arXiv subjects

Willem van Straten

Publications and source records attributed to Willem van Straten.

At least 19 recordsLinked to original sources

arXiv2026

Insights into the Pulsar Timing Array hypothesis space

We present novel insights into the pulsar-noise model space in pulsar timing array (PTA) experiments. Through a comparative analysis of the same Parkes PTA second data release observations processed with the 2020 and 2023 pipelines, we show that data processing materially changes the distribution of posterior support across competing noise hypotheses and increases sensitivity to weak contributions such as the solar wind. A solar-wind component appears in the highest-posterior hypothesis for ten pulsars under the 2023 pipeline, against four under the 2020 pipeline, while the corresponding mean electron density estimates are consistent with PPTA DR3 results. Furthermore, hypothesis-space analysis exposes model competition and degeneracies that are hidden by single-model summaries. Approximately 75% of the pulsars retain substantial posterior support for alternative noise descriptions. Therefore, understanding the nature of the pulsar noise hypothesis space is crucial for robust inference and nanohertz gravitational-wave background searches.

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arXiv2026

Enhancing evidence estimation through informed probability density approximation

We introduce the Morph approximation, a class of product approximations of probability densities that selects low-order disjoint parameter blocks by maximizing the sum of their total correlations. We use the posterior approximation via Morph as the importance distribution in optimal bridge sampling. We denote this procedure by MorphZ, which serves as a post-processing estimator of the marginal likelihood. The MorphZ estimator requires only posterior samples, and is fully agnostic regarding the choice of sampler. We evaluate MorphZ's performance across statistical benchmarks, pulsar timing array (PTA) models, compact binary coalescence (CBC) gravitational-wave (GW) simulations and the GW150914 event. Across these applications, spanning low to high dimensionalities, MorphZ yields accurate evidence estimates at substantially reduced computational cost relative to standard approaches. We have found that when these approaches fail to provide accurate estimates, MorphZ has proven to either resolve the estimation failure or significantly improve the results. Its bridge sampling relative error diagnostic provides conservative uncertainty estimates. Because MorphZ operates directly on posterior draws, it complements exploration-oriented samplers by enabling fast and reliable evidence estimation, while it can be seamlessly integrated into existing inference workflows.

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arXiv2026

An Introduction to Single-Antenna Radio Astronomical Polarimetry

This tutorial reviews the mathematical foundations of single-antenna radio polarimetry with the aim of fostering a conceptual understanding of the relationships between a physical description of signal propagation (gain, delay, reflection, down-conversion, etc.), the corresponding transformations of the electric field vector, and the equivalent operations on the Stokes parameters. The adopted framework is based on the work of Britton (2000) and Hamaker (2000) and applied to analyze the signal path described by Hamaker et al. (1996) with additional corrections for phase convention and reflection. Some objective criteria for selecting a model of the instrumental response are introduced and discussed, along with some practical guidelines that facilitate polarimetric calibration. Further relevant background material and lengthier mathematical proofs are included in the appendix, which introduces the vector, matrix, and tensor notation and concepts of linear algebra used in this work. The appendix also reviews some of the basics of analog and digital signal processing that are relevant to radio astronomy, and discusses some numerical instabilities that arise when modeling observations.

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arXiv2025

The polarimetric response of the Nançay Radio Telescope and its impact on precision pulsar timing

In \citet{Guillemot2023} we presented a new method for calibrating pulsar observations conducted with the Nançay decimetric Radio Telescope (NRT), which significantly improved NRT polarimetric measurements and pulsar timing quality for data taken after this method was developed, in November 2019. Results hinted at a dependence of the polarimetric response of the NRT on the observed direction. We investigated this potential dependence, since unaccounted variations of the instrumental response could degrade polarimetric measurements. Additionally, we aimed to develop a method for properly calibrating NRT pulsar observations conducted before November 2019. We conducted three series of observations of bright pulsars over wide declination ranges, in a special observation mode in which the feed horn rotates by $\sim$ 180$^\circ$ degrees across the observation, enabling us to determine the full polarimetric response of the NRT while modeling potential variations of calibration parameters with hour angle and declination. In addition, we used the METM technique to improve the calibration of pre-November 2019 data. From the analysis of the series of observations of bright pulsars with horn rotation, we found that the polarimetric response of the NRT does not appear to vary with hour angle or declination. On the other hand, the new METM-based calibration method appears to significantly improve the calibration of pre-November 2019 data. By analyzing NRT data on a selection of millisecond pulsars we found that the new polarimetric profiles are more homogeneous, they generally have larger signal-to-noise ratios, and found that the TOA data for these MSPs are more accurate and contain lower levels of noise, especially when combining the new calibration method with the \textit{Matrix Template Matching} (MTM) method for extracting TOAs from pulsar observations.

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arXiv2025

Generalized Steppingstone Sampling: Efficient marginal likelihood estimation in gravitational wave analysis of Pulsar Timing Array data

Globally, Pulsar Timing Array (PTA) experiments have revealed evidence supporting an existing gravitational wave background (GWB) signal in the PTA data set. Apart from acquiring more observations, the sensitivity of PTA experiments can be increased by improving the accuracy of the noise modeling. In PTA data analysis, noise modeling is conducted primarily using Bayesian statistics, relying on the marginal likelihood and the Bayes factor to assess the evidence. We introduce generalized steppingstone (GSS) as an efficient and accurate marginal likelihood estimation method for the PTA-Bayesian framework. This method enables low-cost estimates with high accuracy, especially when comparing expensive models such as the Hellings-Downs (HD) model or the overlap reduction function model (ORF). We demonstrate the efficiency and the accuracy of GSS for model selection and evidence calculation by reevaluating the evidence of previous analyses from the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) 15 yr data set and the European PTA (EPTA) second data release. We find similar evidence for the GWB compared to the one reported by the NANOGrav 15-year data set. Compared to the evidence reported for the EPTA second data release, we find a substantial increase in evidence supporting GWB across all data sets.

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arXiv2025

Bow Shock and Local Bubble Plasma Unveiled by the Scintillating Millisecond Pulsar J0437$-$4715

The ionized interstellar medium contains au-scale (and below) structures that scatter radio waves from pulsars, resulting in scintillation. Power spectral analysis of scintillation often shows parabolic arcs, with curvatures that encode the locations and kinematics of the pulsar, Earth, and interstellar plasma. Here we report the discovery of 25 distinct plasma structures in the direction of the brilliant millisecond pulsar, PSR J0437-4715, in observations obtained with the MeerKAT radio telescope. Four arcs reveal structures within 5000 au of the pulsar, from a series of shocks induced as the pulsar and its wind interact with the ambient interstellar medium. The measured radial distance and velocity of the main shock allows us to solve the shock geometry and space velocity of the pulsar in three dimensions, while the velocity of another structure unexpectedly indicates a back flow from the direction of the shock or pulsar-wind tail. The remaining 21 arcs represent a surprising abundance of structures sustained by turbulence within the Local Bubble -- a region of the interstellar medium thought to be depleted of gas by a series of supernova explosions about 14 Myr ago. The Local Bubble is cool enough in areas for sub-au density fluctuations to arise from turbulence.

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arXiv2025

Tackling artefacts in the timing of relativistic pulsar binaries: towards the SKA

Common signal-processing approximations produce artefacts when timing pulsars in relativistic binary systems, especially edge-on systems with tight orbits, such as the Double Pulsar. In this paper, we use extensive simulations to explore various patterns that arise from the inaccuracies of approximations made when correcting dispersion and Shapiro delay. In a relativistic binary, the velocity of the pulsar projected onto the line-of-sight varies significantly on short time scales, causing rapid changes in the apparent pulsar spin frequency, which is used to convert dispersive delays to pulsar rotational phase shifts. A well-known example of the consequences of this effect is the artificial variation of dispersion measure (DM) with binary phase, first observed in the Double Pulsar 20 years ago. We show that ignoring the Doppler shift of the spin frequency when computing the dispersive phase shift exactly reproduces the shape and magnitude of the reported DM variations. We also simulate and study two additional effects of much smaller magnitude, which are caused by the assumption that the spin frequency used to correct dispersion is constant over the duration of the sub-integration and over the observed bandwidth. We show that failure to account for these two effects leads to orbital phase-dependent dispersive smearing that leads to apparent orbital DM variations. The functional form of the variation depends on the orbital eccentricity. In addition, we find that a polynomial approximation of the timing model is unable to accurately describe the Shapiro delay of edge-on systems with orbits less than 4 hours, which poses problems for the measurements of timing parameters, most notably the Shapiro delay. This will be a potential issue for sensitive facilities like the FAST and the forthcoming Square Kilometre Array (SKA); therefore, a more accurate phase predictor is indispensable.

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arXiv2024

Precision orbital dynamics from interstellar scintillation arcs for PSR J0437-4715

Intensity scintillations of radio pulsars are known to originate from interference between waves scattered by the electron density irregularities of interstellar plasma, often leading to parabolic arcs in the two-dimensional power spectrum of the recorded dynamic spectrum. The degree of arc curvature depends on the distance to the scattering plasma and its transverse velocity with respect to the line-of-sight. We report the observation of annual and orbital variations in the curvature of scintillation arcs over a period of 16 years for the bright millisecond pulsar, PSR J0437-4715. These variations are the signature of the relative transverse motions of the Earth, pulsar, and scattering medium, which we model to obtain precise measurements of parameters of the pulsar's binary orbit and the scattering medium itself. We observe two clear scintillation arcs in most of our $>$5000 observations and we show that they originate from scattering by thin screens located at distances $D_1 = 89.8 \pm 0.4$ pc and $D_2 = 124 \pm 3$ pc from Earth. The best-fit scattering model we derive for the brightest arc yields the pulsar's orbital inclination angle $i = 137.1 \pm 0.3^\circ$, and longitude of ascending node, $Ω=206.3\pm0.4^\circ$. Using scintillation arcs for precise astrometry and orbital dynamics can be superior to modelling variations in the diffractive scintillation timescale, because the arc curvature is independent of variations in the level of turbulence of interstellar plasma. This technique can be used in combination with pulsar timing to determine the full three-dimensional orbital geometries of binary pulsars, and provides parameters essential for testing theories of gravity and constraining neutron star masses.

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arXiv2024

Reducing instrumental errors in Parkes Pulsar Timing Array data

This paper demonstrates the impact of state-of-the-art instrumental calibration techniques on the precision of arrival times obtained from 9.6 years of observations of millisecond pulsars using the Murriyang 64-m CSIRO Parkes Radio Telescope. Our study focuses on 21-cm observations of 25 high-priority pulsars that are regularly observed as part of the Parkes Pulsar Timing Array (PPTA) project, including those predicted to be the most susceptible to calibration errors. We employ Measurement Equation Template Matching (METM) for instrumental calibration and Matrix Template Matching (MTM) for arrival time estimation, resulting in significantly improved timing residuals with up to a sixfold reduction in white noise compared to arrival times estimated using Scalar Template Matching and conventional calibration based on the Ideal Feed Assumption. The median relative reduction in white noise is 33 percent, and the maximum absolute reduction is 4.5 microseconds. For PSR J0437-4715, METM and MTM reduce the best-fit power-law amplitude (2.7 sigma) and spectral index (1.7 sigma) of the red noise in the arrival time residuals, which can can be tentatively interpreted as mitigation of 1/f noise due to otherwise unmodeled steps in polarimetric response. These findings demonstrate the potential to directly enhance the sensitivity of pulsar timing array experiments through more accurate methods of instrumental calibration and arrival time estimation.

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arXiv2023

Improving pulsar polarization and timing measurements with the Nançay Radio Telescope

Accurate polarimetric calibration of the radio pulse profiles from pulsars is crucial for studying their radiation properties at these wavelengths. Inaccurate calibration can also distort recorded pulse profiles, introducing noise in time of arrival (TOA) data and thus degrading pulsar timing analyses. One method for determining the full polarimetric response of a given telescope is to conduct observations of bright polarized pulsars over wide ranges of parallactic angles, to sample different orientations of their polarization angle and determine the cross-couplings between polarization feeds. The Nançay decimetric Radio Telescope (NRT) is a 94m equivalent meridian telescope, capable of tracking a given pulsar for approximately one hour around transit. In November 2019, we began conducting regular observations of the bright and highly linearly polarized pulsar PSR~J0742$-$2822, in a special mode where the feed horn rotates by $\sim 180^\circ$ over the course of the one hour observation, mimicking wide parallactic angle variations and enabling us to determine the polarimetric response of the NRT at 1.4~GHz. The improved polarimetric response of the NRT as determined from these observations was applied to observations of a selection of MSPs with published polarimetric properties. We find that the new polarimetric profiles and polarization position angles are consistent with previous findings, unlike NRT polarimetric results obtained with the previously used method of calibration. The analysis of timing data on J1730$-$2304, J1744$-$1134, and J1857+0953 shows that the new calibration method improves the quality of the timing, and the Matrix Template Matching (MTM) method proves very effective at reducing noise from imperfect calibration. For pulsars with sufficient degrees of polarization, the MTM method appears to be the preferred method for extracting TOAs from NRT observations.

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arXiv2022

The MeerKAT Pulsar Timing Array: First Data Release

We present the first 2.5 years 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 two weeks using the L-band receiver that records data from 856-1712 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, as well as 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 $< 1 μ\rm{s}$. 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.

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arXiv2022

Independent discovery of a nulling pulsar with unusual sub-pulse drifting properties with the Murchison Widefield Array

We report the independent discovery of PSR J0027-1956 with the Murchison Widefield Array (MWA) in the ongoing Southern-sky MWA Rapid Two-meter (SMART) pulsar survey. J0027-1956 has a period of ~1.306 s, a dispersion measure (DM) of ~20.869 pc cm^-3 , and a nulling fraction of ~77%. This pulsar highlights the advantages of the survey's long dwell times (~80 min), which, when fully searched, will be sensitive to the expected population of similarly bright, intermittent pulsars with long nulls. A single-pulse analysis in the MWA's 140-170 MHz band also reveals a complex sub-pulse drifting behavior, including both rapid changes of the drift rate characteristic of mode switching pulsars, as well as a slow, consistent evolution of the drift rate within modes. In some longer drift sequences, interruptions in the otherwise smooth drift rate evolution occur preferentially at a particular phase, typically lasting a few pulses. These properties make this pulsar an ideal test bed for prevailing models of drifting behavior such as the carousel model.

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arXiv2022

Detection of frequency-dependent dispersion measure toward the millisecond pulsar PSR J2241-5236 from contemporaneous wide-band observations

Making precise measurements of pulsar dispersion measures (DMs) and applying suitable corrections for them is amongst the major challenges in high-precision timing programmes such as pulsar timing arrays (PTAs). While the advent of wide-band pulsar instrumentation can enable more precise DM measurements and thence improved timing precision, it also necessitates doing careful assessments of frequency-dependent (chromatic) DMs that was theorised by Cordes et al. (2016). Here we report the detection of such an effect in broadband observations of the millisecond pulsar PSR J2241-5236, a high-priority target for current and future PTAs. The observations were made contemporaneously using the wide-band receivers and capabilities now available at the Murchison Widefield Array (MWA), the upgraded Giant Metrewave Radio Telescope (uGMRT) and the Parkes telescopes, thus providing an unprecedentedly large frequency coverage from 80 MHz to 4 GHz. Our analysis shows the measurable changes in DM that scale with the observing frequency ($ν$) as $\rm δDM \sim ν^{2.5 \pm 0.1}$. We discuss the potential implications of such a frequency dependence in the measured DMs, and the likely impact on the timing noise budget, and comment on the usefulness of low-frequency observations in advancing PTA efforts.

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arXiv2021

The Thousand-Pulsar-Array programme on MeerKAT III: Giant pulse characteristics of PSR J0540$-$6919

PSR J0540$-$6919 is the second-most energetic radio pulsar known and resides in the Large Magellanic Cloud. Like the Crab pulsar it is observed to emit giant radio pulses (GPs). We used the newly-commissioned PTUSE instrument on the MeerKAT radio telescope to search for GPs across three observations. In a total integration time of 5.7 hrs we detected 865 pulses above our 7$σ$ threshold. With full polarisation information for a subset of the data, we estimated the Faraday rotation measure, $\rm{RM}=-245.8 \pm 1.0$ rad m$^{-2}$ toward the pulsar. The brightest of these pulses is $\sim$ 60% linearly polarised but the pulse-to-pulse variability in the polarisation fraction is significant. We find that the cumulative GP flux distribution follows a power law distribution with index $-2.75 \pm 0.02$. Although the detected GPs make up only $\sim$ 10% of the mean flux, their average pulse shape is indistinguishable from the integrated pulse profile, and we postulate that there is no underlying emission. The pulses are scattered at L-band frequencies with the brightest pulse exhibiting a scattering time-scale of $τ= 0.92 \pm 0.02$ 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.

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arXiv2020

Constraints from globular cluster pulsars on the magnetic field in the Galactic halo

The Galactic magnetic field plays an important role in the evolution of the Galaxy, but its small-scale behaviour is still poorly known. It is also unknown whether it permeates the halo of the Galaxy or not. By using observations of pulsars in the halo globular cluster 47 Tucanae, we probed the Galactic magnetic field at arcsecond scales for the first time and discovered an unexpected large gradient in the component of the magnetic field parallel to the line of sight. This gradient is aligned with a direction perpendicular to the Galactic disk and could be explained by magnetic fields amplified to some 60 μG within the globular cluster. This scenario supports the existence of a magnetized outflow that extends from the Galactic disk to the halo and interacts with the studied globular cluster.

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arXiv2019

Parkes Pulsar Timing Array constraints on ultralight scalar-field dark matter

It is widely accepted that dark matter contributes about a quarter of the critical mass-energy density in our Universe. The nature of dark matter is currently unknown, with the mass of possible constituents spanning nearly one hundred orders of magnitude. The ultralight scalar field dark matter, consisting of extremely light bosons with $m \sim 10^{-22}$ eV and often called "fuzzy" dark matter, provides intriguing solutions to some challenges at sub-Galactic scales for the standard cold dark matter model. As shown by Khmelnitsky and Rubakov, such a scalar field in the Galaxy would produce an oscillating gravitational potential with nanohertz frequencies, resulting in periodic variations in the times of arrival of radio pulses from pulsars. The Parkes Pulsar Timing Array (PPTA) has been monitoring 20 millisecond pulsars at two to three weeks intervals for more than a decade. In addition to the detection of nanohertz gravitational waves, PPTA offers the opportunity for direct searches for fuzzy dark matter in an astrophysically feasible range of masses. We analyze the latest PPTA data set which includes timing observations for 26 pulsars made between 2004 and 2016. We perform a search in this data set for evidence of ultralight dark matter in the Galaxy using Bayesian and Frequentist methods. No statistically significant detection has been made. We therefore place upper limits on the local dark matter density. Our limits, improving on previous searches by a factor of two to five, constrain the dark matter density of ultralight bosons with $m \leq 10^{-23}$ eV to be below $6\,\text{GeV}\,\text{cm}^{-3}$ with 95\% confidence in the Earth neighborhood. Finally, we discuss the prospect of probing the astrophysically favored mass range $m \gtrsim 10^{-22}$ eV with next-generation pulsar timing facilities.

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arXiv2017

Toward an Empirical Theory of Pulsar Emission XII: Exploring the Physical Conditions in Millisecond Pulsar Emission Regions

The five-component profile of the 2.7-ms pulsar J0337+1715 appears to exhibit the best example to date of a core/double-cone emission-beam structure in a millisecond pulsar (MSP). Moreover, three other MSPs, the Binary Pulsar B1913+16, B1953+29 and J1022+1001, seem to exhibit core/single-cone profiles. These configurations are remarkable and important because it has not been clear whether MSPs and slow pulsars exhibit similar emission-beam configurations, given that they have considerably smaller magnetospheric sizes and magnetic field strengths. MSPs thus provide an extreme context for studying pulsar radio emission. Particle currents along the magnetic polar flux tube connect processes just above the polar cap through the radio-emission region to the light-cylinder and the external environment. In slow pulsars radio-emission heights are typically about 500 km around where the magnetic field is nearly dipolar, and estimates of the physical conditions there point to radiation below the plasma frequency and emission from charged solitons by the curvature process. We are able to estimate emission heights for the four MSPs and carry out a similar estimation of physical conditions in their much lower emission regions. We find strong evidence that MSPs also radiate by curvature emission from charged solitons.

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arXiv2017

The Statistics of Radio Astronomical Polarimetry: Disjoint, Superposed, and Composite Samples

A statistical framework is presented for the study of the orthogonally polarized modes of radio pulsar emission via the covariances between the Stokes parameters. To accommodate the typically heavy-tailed distributions of single-pulse radio flux density, the fourth-order joint cumulants of the electric field are used to describe the superposition of modes with arbitrary probability distributions. The framework is used to consider the distinction between superposed and disjoint modes with particular attention to the effects of integration over finite samples. If the interval over which the polarization state is estimated is longer than the timescale for switching between two or more disjoint modes of emission, then the modes are unresolved by the instrument. The resulting composite sample mean exhibits properties that have been attributed to mode superposition, such as depolarization. Because the distinction between disjoint modes and a composite sample of unresolved disjoint modes depends on the temporal resolution of the observing instrumentation, the arguments in favour of superposed modes of pulsar emission are revisited and observational evidence for disjoint modes is described. In principle, the four-dimensional covariance matrix that describes the distribution of sample mean Stokes parameters can be used to distinguish between disjoint modes, superposed modes, and a composite sample of unresolved disjoint modes. More comprehensive and conclusive interpretation of the covariance matrix requires more detailed consideration of various relevant phenomena, including temporally correlated subpulse modulation (e.g. jitter), statistical dependence between modes (e.g. covariant intensities and partial coherence), and multipath propagation effects (e.g. scintillation and scattering). Unpublished supplementary material is appended after the bibliography.

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