No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Study on the One-year Accuracy of Pulsar Time-scale
Abstract
Determining accurate pulsar timing model parameters is essential for establishing TT(PT), a realization of Terrestrial Time(TT) based on a pulsar time-scale(PT). This study discusses the impact of different data spans on the accuracy of pulsar timing model parameters when determining pulsar timing model parameters. Using observations of PSR J0437-4715, J1909-3744, J1713+0747, and J1744-1134 from the second data release of the International Pulsar Timing Array (IPTA II, Version A), we compare the accuracy of the timing model parameters determined by these observations with different data spans. The results show for PSR J0437-4715, J1713+0747, and J1909-3744, the amplitude fluctuations of rotational frequency remain within , , and Hz, respectively, when the data spans for determining pulsar timing model parameters exceed 13, 14, and 6 years. Additionally, the one-year accuracy of TT(PT) is crucial for its application in timekeeping. By comparing the frequency deviations of TT(PT) relative to TT(BIPM) under both ideal () and actual () conditions across different data spans, we find that when the data span reaches the duration above, the accuracy of TT(PT) surpasses that of TT(TAI) under ideal conditions, slightly inferior under actual conditions. This suggests with improved observational technologies, the accuracy of TT(PT) can be further enhanced.
... The possibility of establishing time scales with high-stability performance by combining pulsars and atomic clocks was discussed in [21,22]. The authors in [23] discussed the impact of different data spans on the accuracy of pulsar timing model parameters. The authors in [24] intended to utilize millisecond pulsar timing to enhance and maintain a space-time benchmark for future deep space application. ...
To provide autonomous and accurate time service for deep space missions, a pulsar observation–based timekeeping method is documented in this paper, which utilizes pulsars as the time information source. Firstly, the pulsar observation noise is remodeled as the combination of the Gaussian noise and colored noise, and the detailed expression of the colored noise is presented in the paper. An improved Grey model (GM) is proposed to describe the onboard clock evolution, which models the grey action quantity as a time-varying coefficient and attenuates the dependence of the model on the initial state. On the basis of the modified observation noise model and GM, an unscented Kalman filtering (UKF) is adopted to estimate the onboard clock error. Numerical experiments are conducted to analyze the validity of the proposed method and the impact of spacecraft positioning error and pulsar selection on the proposed method. The proposed method offers an autonomous solution for onboard timekeeping in deep space missions.
An improved Wiener filtration (WF) method for constructing the ensemble pulsar timescale (EPT) is proposed to solve the existing problems in the WF algorithm. The improvements consist of three parts: (i) adjusting the cross-power spectral density (PSD) and PSD estimated by the weighted average (WA) algorithm by the concept of the energy upper limit; (ii) combining the cross-PSD and WA PSD to form a different signal modulus for the filtration of residuals of different noise levels; and (iii) setting a weight for each residual by a more general algorithm based on the concept of effective power. We use this improved WF method to construct the EPT by both simulated data and observational data from the second data release of the International Pulsar Timing Array. The results from the simulated data indicate this improved WF can successfully suppress the noise level and reform the common signal. For observational data, this method also successfully detects the fluctuations of International Atomic Time (TAI) and local atomic time TA(NTSC). A stability analysis shows that an EPT will have the potential to reveal the instability of TAI on a scale longer than 7 yr. This improved WF method can also be used to detect and monitor the noise in pulsar timing residuals inversely, by using multiple atomic time standards, which can in turn improve the EPT.
We present the pulse arrival times and high-precision dispersion measure estimates for 14 millisecond pulsars observed simultaneously in the 300 500 MHz and 1260 1460 MHz frequency bands using the upgraded Giant Metrewave Radio Telescope. The data spans over a baseline of 3.5 years (2018-2021), and is the first official data release made available by the Indian Pulsar Timing Array collaboration. This data release presents a unique opportunity for investigating the interstellar medium effects at low radio frequencies and their impact on the timing precision of pulsar timing array experiments. In addition to the dispersion measure time series and pulse arrival times obtained using both narrowband and wideband timing techniques, we also present the dispersion measure structure function analysis for selected pulsars. Our ongoing investigations regarding the frequency dependence of dispersion measures have been discussed. Based on the preliminary analysis for five millisecond pulsars, we do not find any conclusive evidence of chromaticity in dispersion measures. Data from regular simultaneous two-frequency observations are presented for the first time in this work. This distinctive feature leads us to the highest precision dispersion measure estimates obtained so far for a subset of our sample. Simultaneous multi-band upgraded Giant Metrewave Radio Telescope observations in 300 500 MHz and 1260 1460 MHz are crucial for high-precision dispersion measure estimation and for the prospect of expanding the overall frequency coverage upon the combination of data from the various Pulsar Timing Array consortia in the near future. Parts of the data presented in this work are expected to be incorporated into the upcoming third data release of the International Pulsar Timing Array.
A new algorithm for the clock frequency prediction used to calculate the Echelle Atomique Libre (EAL) is presented. The mathematical model adopted in the new prediction algorithm takes into account the effect of the frequency drift of the H-masers. We demonstrate that there is an improvement in long-term stability of EAL by using the new prediction algorithm.
Unusual signals from pulsating radio sources have been recorded at the Mullard Radio Astronomy Observatory. The radiation seems to come from local objects within the galaxy, and may be associated with oscillations of white dwarf or neutron stars.
The algorithm of the ensemble pulsar time based on the optimal Wiener filtration method has been constructed. This algorithm allows the separation of the contributions to the post-fit pulsar timing residuals of the atomic clock and pulsar itself. Filters were designed with the use of the cross- and autocovariance functions of the timing residuals. The method has been applied to the timing data of millisecond pulsars PSR B1855+09 and PSR B1937+21 and allowed the filtering out of the atomic scale component from the pulsar data. Direct comparison of the terrestrial time TT(BIPM06) and the ensemble pulsar time PT revealed that fractional instability of TT(BIPM06)--PT is equal to . Based on the statistics of TT(BIPM06)--PT a new limit of the energy density of the gravitational wave background was calculated to be equal to . Comment: 6 pages, 5 figures
The main goal of pulsar timing array experiments is to detect correlated signals such as nanohertz-frequency gravitational waves. Pulsar timing data collected in dense monitoring campaigns can also be used to study the stars themselves, their binary companions, and the intervening ionized interstellar medium. Timing observations are extraordinarily sensitive to changes in path-length between the pulsar and the Earth, enabling precise measurements of the pulsar positions, distances and velocities, and the shapes of their orbits. Here we present a timing analysis of 25 pulsars observed as part of the Parkes Pulsar Timing Array (PPTA) project over time spans of up to 24 yr. The data are from the second data release of the PPTA, which we have extended by including legacy data. We make the first detection of Shapiro delay in four Southern pulsars (PSRs J1017−7156, J1125−6014, J1545−4550, and J1732−5049), and of parallax in six pulsars. The prominent Shapiro delay of PSR J1125−6014 implies a neutron star mass of Mp = 1.5 ± 0.2 M⊙ (68 per cent credibility interval). Measurements of both Shapiro delay and relativistic periastron advance in PSR J1600−3053 yield a large but uncertain pulsar mass of M⊙ (68 per cent credibility interval). We measure the distance to PSR J1909−3744 to a precision of 10 lyr, indicating that for gravitational wave periods over a decade, the pulsar provides a coherent baseline for pulsar timing array experiments.
We search for an isotropic stochastic gravitational-wave background (GWB) in the 12.5 yr pulsar-timing data set collected by the North American Nanohertz Observatory for Gravitational Waves. Our analysis finds strong evidence of a stochastic process, modeled as a power law, with common amplitude and spectral slope across pulsars. Under our fiducial model, the Bayesian posterior of the amplitude for an f −2/3 power-law spectrum, expressed as the characteristic GW strain, has median 1.92 × 10 ⁻¹⁵ and 5%–95% quantiles of 1.37–2.67 × 10 ⁻¹⁵ at a reference frequency of f yr = 1 yr − 1 ; the Bayes factor in favor of the common-spectrum process versus independent red-noise processes in each pulsar exceeds 10,000. However, we find no statistically significant evidence that this process has quadrupolar spatial correlations, which we would consider necessary to claim a GWB detection consistent with general relativity. We find that the process has neither monopolar nor dipolar correlations, which may arise from, for example, reference clock or solar system ephemeris systematics, respectively. The amplitude posterior has significant support above previously reported upper limits; we explain this in terms of the Bayesian priors assumed for intrinsic pulsar red noise. We examine potential implications for the supermassive black hole binary population under the hypothesis that the signal is indeed astrophysical in nature.
We report here on initial results from the Thousand-Pulsar-Array (TPA) programme, part of the Large Survey Project ‘MeerTime’ on the MeerKAT telescope. The interferometer is used in the tied-array mode in the band from 856 to 1712 MHz, and the wide band coupled with the large collecting area and low receiver temperature make it an excellent telescope for the study of radio pulsars. The TPA is a 5 year project, which aims at to observing (a) more than 1000 pulsars to obtain high-fidelity pulse profiles, (b) some 500 of these pulsars over multiple epochs, and (c) long sequences of single-pulse trains from several hundred pulsars. The scientific outcomes from the programme will include the determination of pulsar geometries, the location of the radio emission within the pulsar magnetosphere, the connection between the magnetosphere and the crust and core of the star, tighter constraints on the nature of the radio emission itself, as well as interstellar medium studies. First, results presented here include updated dispersion measures, 26 pulsars with Faraday rotation measures derived for the first time, and a description of interesting emission phenomena observed thus far.
We have constructed a new time-scale, TT(IPTA16), based on observations of radio pulsars presented in the first data release from the International Pulsar Timing Array (IPTA). We used two analysis techniques with independent estimates of the noise models for the pulsar observations and different algorithms for obtaining the pulsar time-scale. The two analyses agree within the estimated uncertainties and both agree with TT(BIPM17), a post-corrected time-scale produced by the Bureau International des Poids et Mesures (BIPM). We show that both methods could detect significant errors in TT(BIPM17) if they were present. We estimate the stability of the atomic clocks from which TT(BIPM17) is derived using observations of four rubidium fountain clocks at the US Naval Observatory. Comparing the power spectrum of TT(IPTA16) with that of these fountain clocks suggests that pulsar-based time-scales are unlikely to contribute to the stability of the best time-scales over the next decade, but they will remain a valuable independent check on atomic time-scales. We also find that the stability of the pulsar-based time-scale is likely to be limited by our knowledge of solar-system dynamics, and that errors in TT(BIPM17) will not be a limiting factor for the primary goal of the IPTA, which is to search for the signatures of nano-Hertz gravitational waves.
In this paper, we describe the International Pulsar Timing Array second data release, which includes recent pulsar timing data obtained by three regional consortia: the European Pulsar Timing Array, the North American Nanohertz Observatory for Gravitational Waves, and the Parkes Pulsar Timing Array. We analyse and where possible combine high-precision timing data for 65 millisecond pulsars which are regularly observed by these groups. A basic noise analysis, including the processes which are both correlated and uncorrelated in time, provides noise models and timing ephemerides for the pulsars. We find that the timing precisions of pulsars are generally improved compared to the previous data release, mainly due to the addition of new data in the combination. The main purpose of this work is to create the most up-to-date IPTA data release. These data are publicly available for searches for low-frequency gravitational waves and other pulsar science.
We report on the high-precision timing of 42 radio millisecond pulsars (MSPs) observed by the European Pulsar Timing Array
(EPTA). This EPTA Data Release 1.0 extends up to mid-2014 and baselines range from 7-18 years. It forms the basis for the
stochastic gravitational-wave background, anisotropic background, and continuous-wave limits recently presented by the EPTA
elsewhere. The Bayesian timing analysis performed with TempoNest yields the detection of several new parameters: seven parallaxes,
nine proper motions and, in the case of six binary pulsars, an apparent change of the semi-major axis. We find the NE2001
Galactic electron density model to be a better match to our parallax distances (after correction from the Lutz-Kelker bias)
than the M2 and M3 models by Schnitzeler (2012). However, we measure an average uncertainty of 80% (fractional) for NE2001,
three times larger than what is typically assumed in the literature. We revisit the transverse velocity distribution for a
set of 19 isolated and 57 binary MSPs and find no statistical difference between these two populations. We detect Shapiro
delay in the timing residuals of PSRs J1600−3053 and J1918−0642, implying pulsar and companion masses , and , , respectively. Finally, we use the measurement of the orbital period derivative to set a stringent constraint on the distance
to PSRs J1012+5307 and J1909−3744, and set limits on the longitude of ascending node through the search of the annual-orbital
parallax for PSRs J1600−3053 and J1909−3744.
The highly stable spin of neutron stars can be exploited for a variety of (astro-)physical investigations. In particular arrays
of pulsars with rotational periods of the order of milliseconds can be used to detect correlated signals such as those caused
by gravitational waves. Three such “Pulsar Timing Arrays” (PTAs) have been set up around the world over the past decades and
collectively form the “International” PTA (IPTA). In this paper, we describe the first joint analysis of the data from the
three regional PTAs, i.e. of the first IPTA data set. We describe the available PTA data, the approach presently followed
for its combination and suggest improvements for future PTA research. Particular attention is paid to subtle details (such
as underestimation of measurement uncertainty and long-period noise) that have often been ignored but which become important
in this unprecedentedly large and inhomogeneous data set. We identify and describe in detail several factors that complicate
IPTA research and provide recommendations for future pulsar timing efforts. The first IPTA data release presented here (and
available online) is used to demonstrate the IPTA's potential of improving upon gravitational-wave limits placed by individual
PTAs by a factor of ∼2 and provides a 2 − σ limit on the dimensionless amplitude of a stochastic GWB of 1.7 × 10−15 at a frequency of 1 yr−1. This is 1.7 times less constraining than the limit placed by (Shannon et al. 2015), due mostly to the more recent, high-quality
data they used.
The radio properties of 4C21.53 have been an enigma for many years. First, the object displays interplanetary scintillations (IPS) at 81 MHz, indicating structure smaller than 1 are s, despite its low galactic latitude (-0.3°) 1. IPS modulation is rare at low latitudes because of interstellar angular broadening. Second, the source has an extremely steep (∼v -2) spectrum at decametric wavelengths 2. This combination of properties suggested that 4C21.53 was either an undetected pulsar or a member of some new class of objects. This puzzle may be resolved by the discovery and related observations of a fast pulsar, 1937 +214, with a period of 1.558 ms in the constellation Vulpecula only a few degrees from the direction to the original pulsar, 1919+21. The existence of such a fast pulsar with no evidence either of a new formation event or of present energy losses raises new questions about the origin and evolution of pulsars.
Contemporary pulsar-timing experiments have reached a sensitivity level
where systematic errors introduced by existing analysis procedures are
limiting the achievable science. We have developed TEMPO2, a new
pulsar-timing package that contains propagation and other relevant
effects implemented at the 1-ns level of precision (a factor of ~100
more precise than previously obtainable). In contrast with earlier
timing packages, TEMPO2 is compliant with the general relativistic
framework of the IAU 1991 and 2000 resolutions and hence uses the
International Celestial Reference System, Barycentric Coordinate Time
and up-to-date precession, nutation and polar motion models. TEMPO2
provides a generic and extensible set of tools to aid in the analysis
and visualization of pulsar-timing data. We provide an overview of the
timing model, its accuracy and differences relative to earlier work. We
also present a new scheme for predictive use of the timing model that
removes existing processing artefacts by properly modelling the
frequency dependence of pulse phase.
Long-term studies of pulse arrival times for the pulsar PSR 1937+21 yield estimates of the pulsar's period, spin-down rate and celestial coordinates with unprecedented accuracies. Over periods exceeding six months, this pulsar is at least comparable in stability to the best man-made atomic clocks. The results place a limit on the energy density of low-frequency gravitational waves in the Universe.
Using observations of pulsars from the Parkes Pulsar Timing Array (PPTA)
project we develop the first pulsar-based timescale that has a precision
comparable to the uncertainties in international atomic timescales. Our
ensemble of pulsars provides an Ensemble Pulsar Scale (EPS) analogous to the
free atomic timescale Echelle Atomique Libre (EAL). The EPS can be used to
detect fluctuations in atomic timescales and therefore can lead to a new
realisation of Terrestrial Time, TT(PPTA11). We successfully follow features
known to affect the frequency of the International Atomic Timescale (TAI) and
we find marginally significant differences between TT(PPTA11) and TT(BIPM11).
We discuss the various phenomena that lead to a correlated signal in the pulsar
timing residuals and therefore limit the stability of the pulsar timescale.
Pulsar timing observations are usually analysed with least-square-fitting
procedures under the assumption that the timing residuals are uncorrelated
(statistically "white"). Pulsar observers are well aware that this assumption
often breaks down and causes severe errors in estimating the parameters of the
timing model and their uncertainties. Ad hoc methods for minimizing these
errors have been developed, but we show that they are far from optimal.
Compensation for temporal correlation can be done optimally if the covariance
matrix of the residuals is known using a linear transformation that whitens
both the residuals and the timing model. We adopt a transformation based on the
Cholesky decomposition of the covariance matrix, but the transformation is not
unique. We show how to estimate the covariance matrix with sufficient accuracy
to optimize the pulsar timing analysis. We also show how to apply this
procedure to estimate the spectrum of any time series with a steep red
power-law spectrum, including those with irregular sampling and variable error
bars, which are otherwise very difficult to analyse.
The author describes the role pulsars might play in time and
frequency technology. Millisecond pulsars are rapidly rotating neutron
stars: some 20 km in diameter, 1.4 times as massive as the Sun, and
spinning as fast as several thousand radians per second. Radio noise
generated in a pulsar's magnetosphere by a highly beamed process is
detectable over interstellar distances, as a periodic sequence of
pulses. High-precision comparisons between pulsar time and terrestrial
atomic time show that over intervals of several years, some millisecond
pulsars have fractional stabilities comparable to those of the best
atomic clocks. The author briefly reviews the physics of pulsars,
discusses the techniques of pulsar timing measurements, and summarizes
the results of careful studies of pulsar stabilities
Tempo2 is a new software package for the analysis of pulsar pulse times of arrival. In this paper we describe in detail the timing model used by tempo2, and discuss limitations on the attainable precision. In addition to the intrinsic slow-down behaviour of the pulsar, tempo2 accounts for the effects of a binary orbital motion, the secular motion of the pulsar or binary system, interstellar, Solar system and ionospheric dispersion, observatory motion (including Earth rotation, precession, nutation, polar motion and orbital motion), tropospheric propagation delay, and gravitational time dilation due to binary companions and Solar system bodies. We believe the timing model is accurate in its description of predictable systematic timing effects to better than one nanosecond, except in the case of relativistic binary systems where further theoretical development is needed. The largest remaining sources of potential error are measurement error, interstellar scattering, Solar system ephemeris errors, atomic clock instability and gravitational waves. Comment: 30 pages, 3 figures, accepted for publication in MNRAS