Signals from radio pulsars show a wavelength-dependent delay due to dispersion in the interstellar plasma. At a typical observing wavelength, this delay can vary by tens of microseconds on 5-yr ...time-scales, far in excess of signals of interest to pulsar timing arrays, such as that induced by a gravitational wave background. Measurement of these delay variations is not only crucial for the detection of such signals, but also provides an unparalleled measurement of the turbulent interstellar plasma at astronomical unit (au) scales.
In this paper we demonstrate that without consideration of wavelength-independent red noise, 'simple' algorithms to correct for interstellar dispersion can attenuate signals of interest to pulsar timing arrays. We present a robust method for this correction, which we validate through simulations, and apply it to observations from the Parkes Pulsar Timing Array. Correction for dispersion variations comes at a cost of increased band-limited white noise. We discuss scheduling to minimize this additional noise, and factors, such as scintillation, that can exacerbate the problem.
Comparison with scintillation measurements confirms previous results that the spectral exponent of electron density variations in the interstellar medium often appears steeper than expected. We also find a discrete change in dispersion measure of PSR J1603−7202 of ∼2 × 10−3 cm−3 pc for about 250 d. We speculate that this has a similar origin to the 'extreme scattering events' seen in other sources. In addition, we find that four pulsars show a wavelength-dependent annual variation, indicating a persistent gradient of electron density on an au spatial scale, which has not been reported previously.
The analysis of high-precision timing observations of an array of ∼20 millisecond pulsars (a so-called ‘timing array’) may ultimately result in the detection of a stochastic gravitational-wave ...background. The feasibility of such a detection and the required duration of this type of experiment are determined by the achievable rms of the timing residuals and the timing stability of the pulsars involved. We present results of the first long-term, high-precision timing campaign on a large sample of millisecond pulsars used in gravitational-wave detection projects. We show that the timing residuals of most pulsars in our sample do not contain significant low-frequency noise that could limit the use of these pulsars for decade-long gravitational-wave detection efforts. For our most precisely timed pulsars, intrinsic instabilities of the pulsars or the observing system are shown to contribute to timing irregularities on a 5-year time-scale below the 100 ns level. Based on those results, realistic sensitivity curves for planned and ongoing timing array efforts are determined. We conclude that prospects for detection of a gravitational-wave background through pulsar timing array efforts within 5 years to a decade are good.
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 super(S) sub(g) super(MBH) 1/(8 yr) h super(2) , 1.9 x 10 super(-8) for an astrophysical background that is 5 times more stringent than the earlier limit of 1.1 x 10 super(-7) found by Kaspi and colleagues. We also provide limits on a background due to relic gravitational waves and cosmic strings of super(r) sub(g) super(elic) 1/(8 yr) h super(2) ,< 2.0 x 10 super(-8) and super(c) sub(g) super(s) 1/(8 yr) h super(2) , 1.9 x 10 super(-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 acosmic string background.
We present the sensitivity of the Parkes Pulsar Timing Array to gravitational waves (GWs) emitted by individual supermassive black hole binary systems in the early phases of coalescing at the cores ...of merged galaxies. Our analysis includes a detailed study of the effects of fitting a pulsar timing model to non-white timing residuals. Pulsar timing is sensitive at nanoHertz frequencies and hence complementary to Laser Interferometer Gravitational-Wave Observatory and Laser Interferometer Space Antenna. We place a sky-averaged constraint on the merger rate of nearby (z < 0.6) black hole binaries in the early phases of coalescence with a chirp mass of 1010 M☉ of less than one merger every 7 yr. The prospects for future GW astronomy of this type with the proposed Square Kilometre Array telescope are discussed.
Polarization observations of 20 millisecond pulsars Yan, W. M.; Manchester, R. N.; van Straten, W. ...
Monthly notices of the Royal Astronomical Society,
July 2011, Letnik:
414, Številka:
3
Journal Article
Recenzirano
Odprti dostop
Polarization profiles are presented for 20 millisecond pulsars that are being observed as part of the Parkes Pulsar Timing Array project. The observations used the Parkes multibeam receiver with a ...central frequency of 1369 MHz and the Parkes digital filter bank pulsar signal-processing system PDFB2. Because of the large total observing time, the summed polarization profiles have very high signal-to-noise ratios and show many previously undetected profile features. 13 of the 20 pulsars show emission over more than half of the pulse period. Polarization variations across the profiles are complex, and the observed position angle variations are generally not in accord with the rotating vector model for pulsar polarization. Nevertheless, the polarization properties are broadly similar to those of normal (non-millisecond) pulsars, suggesting that the basic radio emission mechanism is the same in both classes of pulsar. The results support the idea that radio emission from millisecond pulsars originates high in the pulsar magnetosphere, probably close to the emission regions for high-energy X-ray and gamma-ray emission. Rotation measures were obtained for all 20 pulsars, eight of which had no previously published measurements.
We search for the signature of an isotropic stochastic gravitational-wave background in pulsar timing observations using a frequency-domain correlation technique. These observations, which span ...roughly 12 yr, were obtained with the 64-m Parkes radio telescope augmented by public domain observations from the Arecibo Observatory. A wide range of signal processing issues unique to pulsar timing and not previously presented in the literature are discussed. These include the effects of quadratic removal, irregular sampling and variable errors which exacerbate the spectral leakage inherent in estimating the steep red spectrum of the gravitational-wave background. These observations are found to be consistent with the null hypothesis that no gravitational-wave background is present, with 76 per cent confidence. We show that the detection statistic is dominated by the contributions of only a few pulsars because of the inhomogeneity of this data set. The issues of detecting the signature of a gravitational-wave background with future observations are discussed.
Abstract
Many radio galaxies show the presence of dense and dusty gas near the active nucleus. This can be traced by both 21 cm H i absorption and soft X-ray absorption, offering new insight into the ...physical nature of the circumnuclear medium of these distant galaxies. To better understand this relationship, we investigate soft X-ray absorption as an indicator for the detection of associated H i absorption, as part of preparation for the First Large Absorption Survey in H i to be undertaken with the Australian Square Kilometre Array Pathfinder (ASKAP). We present the results of our pilot study using the Boolardy Engineering Test Array, a precursor to ASKAP, to search for new absorption detections in radio sources brighter than 1 Jy that also feature soft X-ray absorption. Based on this pilot survey, we detected H i absorption towards the radio source PKS 1657−298 at a redshift of z = 0.42. This source also features the highest X-ray absorption ratio of our pilot sample by a factor of 3, which is consistent with our general findings that X-ray absorption predicates the presence of dense neutral gas. By comparing the X-ray properties of active galactic nuclei with and without detection of H i absorption at radio wavelengths, we find that X-ray hardness ratio and H i absorption optical depth are correlated at a statistical significance of 4.71σ. We conclude by considering the impact of these findings on future radio and X-ray absorption studies.
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