ABSTRACT
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 $M_p = 2.06^{+0.44}_{-0.41}$ 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.
ABSTRACT
We present the first 2.5 yr of data from the MeerKAT Pulsar Timing Array (MPTA), part of MeerTime, a MeerKAT Large Survey Project. The MPTA aims to precisely measure pulse arrival times from ...an ensemble of 88 pulsars visible from the Southern hemisphere, with the goal of contributing to the search, detection, and study of nanohertz-frequency gravitational waves as part of the International Pulsar Timing Array. This project makes use of the MeerKAT telescope and operates with a typical observing cadence of 2 weeks using the L-band receiver that records data from 856 to 1712 MHz. We provide a comprehensive description of the observing system, software, and pipelines used and developed for the MeerTime project. The data products made available as part of this data release are from the 78 pulsars that had at least 30 observations between the start of the MeerTime programme in February 2019 and October 2021. These include both sub-banded and band-averaged arrival times and the initial timing ephemerides, noise models, and the frequency-dependent standard templates (portraits) used to derive pulse arrival times. After accounting for detected noise processes in the data, the frequency-averaged residuals of 67 of the pulsars achieved a root-mean-square residual precision of $\lt 1 \, \mu \rm {s}$. We also present a novel recovery of the clock correction waveform solely from pulsar timing residuals and an exploration into preliminary findings of interest to the international pulsar timing community. The arrival times, standards, and full Stokes parameter-calibrated pulsar timing archives are publicly available.
ABSTRACT
PSR J0540−6919 is the second-most energetic radio pulsar known and resides in the Large Magellanic Cloud. Like the Crab pulsar, it is observed to emit giant radio pulses (GPs). We used the ...newly commissioned PTUSE instrument on the MeerKAT radio telescope to search for GPs across three observations. In a total integration time of 5.7 h, we detected 865 pulses above our 7σ threshold. With full polarization information for a subset of the data, we estimated the Faraday rotation measure, $\rm {RM}=-245.8 \pm 1.0$ rad m−2 towards the pulsar. The brightest of these pulses is ∼60 per cent linearly polarized but the pulse-to-pulse variability in the polarization fraction is significant. We find that the cumulative GP flux distribution follows a power-law distribution with index −2.75 ± 0.02. Although the detected GPs make up only ∼10 per cent of the mean flux, their average pulse shape is indistinguishable from the integrated pulse profile, and we postulate that, unlike in the Crab pulsar, there are no additional regular emission components. The pulses are scattered at L-band frequencies with the brightest pulse exhibiting a scattering time-scale of τ = 0.92 ± 0.02 ms at 1.2 GHz. We find several of the giants display very narrow-band flux knots similar to those seen in many Fast Radio Bursts, which we assert cannot be due to scintillation or plasma lensing. The GP time-of-arrival distribution is found to be Poissonian on all but the shortest time-scales where we find four GPs in six rotations, which if GPs are statistically independent is expected to occur in only 1 of 7000 observations equivalent to our data.
ABSTRACT
Pulse-to-pulse profile shape variations introduce correlations in pulsar times of arrival across radio frequency measured at the same observational epoch. This leads to a broad-band noise in ...excess of radiometer noise, which is termed as pulse jitter noise. The presence of jitter noise limits the achievable timing precision and decreases the sensitivity of pulsar timing data sets to signals of interest such as nanohertz-frequency gravitational waves. Current white noise models used in pulsar timing analyses attempt to account for this, assuming complete correlation of uncertainties through the arrival times collected in a unique observation and no frequency dependence of jitter (which corresponds to a rank-one covariance matrix). However, previous studies show that the brightest millisecond pulsar at decimetre wavelengths, PSR J0437−4715, shows decorrelation and frequency dependence of jitter noise. Here, we present a detailed study of the decorrelation of jitter noise in PSR J0437−4715 and implement a new technique to model it. We show that the rate of decorrelation due to jitter can be expressed as a power law in frequency. We analyse the covariance matrix associated with the jitter noise process and find that a higher rank approximation is essential to account for the decorrelation and to account for frequency dependence of jitter noise. We show that the use of this novel method significantly improves the estimation of other chromatic noise parameters such as dispersion measure variations. However, we find no significant improvement in errors and estimation of other timing model parameters suggesting that current methods are not biased for other parameters, for this pulsar due to this misspecification. We show that pulse energy variations show a similar decorrelation to the jitter noise, indicating a common origin for both observables.
ABSTRACT
We have determined positions, proper motions, and parallaxes of 77 millisecond pulsars (MSPs) from ∼3 yr of MeerKAT radio telescope observations. Our timing and noise analyses enable us to ...measure 35 significant parallaxes (12 of them for the first time) and 69 significant proper motions. Eight pulsars near the ecliptic have an accurate proper motion in ecliptic longitude only. PSR J0955−6150 has a good upper limit on its very small proper motion (<0.4 mas yr−1). We used pulsars with accurate parallaxes to study the MSP velocities. This yields 39 MSP transverse velocities, and combined with MSPs in the literature (excluding those in Globular Clusters) we analyse 66 MSPs in total. We find that MSPs have, on average, much lower velocities than normal pulsars, with a mean transverse velocity of only 78(8) km s−1 (MSPs) compared with 246(21) km s−1 (normal pulsars). We found no statistical differences between the velocity distributions of isolated and binary MSPs. From Galactocentric cylindrical velocities of the MSPs, we derive 3D velocity dispersions of σρ, σϕ, σz = 63(11), 48(8), 19(3) km s−1. We measure a mean asymmetric drift with amplitude 38(11) km s−1, consistent with expectation for MSPs, given their velocity dispersions and ages. The MSP velocity distribution is consistent with binary evolution models that predict very few MSPs with velocities >300 km s−1 and a mild anticorrelation of transverse velocity with orbital period.
ABSTRACT
We present a flux density study of 89 millisecond pulsars (MSPs) regularly monitored as part of the MeerKAT Pulsar Timing Array (MPTA) using the L-Band receiver with an approximately two ...week cadence between 2019 and 2022. For each pulsar, we have determined the mean flux densities at each epoch in eight ∼97 MHz sub-bands ranging from 944 to 1625 MHz. From these we have derived their modulation indices, their average and peak-to-median flux densities in each sub-band, as well as their mean spectral indices across the entire frequency range. We find that the vast majority of the MSPs have spectra that are well described by a simple power law, with a mean spectral index of –1.86(6). Using the temporal variation of the flux densities, we measured the structure functions and determined the refractive scintillation time-scale for seven. The structure functions provide strong evidence that the intrinsic radio luminosities of MSPs are stable. As a population, the average modulation index at 20 cm wavelengths peaks near unity at dispersion measures (DMs) of ∼20 pc cm−3 and by a DM of 100 pc cm−3 are closer to 0.2, due to refractive scintillation. We find that timing arrays can improve their observing efficiency by reacting to scintillation maxima, and that 20 cm FRB surveys should prioritize highly scintillating mid-latitude regions of the Galactic sky where they will find ∼30 per cent more events and bursts at greater distances.
ABSTRACT
Precision timing of millisecond pulsars (MSPs) in binary systems enables observers to detect the relativistic Shapiro delay induced by space–time curvature. When favourably aligned, this ...enables constraints to be placed on the component masses and system orientation. Here, we present the results of timing campaigns on seven binary MSPs observed with the 64-antenna MeerKAT radio telescope that show evidence of Shapiro delay: PSRs J0101−6422, J1101−6424, J1125−6014, J1514−4946, J1614−2230, J1732−5049, and J1909−3744. Evidence for Shapiro delay was found in all of the systems, and for three the orientations and data quality enabled strong constraints on their orbital inclinations and component masses. For PSRs J1125−6014, J1614−2230, and J1909−3744, we determined pulsar masses to be $M_{\rm p} = 1.68\pm 0.17$, $1.94\pm 0.03$, and $1.45 \pm 0.03 \, {\rm M_{\odot }}$, and companion masses to be $M_{\rm c} = 0.33\pm 0.02$, $0.495\pm 0.005$, and $0.205 \pm 0.003 \, {\rm M_{\odot }}$, respectively. This provides the first independent confirmation of PSR J1614−2230’s mass, one of the highest known. The Shapiro delays measured for PSRs J0101−6422, J1101−6424, J1514−4946, and J1732−5049 were only weak, and could not provide interesting component mass limits. Despite a large number of MSPs being routinely timed, relatively few have accurate masses via Shapiro delays. We use simulations to show that this is expected, and provide a formula for observers to assess how accurately a pulsar mass can be determined. We also discuss the observed correlation between pulsar companion masses and spin period, and the anticorrelation between recycled pulsar mass and their companion masses.
MeerTime is a five-year Large Survey Project to time pulsars with MeerKAT, the 64-dish South African precursor to the Square Kilometre Array. The science goals for the programme include timing ...millisecond pulsar (MSPs) to high precision (
${<} 1 \unicode{x03BC} \mathrm{s}$
) to study the Galactic MSP population and to contribute to global efforts to detect nanohertz gravitational waves with the International Pulsar Timing Array (IPTA). In order to plan for the remainder of the programme and to use the allocated time most efficiently, we have conducted an initial census with the MeerKAT ‘L-band’ receiver of 189 MSPs visible to MeerKAT and here present their dispersion measures, polarisation profiles, polarisation fractions, rotation measures, flux density measurements, spectral indices, and timing potential. As all of these observations are taken with the same instrument (which uses coherent dedispersion, interferometric polarisation calibration techniques, and a uniform flux scale), they present an excellent resource for population studies. We used wideband pulse portraits as timing standards for each MSP and demonstrated that the MeerTime Pulsar Timing Array (MPTA) can already contribute significantly to the IPTA as it currently achieves better than
$1\,\unicode{x03BC}\mathrm{s}$
timing accuracy on 89 MSPs (observed with fortnightly cadence). By the conclusion of the initial five-year MeerTime programme in 2024 July, the MPTA will be extremely significant in global efforts to detect the gravitational wave background with a contribution to the detection statistic comparable to other long-standing timing programmes.
Pulse-to-pulse profile shape variations introduce correlations in pulsar
times of arrival (TOAs) across radio frequency measured at the same
observational epoch. This leads to a broadband noise in ...excess of radiometer
noise, which is termed pulse jitter noise. The presence of jitter noise limits
the achievable timing precision and decreases the sensitivity of pulsar-timing
data sets to signals of interest such as nanohertz-frequency gravitational
waves. Current white noise models used in pulsar timing analyses attempt to
account for this, assuming complete correlation of uncertainties through the
arrival times collected in a unique observation and no frequency dependence of
jitter (which corresponds to a rank-one covariance matrix). However, previous
studies show that the brightest millisecond pulsar at decimetre wavelengths,
PSR J0437$-$4715, shows decorrelation and frequency dependence of jitter noise.
Here we present a detailed study of the decorrelation of jitter noise in PSR
J0437$-$4715 and implement a new technique to model it. We show that the rate
of decorrelation due to jitter can be expressed as a power-law in frequency. We
analyse the covariance matrix associated with the jitter noise process and find
that a higher-rank-approximation is essential to account for the decorrelation
and to account for frequency dependence of jitter noise. We show that the use
of this novel method significantly improves the estimation of other chromatic
noise parameters such as dispersion measure variations. However, we find no
significant improvement in errors and estimation of other timing model
parameters suggesting that current methods are not biased for other parameters,
for this pulsar due to this misspecification. We show that pulse energy
variations show a similar decorrelation to the jitter noise, indicating a
common origin for both observables.
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