or other neutron stars make ideal laboratories for testing the predictions of gravitational radiation and self-gravitational effects. We report new timing measurements of the pulsar-white-dwarf binary PSR J1141-6545. The orbit is found to be decaying at a rate of 1.04{+-}0.06 times the general relativistic prediction and the Shapiro delay is consistent with the orbital inclination angle derived from scintillation measurements. The system provides a unique testbed for tensor-scalar theories of gravity. Our measurements place stringent constraints in the theory space, with a limit of {alpha}{sub 0}{sup 2}<2.1x10{sup -5} for weakly nonlinear coupling and an asymptotic limit of {alpha}{sub 0}{sup 2}<3.4x10{sup -6} for strongly nonlinear coupling (where {alpha}{sub 0} is the linear coupling strength of matter to an underlying scalar field), which is nearly 3 times smaller than the Cassini bound ({alpha}{sub 0}{sup 2}{approx_equal}10{sup -5})

tempo2: a new pulsar timing package – III. Gravitational wave simulation

Analysis of pulsar timing data sets may provide the first direct detection of gravitational waves. This paper, the third in a series describing the mathematical framework implemented into the tempo2 pulsar timing package, reports on using tempo2 to simulate the timing residuals induced by gravitational waves. The tempo2 simulations can be used to provide upper bounds on the amplitude of an isotropic, stochastic, gravitational wave background in our Galaxy and to determine the sensitivity of a given pulsar timing experiment to individual, supermassive, binary black hole systems.

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 -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.

Limits on the mass, velocity and orbit of PSR J1933−6211

We present a high-precision timing analysis of PSR J1933

Measuring the mass of solar system planets using pulsar timing

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