Optimizing Pulsar Timing Arrays to Maximize Gravitational Wave Single Source Detection: a First Cut

Pulsar Timing Arrays (PTAs) use high accuracy timing of a collection of low timing noise pulsars to search for gravitational waves in the microhertz to nanohertz frequency band. The sensitivity of such a PTA depends on (a) the direction of the gravitational wave source, (b) the timing accuracy of the pulsars in the array and (c) how the available observing time is allocated among those pulsars. Here, we present a simple way to calculate the sensitivity of the PTA as a function of direction of a single GW source, based only on the location and root-mean-square residual of the pulsars in the array. We use this calculation to suggest future strategies for the current North American Nanohertz Observatory for Gravitational Waves (NANOGrav) PTA in its goal of detecting single GW sources. We also investigate the affects of an additional pulsar on the array sensitivity, with the goal of suggesting where PTA pulsar searches might be best directed. We demonstrate that, in the case of single GW sources, if we are interested in maximizing the volume of space to which PTAs are sensitive, there exists a slight advantage to finding a new pulsar near where the array is already most sensitive. Further, the study suggests that more observing time should be dedicated to the already low noise pulsars in order to have the greatest positive effect on the PTA sensitivity. We have made a web-based sensitivity mapping tool available at http://gwastro.psu.edu/ptasm.Comment: 14 pages, 3 figures, accepted by Ap

The Evolution of PSR J0737-3039B and a Model for Relativistic Spin Precession

We present the evolution of the radio emission from the 2.8-s pulsar of the double pulsar system PSR J0737-3039A/B. We provide an update on the Burgay et al. (2005) analysis by describing the changes in the pulse profile and flux density over five years of observations, culminating in the B pulsar's radio disappearance in 2008 March. Over this time, the flux density decreases by 0.177 mJy/yr at the brightest orbital phases and the pulse profile evolves from a single to a double peak, with a separation rate of 2.6 deg/yr. The pulse profile changes are most likely caused by relativistic spin precession, but can not be easily explained with a circular hollow-cone beam as in the model of Clifton & Weisberg (2008). Relativistic spin precession, coupled with an elliptical beam, can model the pulse profile evolution well. This particular beam shape predicts geometrical parameters for the two bright orbital phases which are consistent and similar to those derived by Breton et al. (2008). However, the observed decrease in flux over time and B's eventual disappearance cannot be easily explained by the model and may be due to the changing influence of A on B.Comment: 20 pages, 18 figures, Accepted by ApJ on 2 August 201

Limits on the Stochastic Gravitational Wave Background from the North American Nanohertz Observatory for Gravitational Waves

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.Comment: To be submitted to Ap

A precise mass measurement of the intermediate-mass binary pulsar PSR J1802-2124

PSR J1802-2124 is a 12.6-ms pulsar in a 16.8-hour binary orbit with a relatively massive white dwarf (WD) companion. These properties make it a member of the intermediate-mass class of binary pulsar (IMBP) systems. We have been timing this pulsar since its discovery in 2002. Concentrated observations at the Green Bank Telescope, augmented with data from the Parkes and Nancay observatories, have allowed us to determine the general relativistic Shapiro delay. This has yielded pulsar and white dwarf mass measurements of 1.24(11) and 0.78(4) solar masses (68% confidence), respectively. The low mass of the pulsar, the high mass of the WD companion, the short orbital period, and the pulsar spin period may be explained by the system having gone through a common-envelope phase in its evolution. We argue that selection effects may contribute to the relatively small number of known IMBPs.Comment: 9 pages, 4 figures, 3 tables, accepted for publication in the Astrophysical Journa

Placing limits on the stochastic gravitational-wave background using European Pulsar Timing Array data

Direct detection of low-frequency gravitational waves ($10^{-9} - 10^{-8}$ Hz) is the main goal of pulsar timing array (PTA) projects. One of the main targets for the PTAs is to measure the stochastic background of gravitational waves (GWB) whose characteristic strain is expected to approximately follow a power-law of the form $h_c(f)=A (f/\hbox{yr}^{-1})^{\alpha}$, where $f$ is the gravitational-wave frequency. In this paper we use the current data from the European PTA to determine an upper limit on the GWB amplitude $A$ as a function of the unknown spectral slope $\alpha$ with a Bayesian algorithm, by modelling the GWB as a random Gaussian process. For the case $\alpha=-2/3$, which is expected if the GWB is produced by supermassive black-hole binaries, we obtain a 95% confidence upper limit on $A$ of $6\times 10^{-15}$, which is 1.8 times lower than the 95% confidence GWB limit obtained by the Parkes PTA in 2006. Our approach to the data analysis incorporates the multi-telescope nature of the European PTA and thus can serve as a useful template for future intercontinental PTA collaborations.Comment: 14 pages, 8 figures, 3 tables, mnras accepte

Tests of general relativity from timing the double pulsar

The double pulsar system, PSR J0737-3039A/B, is unique in that both neutron stars are detectable as radio pulsars. This, combined with significantly higher mean orbital velocities and accelerations when compared to other binary pulsars, suggested that the system would become the best available testbed for general relativity and alternative theories of gravity in the strong-field regime. Here we report on precision timing observations taken over the 2.5 years since its discovery and present four independent strong-field tests of general relativity. Use of the theory-independent mass ratio of the two stars makes these tests uniquely different from earlier studies. By measuring relativistic corrections to the Keplerian description of the orbital motion, we find that the ``post-Keplerian'' parameter s agrees with the value predicted by Einstein's theory of general relativity within an uncertainty of 0.05%, the most precise test yet obtained. We also show that the transverse velocity of the system's center of mass is extremely small. Combined with the system's location near the Sun, this result suggests that future tests of gravitational theories with the double pulsar will supersede the best current Solar-system tests. It also implies that the second-born pulsar may have formed differently to the usually assumed core-collapse of a helium star.Comment: Appeared in Science Express, Sept. 14, 2006. Includes supporting materia

The High Time Resolution Universe Survey II: Discovery of 5 Millisecond Pulsars

We present the discovery of 5 millisecond pulsars found in the mid-Galactic latitude portion of the High Time Resolution Universe (HTRU) Survey. The pulsars have rotational periods from ~2.3 to ~7.5 ms, and all are in binary systems with orbital periods ranging from ~0.3 to ~150 d. In four of these systems, the most likely companion is a white dwarf, with minimum masses of ~0.2 Solar Masses. The other pulsar, J1731-1847, has a very low mass companion and exhibits eclipses, and is thus a member of the "black widow" class of pulsar binaries. These eclipses have been observed in bands centred near frequencies of 700, 1400 and 3000 MHz, from which measurements have been made of the electron density in the eclipse region. These measurements have been used to examine some possible eclipse mechanisms. The eclipse and other properties of this source are used to perform a comparison with the other known eclipsing and "black widow" pulsars. These new discoveries occupy a short-period and high-dispersion measure (DM) region of parameter space, which we demonstrate is a direct consequence of the high time and frequency resolution of the HTRU survey. The large implied distances to our new discoveries makes observation of their companions unlikely with both current optical telescopes and the Fermi Gamma-ray Space Telescope. The extremely circular orbits make any advance of periastron measurements highly unlikely. No relativistic Shapiro delays are obvious in any of the systems, although the low flux densities would make their detection difficult unless the orbits were fortuitously edge-on.Comment: 11 pages, 5 figures, 4 tables, for publication in MNRA

The High Time Resolution Universe Survey VI: An Artificial Neural Network and Timing of 75 Pulsars

We present 75 pulsars discovered in the mid-latitude portion of the High Time Resolution Universe survey, 54 of which have full timing solutions. All the pulsars have spin periods greater than 100 ms, and none of those with timing solutions are in binaries. Two display particularly interesting behaviour; PSR J1054-5944 is found to be an intermittent pulsar, and PSR J1809-0119 has glitched twice since its discovery. In the second half of the paper we discuss the development and application of an artificial neural network in the data-processing pipeline for the survey. We discuss the tests that were used to generate scores and find that our neural network was able to reject over 99% of the candidates produced in the data processing, and able to blindly detect 85% of pulsars. We suggest that improvements to the accuracy should be possible if further care is taken when training an artificial neural network; for example ensuring that a representative sample of the pulsar population is used during the training process, or the use of different artificial neural networks for the detection of different types of pulsars.Comment: 15 pages, 8 figure