We report the detection of a glitch in the millisecond pulsar (MSP) PSR J0900-3144, which is included in the European, MeerKAT, and Parkes pulsar timing array (PTA) experiments. The data set combines observations from the MeerKAT, Nanc,ay, Lovell, and Murriyang telescopes, spanning a total baseline of approximately 14 yr. The glitch occurred on MJD 59942(17), with a measured fractional spin frequency step of Av(g) /v = 1 . 15(13) x 10(-12). This event represents the third glitch detected in an MSP, following those in PSRs B1821-24A and J0613-0200. Although smaller in amplitude than the previous two, the glitch in PSR J0900-3144 is of a comparable order of magnitude. The updated MSP glitch rate is 2 . 5(1) x 10(-3 )glitches per pulsar per year, which suggests it is likely that current PTAs will detect another MSP glitch within 5 yr. Using simulations, we demonstrate that such small glitches can go undetected, especially in short data sets such as those from new PTAs, and can bias the inferred achromatic noise model parameters, potentially leading to the down-weighting of the pulsar in gravitational wave background searches.

Heliospheric density variations impart delays on pulse times of arrivals from millisecond pulsars. Improper modelling of these variations may affect gravitational wave detection and characterisation by pulsar timing arrays (PTAs). Currently, PTAs typically employ a time-varying, spherically symmetric heliosphere model, which does not capture the full spatial and temporal complexity of the heliosphere. Instead, we investigate whether a three-dimensional, time-dependent model of the inner heliosphere from interplanetary scintillation (IPS) measurements - the IPS-UCSD model - can be employed to mitigate the solar wind in PTA analyses. We applied the IPS-UCSD model to the MeerKAT PTA 4.5-year dataset to assess whether it could correct for heliospheric density variations, and the impact on GW sensitivity compared to a spherically-symmetric model. We find that the model does not accurately correct for heliosphere-induced timing distortions, leading to bias in recovered GW parameters. Using simulations, we show that the spherically symmetric heliosphere model also fails to fully capture heliospheric density variations like those in the IPS-UCSD model. However, if interstellar dispersion measure (DM) variations are also modelled, then the heliospheric model errors are partially absorbed by DM variations, reducing contamination of the GW signal. Therefore we find that a time-varying spherically symmetric model is sufficient to mitigate the effect of heliospheric time delays on recovered GW results at typical PTA radio frequencies, provided other signal components are also modelled. We propose that the most precisely timed pulsars may be used to improve data-driven heliospheric density models in the future.

A careful characterisation of the noise processes in pulsar timing data is a prerequisite for pulsar timing array experiments. While single-pulsar noise analyses are crucial for both gravitational-wave searches and astrophysical studies, they are often computationally intensive and rely on running and comparing multiple fixed noise models. We present tPTABilby, a transdimensional Bayesian inference framework for single-pulsar noise analysis built on the Bilby library. The method flexibly models a wide range of noise processes like radiometer noise, pulse-phase jitter, intrinsic red noise, dispersion measure variations, and chromatic interstellar medium effects. By employing transdimensional sampling, tPTABilby simultaneously infers the number and type of active noise sources, providing a unified treatment of model selection and parameter estimation. We validate the methodology through simulations with known injected noise models, demonstrating accurate recovery of model probabilities and calibrated posterior distributions. We then apply this approach to a single pulsar, PSR J1713+0747, from a MeerKAT Pulsar Timing Array (MPTA) dataset, analysing the data with both tPTABilby and Enterprise, and subsequently compare the results with existing MPTA analyses through posterior predictive checks of the inferred noise spectra. Our results highlight the flexibility of transdimensional approaches to single-pulsar noise analysis, demonstrating consistency with standard fixed-model methods while providing a more statistically robust framework, and present tPTABilby as a simple and reproducible approach for PTA inference.

The cold dark matter model successfully describes the Universe on large scales, yet faces challenges at subgalactic scales. Ultralight dark matter (ULDM), with particle masses around 10-22 eV, offers a promising solution to these small-scale issues. Pulsar timing arrays (PTAs), designed to detect nanohertz gravitational waves, can also provide a sensitive probe for ULDM signals. In this work, we perform a Bayesian search for ULDM using PTA datasets, focusing on two types of signals: the oscillatory gravitational potential from scalar ULDM and the fifth-force interaction mediated by dark photon dark matter (DPDM). We incorporate pulsar distances in the analysis to better model the ULDM density. No statistically significant evidence for ULDM has been found; therefore, we place 95% confidence level upper limits on the relevant parameters. For scalar ULDM, our analysis does not exclude the scenario in which ULDM constitutes all of dark matter. The constraints from PPTA-DR3 show significant improvements over the earlier PPTA-DR2 (2018 Preview) across most of the mass range, and are consistent with the recent uncorrelated limits from other PTAs. We also present for the first time the DPDM constraints using EPTA data. The obtained bounds on the DPDM from the EPTA-DR2 and PPTA-DR3 are comparable to existing constraints.

A subparsec supermassive black hole binary (SMBHB) at the center of the galaxy 3C 66B is a promising candidate for continuous gravitational-wave searches with pulsar timing arrays (PTAs). In this work, we search for such a signal in the third data release of the Parkes Pulsar Timing Array. Matching our priors to estimates of binary parameters from electromagnetic (EM) observations, we find a log Bayes factor lnB=-0.0027(7) , highlighting that the source can be neither confirmed nor ruled out. We place upper limits at 95% credibility on the chirp mass M < 6.90 & times; 10(8)M(circle dot) , and on the characteristic strain amplitude log(10)(h(0))<-14.44 . This partially rules out the parameter space suggested by EM observations of 3C 66B. We also independently reproduce the calculation of the chirp mass with 3 mm flux monitor data of the unresolved core of 3C 66B. Based on this, we outline a new methodology for constructing a joint likelihood of EM and gravitational-wave data from SMBHBs. Finally, we suggest that targeted searches may allow firmly established SMBHB candidates to be treated as standard sirens, for complementary constraints on the Universe's expansion rate.

We conduct the first-ever pulsar polarization array (PPA) analysis to detect the ultralight axionlike dark matter (ALDM) using the polarization data of 22 millisecond pulsars from the third data release of Parkes pulsar timing array. As one of the major dark matter candidates, the ultralight ALDM exhibits a pronounced wave nature on astronomical scales and offers a promising solution to small-scale structure issues within local galaxies. While the linearly polarized pulsar light travels through the ALDM galactic halo, its position angle (PA) can be subject to an oscillation induced by the ALDM Chern-Simons coupling with electromagnetic field. The PPA is thus especially suited for detecting the ultralight ALDM by correlating polarization data across the arrayed pulsars. To accomplish this task, we develop an advanced Bayesian analysis framework that allows us to construct pulsar PA residual time series, model noise contributions properly and search for pulsar cross-correlations. We find that for an ALDM density of ρ_{0}=0.4  GeV/cm^{3}, the Parkes PPA offers the best global limits on the ALDM Chern-Simons coupling, namely ≲10^{-13.5}-10^{-12.2}  GeV^{-1}, for the mass range of 10^{-22}-10^{-21}  eV. The crucial role of pulsar cross-correlation in recognizing the nature of the derived limits is also highlighted.

We present JUG (JAX-based Unified pulsar timinG), a JAX-based, fully independent pulsar timing package emphasising speed and ease of use, designed to confidently handle the increasingly large and complex pulsar timing array datasets that are being created in the pulsar timing field. JUG implements the entire pulsar timing pipeline itself, from data handling and clock corrections through to the timing model and fitting, without relying on other timing software. It enables Pythonic programming at the speed of compiled code, is GPU-capable, and can be operated via a Python API or an interactive GUI. A user of JUG can interactively explore data, fit timing models with complex stochastic noise, model deterministic signals such as continuous gravitational waves, and obtain accurate point estimates of the parameters of the stochastic processes present, thereby bridging frequentist timing and Bayesian noise analysis. JUG is faster than PINT by more than fifty times and is comparably fast to Tempo2, can handle millions of arrival times, agrees with PINT at the picosecond level, and can reliably recover known timing model and noise parameter values. In this paper we describe its design, performance, and validation, and demonstrate its advantages for pulsar timing data analysis.

We report the discovery of a new long-period radio transient, ASKAP J142431.2-612611, with a 36 minute period, identified in the Australian SKA Pathfinder Evolutionary Map of the Universe survey. We detected pulsed emission from ASKAP J142431.2-612611 over a period of eight days during follow-up observations with the Australia Telescope Compact Array, after which the source appears to have switched off. No optical or near-infrared counterpart is detected in archival surveys or in targeted Gemini South FLAMINGOS-2 observations. During its active state, the source exhibits a stable pulse profile with fractional polarisation consistent with 100

Standard continuous gravitational-wave searches with pulsar timing arrays (PTAs) neglect cosmological redshift, restricting their applicability to the local Universe. We introduce a redshift-aware PTA framework and apply it to the Parkes PTA Data Release 3, deriving the first direct constraints on supermassive binary black holes (SMBBHs) at the cosmic dawn. Evaluating our limits across a broad redshift range, we observationally establish the non-monotonic mass-redshift exclusion boundary driven by the theoretical “redshift bias", demonstrating how PTAs can effectively probe sources at extreme distances. Redshift-aware targeted searches toward high-redshift systems, including the ultraluminous quasar J0100+2802 (z=6.327) and the JWST-discovered galaxy JADES-GS-z14-0 (z=14.32), rigorously exclude SMBBHs with chirp masses ℳ_c ≳ 10^10 M_⊙ across the nanohertz frequency band. Finally, we demonstrate that high-redshift binaries can be robustly detected and localized, allowing for the accurate measurement of their intrinsic properties. Our results provide the tightest constraints to date on SMBBHs at z > 6 and establish a practical framework for probing early-Universe black hole assembly.

Nearby short-period exoplanet systems may produce detectable stellar radio emission due to sub-Alfvenic star-planet interaction (SPI), but there are no confirmed cases yet. We targeted five slowly rotating M dwarfs with transiting terrestrial planets, observing at gigahertz frequencies throughout their subday orbital periods. We did not detect any bursty SPI-like emission, but detected two stars in quiescence: LHS 3844 (unpolarized) and LHS 1678 (circularly polarized). These detections imply persistent magnetic activity at gigayear ages, especially notable for LHS 1678 given its low photometric variability, and these two systems can serve as targets for radio transit experiments. Our SPI nondetections may be due to radio beaming geometry, a subgigahertz maximum emission frequency, or undetectable flux density. If the last case applies, then the flux density upper limits constrain the exoplanet magnetosphere. GJ 367 b has the tightest constraints-no extended magnetosphere and an exoplanet field < 0.8 G-although these results depend strongly on unknown stellar wind parameters inferred from stellar rotation period. Due to their small orbital distances, our nondetection systems a priori appear to have more favorable conditions for SPI than most radio-detected SPI candidate systems in the literature, a tension that can either be resolved by favorable wind or geometry conditions on the detected candidates or by a non-SPI (stellar activity) explanation for those candidate detections. Our results favor the approach of subgigahertz searches for radio SPI, especially with the sensitivity of new and upcoming facilities such as MeerKAT, and underscore the need for observational and theoretical work to constrain the magnetized stellar wind parameters.

Pulsar timing is used for a variety of applications including tests of fundamental physics, probing the structure of neutron stars, and detecting nanohertz gravitational waves. Development of robust methods and generation of high-quality timing data is therefore of utmost importance. In this paper, we present a new technique for creating high-fidelity templates that can be used to measure the pulse times of arrival with significantly increased precision compared to existing methods. Our framework makes use of all available polarimetric information to generate frequency-dependent models of pulse-shape evolution of all four Stokes parameters. We apply this method to millisecond pulsars observed by the Parkes Pulsar Timing Array and show that it results in timing measurement uncertainties reduced up to $\sim$20-30 per cent. We also present, for the first time, phase- and frequency-resolved polarimetric measurements of millisecond pulsars observed with the Parkes Murriyang ultra-wide-bandwdith low receiver. The data, plots, and codes underlying this analysis are made publicly available.

In recent years, several pulsar timing array collaborations have reported evidence for a nanohertz gravitational wave background (GWB). Such a background signal could be produced by supermassive binary black holes, early-Universe processes such as inflation and phase transitions, or a mixture of both. One way to disentangle different contributions to the GWB is to search for anisotropic signatures. In this work, we search for anisotropy in the GWB using the third data release of the Parkes Pulsar Timing Array. Our analysis employs both the radiometer method and the spherical harmonic basis to characterize the distribution of GWB power across the sky. We calculate the angular power in the lowest five frequency bins and compare it with detection thresholds determined under the null hypothesis of isotropy. In the 5.26 nHz frequency bin, we identify a hotspot in the reconstructed sky map with a p-value of 0.016 (the lowest in our analysis), which we attribute to noise fluctuations. While our search reveals no statistically significant anisotropy, we expect that the precise measurement of angular power spectrum of the GWB will become instrumental in determining the origin of the nanohertz GWB signal.

CU Vir, a magnetic hot star, is the first discovered Main-sequence Radio Pulse emitter (MRP) characterized by its ability to produce periodic radio pulses via electron cyclotron maser emission. Although significant advancements have been made in understanding MRPs, their temporal properties remain mostly unexplored. To overcome this limitation, we conducted a pilot study with the Australia Telescope Compact Array, in which we observed pulses from CU Vir at 36 epochs over 1-3 GHz. In this frequency range, CU Vir produces two ≈ 100% circularly polarized pulses, called `leading' and `trailing' pulses per rotation period. We find significant differences in the variability indices exhibited by the two pulses as a function of frequencies, with the leading pulse showing higher variability throughout our observing band. This result could be explained in the scenario of centrifugal breakout events in the magnetosphere of an oblique rotator causing correlated fluctuations across frequencies, along with intrinsic instabilities associated with coherent emission. In addition, we discover jittering in the arrival phases of pulses that must be considered in future monitoring campaigns. The pulses also exhibit a systematic shift to later arrival times during the course of our observing campaign, allowing us to refine the rotation period to 0.5206882 days. Finally, we estimate that ∼ 30 pulses will be needed to extract global pulse properties for the leading or trailing pulses. This relatively small number strongly motivates more extensive monitoring campaigns of MRPs, both to validate our results, and also to pinpoint the origin of the observed temporal variations.

We perform Bayesian targeted searches for continuous gravitational waves from eccentric supermassive binary black holes (SMBBHs) using the Parkes Pulsar Timing Array third data release (PPTA DR3). Six electromagnetically motivated sky directions are analyzed, including the blazar OJ 287 and five nearby galaxy clusters (Virgo, Fornax, Norma, Hercules, and Coma). No significant signals are found. For OJ 287, by explicitly incorporating orbital eccentricity (up to e(0) = 0.8) to robustly capture signal power spread across multiple harmonics, we constrain the total binary mass to M-tot < 5.25 & times; 10(10)M(circle dot) (95% credible level). We also place upper limits on the chirp mass of potential SMBBHs residing in galaxy clusters. By combining these limits with independent black hole mass estimates, we place novel constraints on the allowed binary mass ratios for potential hosts such as M87 and NGC 4889. Specifically, our results exclude binaries with mass ratios q greater than or similar to 10(-2) at around 10 nHz for these massive systems, effectively ruling out equal-mass black hole mergers in the sampled parameter space. These findings demonstrate the growing power of pulsar timing arrays to probe SMBBH populations.

Long-period transients are a class of periodic pulsed radio source repeating on the minute to hour timescale. Recently, an increasing number of them are being identified as binary systems, specifically white dwarfs with low-mass main-sequence companions. In this work we analyse the most luminous long-period transient discovered to date, ASKAP/DART J1832-0911, with two years of radio data, and propose that it, too, may be a white dwarf system, although in a far more compact orbit than the aforementioned. The pulses are composed of quasi-periodic components which evolve in a systematic way over days and months. The source is highly linearly or elliptically polarised and its brightness enabled very high signal-to-noise measurement of the time-resolved Faraday rotation measure, which was found to vary across pulse phase. The linear polarisation position angle, circular polarised fraction, and spectral index also varied systematically in ways not typical of pulsars and magnetars. We show that an ultra-compact asynchronous polar explains much of the phenomenology of ASKAP/DART J1832-0911, in particular the evolution of the pulse morphology, rotation measure variation, and periodic X-ray emission, although we cannot conclusively prove a binary nature. However, our model makes testable predictions.

We study an inflation model with nonminimal derivative coupling that features a coupling between the derivative of the inflaton field and the Einstein tensor. This model naturally amplifies curvature perturbations at small scales via gravitationally enhanced friction, a mechanism critical for the formation of primordial black holes and the associated production of potentially detectable scalar-induced gravitational waves. We derive analytical expressions for the primordial power spectrum, enabling efficient exploration of the model parameter space without requiring computationally intensive numerical solutions of the Mukhanov-Sasaki equation. Using the third data release of the Parkes Pulsar Timing Array -0.3, and log10 sigma = -8.3 & thorn;0.3-0.6 at 90% confidence level. Our results demonstrate the growing capability of pulsar timing arrays to probe early Universe physics, complementing traditional cosmic microwave background observations by providing unique constraints on inflationary dynamics at small scales.

We present the discovery of PSR J1728-4608, a new redback spider pulsar identified in images from the Australian SKA Pathfinder telescope. PSR J1728-4608 is a millisecond pulsar with a spin period of 2.86 ms, in a 5.05 h orbit with a companion star. The pulsar exhibits a radio spectrum of the form S-nu proportional to nu(alpha), with a measured spectral index of alpha=-1.8(3). It is eclipsed for 42% of its orbit at 888 MHz, and multi-frequency image-domain observations show that the egress duration scales with frequency as a power law with index n=-1.74, where longer duration eclipses are seen at lower frequencies. An optical counterpart is detected in archival Gaia data within 0.5 '' of the radio position. It has a mean G-band magnitude of 18.8 mag, and its light curve displays characteristics consistent with a combination of ellipsoidal modulation and irradiation effects. We also report the nearest Fermi gamma-ray source, located 2 ' away from our source, as a possible association. A radio timing study constrains the intrinsic and orbital properties of the system, revealing orbital period variations that we attribute to changes in the gravitational quadrupole moment of the companion star. At the eclipse boundary, we measure a maximum dispersion measure excess of 2.0 +/- 1.2 pc cm(-3), corresponding to an electron column density of 5.9 +/- 3.6x101(8) cm(-2). Modelling of the eclipse mechanism suggests that synchrotron absorption is the dominant cause of the eclipses observed at radio wavelengths. The discovery and characterisation of systems like PSR J1728-4608 provide valuable insights into pulsar recycling, binary evolution, the nature of companion-driven eclipses, and the interplay between compact objects and their plasma environments.

Coherent, periodic radio emission from pulsars has been widely interpreted as evidence of neutron stars as strongly magnetized compact objects. In recent years, radio pulses have also been detected from white dwarfs (WDs) in tight binary systems, raising the question of whether isolated WDs could similarly host pulsar-like emission. We conducted the most sensitive search to date for coherent radio signals from five isolated, rapidly rotating, and magnetized WDs, using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the Green Bank Telescope (GBT), and the Australia Telescope Compact Array (ATCA). No pulsed or continuum radio emission was detected down to μJy levels. These non-detections place the most stringent observational constraints yet on the existence of isolated WD pulsars. Our results suggest that either such emission is intrinsically weak, narrowly beamed, or requires binary-induced magnetospheric interactions absent in solitary systems. Comparison with the known radio-emitting WDs highlights the critical role of companion interaction in enabling detectable emission. This work expands on prior surveys by targeting sources with the most favorable physical conditions for WD pulsar-like activity and employing highly sensitive, targeted observations. Future observations with next-generation facilities such as the SKA will be essential to explore fainter or sporadic emission from massive, magnetic WDs and to investigate their potential as compact radio transients further.

The millisecond pulsar PSR J1713+0747 is a high-priority target for pulsar timing array experiments due to its long-term timing stability, and bright, narrow pulse profile. In April 2021, PSR J1713+0747 underwent a significant profile change event, observed by several telescopes worldwide. Using the broad bandwidth and polarimetric fidelity of the Ultra-Wideband Low-frequency receiver on Murriyang, CSIRO's Parkes radio telescope, we investigated the long-term spectro-polarimetric behaviour of this profile change in detail. We highlight the broad-bandwidth nature of the event, which exhibits frequency dependence that is inconsistent with cold-plasma propagation effects. We also find that spectral and temporal variations are stronger in one of the orthogonal polarisation modes than the other and observe mild variations (similar to 3 - 5 sigma significance) in circular polarisation above 1 400 MHz following the event. However, the linear polarisation position angle remained remarkably stable in the profile leading edge throughout the event. With over three years of data post-event, we find that the profile has not yet recovered back to its original state, indicating a long-term asymptotic recovery, or a potential reconfiguration of the pulsar's magnetic field. These findings favour a magnetospheric origin of the profile change event over a line-of-sight propagation effect in the interstellar medium.

Pulsar timing arrays (PTAs) are ensembles of millisecond pulsars observed for years to decades. The primary goal of PTAs is to study gravitational-wave astronomy at nanohertz frequencies, with secondary goals of undertaking other fundamental tests of physics and astronomy. Recently, compelling evidence has emerged in established PTA experiments for the presence of a gravitational-wave background. To accelerate a confident detection of such a signal and then study gravitational-wave emitting sources, it is necessary to observe a larger number of millisecond pulsars to greater timing precision. The SKA telescopes, which will be a factor of three to four greater in sensitivity compared to any other southern hemisphere facility, is poised to make such an impact. In this chapter, we motivate an SKAO pulsar timing array (SKAO PTA) experiment. We discuss the classes of gravitational waves present in PTA observations and how an SKAO PTA can detect and study them. We then describe the sources that can produce these signals. We discuss the astrophysical noise sources that must be mitigated to undertake the most sensitive searches. We then describe a realistic PTA experiment implemented with the SKA and place it in context alongside other PTA experiments likely ongoing in the 2030s. We describe the techniques necessary to search for gravitational waves in the SKAO PTA and motivate how very long baseline interferometry can improve the sensitivity of an SKAO PTA. The SKAO PTA will provide a view of the Universe complementary to those of the other large facilities of the 2030s.