Golam Shaifullah

We present high-cadence 10 minute rotation-measure (RM) monitoring of PSR J0814+7429 using the LOw-Frequency ARray, aiming to probe ionospheric variability along the pulsar line of sight (LoS). By separating the ionospheric contribution from the observed RM, we quantitatively reconstruct the diurnal variation of the ionospheric electron density along the pulsar LoS based on the World Magnetic Model. The derived variations exhibit clear solar-driven modulation, including the ionospheric noontime bite-out phenomenon, and show good agreement with the LoS total electron content reconstructed from independent global vertical total electron content maps. These results demonstrate the feasibility of using pulsars as probes of temporal variations in the electron density of the Earth’s ionosphere.

Revealing the nature of the nanohertz gravitational wave (GW) signal recently reported by pulsar timing array (PTA) collaborations around the world is the next goal of low-frequency GW astronomy. The signal likely originates from the incoherent superposition of GWs emitted by a cosmological population of supermassive black hole binaries (SMBHBs). These binaries can be highly eccentric and/or strongly coupled to their nuclear environment, resulting in an attenuation of the overall GW signal at low frequencies. In this paper, we propose to use the correlation properties of the distributed GW power in the sky across the frequency spectrum as a smoking gun for eccentric SMBHBs, thus allowing us to break the spectral degeneracy between eccentricity and environmental effect. The simple underlying idea is that, contrary to circular binaries, eccentric ones emit a broadband spectrum thus resulting in similar sky maps at different frequencies. We first demonstrate the applicability of this simple concept on sky maps constructed directly from the theoretical sky distribution of the gravitational wave background (GWB) power induced by realistic populations of SMBHBs. We then demonstrate the viability of this analysis on simulated square kilometer array (SKA) like PTA data. By statistically comparing sky maps reconstructed from hundreds of injected circular and highly eccentric SMBHB populations, we find that eccentricity can be detected at 3 σ in more than 50% of cases.

Context. The interstellar scintillation observed in radio pulsars arises from interference between electromagnetic waves scattered by electron density fluctuations in the turbulent interstellar plasma, providing a critical tool for probing the small-scale structure of the ionized interstellar medium and the pulsar system itself. Aims. The primary aim of this work is to study long-term scintillation variations for a bright and nearby pulsar, PSR J0814+7429, carried out from September 2013 to September 2023 with the LOw-Frequency ARray (LOFAR) High Band Antennae in the frequency range 120-170 MHz. Methods. We derived the basic scintillation parameters, scintillation bandwidth (Δν _d ), and scintillation timescale (Δτ _d ) from the twodimensional (2D) auto-covariance function of the dynamic spectra that are a 2D matrix of pulse intensity as a function of time and frequency. Results. We present a long-term monitoring study of Δν _d and Δτ _d for PSR J0814+7429, which shows a strong annual variation in the time series of the Δτ _d . From our modeling of the annual variations of scintillation velocities, the scattering screen is anisotropic and located at 0.23 ± 0.02 kpc from the Earth, likely corresponding to the boundary of the Local Bubble.

Pulsar timing arrays (PTAs) are approaching the level of sensitivity required to make a 5- σ detection of the nanohertz stochastic gravitational-wave background (GWB). Thus, it is now crucial to develop a comprehensive understanding of our data and of the outcomes of our analysis pipelines. It is also essential to understand a counterintuitive feature revealed in the recent results from the European Pulsar Timing Array (EPTA) second data release ( DR2 ). When restricting the dataset to its ultimate ∼10.3 years ( DR2new ), the inferred GWB significance increases from ≲2 σ for the full 25-year dataset ( DR2full ), to ≳3.5 σ for DR2new . In this work, we investigate whether this behaviour reflects an anomaly in the data or whether it is a possible outcome of the analysis pipeline. Using realistic DR2 -like simulations, we generated multiple realisations with varying observation time spans and analysed their impact on GWB evidence and parameter estimations. We quantified the evidence using the signal-to-noise ratio (S/N) of a common process with Hellings-Downs spatial correlations (HD S/N). We find that the first ∼10 years of DR2 contribute little to the GWB evidence due to the limited bandwidth of the observation frequency, leading to a significant overlap between the DR2full and DR2new HD S/N distributions. As a consequence, random noise fluctuations result in DR2new producing a higher GWB significance than DR2full in ∼15% of the cases and 5% of the realisations are compatible with the HD S/N of the real DR2full and DR2new . This suggests that the picture observed in the data, while unlikely, remains consistent with a ∼2 σ outcome due to noise fluctuations. We also find that regardless of the significance, the DR2new simulated data yielded biased GWB parameter estimates, primarily due to spectral leakage effects that are disregarded in the analysis and tend to flatten the inferred power-law spectrum. Including leakage in the model returns unbiased parameter estimates, demonstrating that DR2new is reliable when the signal is appropriately modelled. Furthermore, we show that combining EPTA DR2full data with complementary long-baseline observations from NANOGrav and PPTA and with low-frequency observations from LOFAR and NenuFAR significantly bolsters the GWB evidence, along with improved precision and accuracy in the parameter estimation. This outcome supports the case of integrating DR2full within the IPTA framework. Finally, we explored the impact of the observation time span on parameter estimations in greater detail, focusing on long-baseline (25 yr) and short-baseline (5 yr) datasets. We find that short-baseline datasets tend to introduce significant bias towards high amplitudes in the estimation of GWB parameters, while the short time span makes them very ineffective at constraining the GWB slope.

PSR J1455−3330 is a ∼7.98 ms pulsar in a ∼76.17 day nearly circular orbit with a white dwarf companion. In this work, we combine the available Lovell, Nançay decimetric Radio Telescope, Green Bank, and MeerKAT pulsar timing data spanning approximately 30 years to measure the kinematic and relativistic effects of PSR J1455−3330 to constrain its 3D orbital geometry and component masses. We detect a relativistic Shapiro delay signal. We measure a significant orthometric amplitude of h ₃ = 0.307 ^+0.022 _−0.026 μs and an orthometric ratio of ς = 0.551 ^+0.057 _−0.054 . We measured the change in projected semi-major axis, x ˙ = −202.1 ^+2.5 _−2.7 × 10 ⁻¹⁶ s s ⁻¹ , with high significance, parallax, ϖ = 1.11(6) mas, parallax derived distance 0.90(5) kpc, and a precise total proper motion magnitude of 12.432(2) mas yr ⁻¹ . A self-consistent analysis of all kinematic and relativistic effects, assuming general relativity, yielded two solutions: (1) a pulsar mass of M _p = 1.39 ^+0.38 _−0.18 M _⊙ , a companion mass of M _c = 0.293 ^+0.056 _−0.026 M _⊙ , an orbital inclination of i = 63(2)°, and a longitude of the ascending node of Ω = 212(12)° or (2) a pulsar mass of M _p = 1.53 ^+1.10 _−0.22 M _⊙ , a companion mass of M _c = 0.309 ^+0.163 _−0.026 M _⊙ , an orbital inclination of i = 123(4)°, and a longitude of the ascending node of Ω = 334(12)°. All uncertainties represent the 68.27% credibility region. These results strongly favour a helium-dominated white dwarf companion.

Pulsar timing array (PTA) experiments have the potential to unveil continuous gravitational wave (CGW) signals from individual low-redshift massive black hole binaries (MBHBs). Detecting these objects in both gravitational waves (GWs) and the electromagnetic (EM) spectrum will open a new chapter in multimessenger astronomy. We investigate the feasibility of conducting multimessenger studies by combining the CGW detections from an idealized 30 year Square Kilometer Array Mid telescope PTA and the optical data from the forthcoming Legacy Survey of Space and Time (LSST). To this end, we employed the L-Galaxies semi-analytical model applied to the Millennium simulation. We generated 200 different all-sky light cones that include galaxies, massive black holes, and MBHBs whose emission is consistently modeled based on their star formation histories and gas accretion physics. Our results predict an average of ≈33 CGW detections, with signal-to-noise ratios greater than five. The MBHBs associated with the detections are typically at z < 0.5, with masses of ∼ 3 × 10 ⁹ M _⊙ , mass ratios > 0.6, and eccentricities ≲0.2. In terms of EM counterparts, we find less than 15% of these systems to be connected with an active galactic nucleus detectable by LSST, while their host galaxies are easily detectable (< 23 mag) massive ( M _★ > 10 ¹¹ M _⊙ ) ellipticals with typical star formation rates (10 ⁻¹⁵ yr ⁻¹ < sSRF < 10 ⁻¹⁰ yr ⁻¹ ). Although the CGW-EM counterpart association is complicated by poor sky localization (only 35% of these CGWs are localized within 100 deg ² ), the number of galaxy host candidates can be considerably reduced (from thousands to several tens, depending on the CGW S/N) by applying priors based on the galaxy-MBH correlations. However, picking the actual host among these candidates is highly non-trivial, as they occupy a similar region in any optical color-color diagram. Our findings highlight the considerable challenges entailed in opening the low-frequency multimessenger GW sky.

Pulsar Timing Arrays (PTAs) use the remarkable rotational stability of millisecond pulsars (MSPs) to detect nanohertz-frequency gravitational waves (GWs). The International PTA (IPTA) combines datasets from six regional PTAs to form the most sensitive experiment to date, with the release of IPTA Data Release 3 (IPTA-DR3) . This dataset spans 130 pulsars, baselines up to 27 years, and frequency coverage from 30 to 5107 MHz, enabling precise characterisation of both the stochastic GW background (GWB)and potential continuous-wave (CGW) sources. We present an overview of IPTA-DR3 construction, including data combination pipelines, white and red noise modelling, and likelihood-based inference. We summarise current evidence for correlated GW signals and outline prospects for individual SMBHB detection, multi-messenger follow-up, and probing cosmological GW sources with next-generation PTAs.

The ionised media that permeate the Milky Way have been active topics of research since the discovery of pulsars in 1967. In fact, pulsars allow one to study several aspects of said plasma, such as their column density, turbulence, scattering measures, and discrete, intervening structures in between the neutron star and the observer, and aspects of the magnetic field throughout. Such sources of information allow us to characterise the electron distribution in the terrestrial ionosphere, the Solar Wind, and our Galaxy and the impact on other experiments involving pulsars such as Pulsar Timing Arrays. In this article, we review the state-of-the-art of plasma research using pulsars, the aspects that should be taken into consideration for optimal plasma studies, and we provide future perspectives on improvements to those enabled by the SKA.

The ionised media that permeate the Milky Way have been active topics of research since the discovery of pulsars in 1967. In fact, pulsars allow one to study several aspects of said plasma, such as their column density, turbulence, scattering measures, and discrete, intervening structures between the neutron star and the observer, as well as aspects of the magnetic field throughout. Such sources of information allow us to characterise the electron distribution in the terrestrial ionosphere, the Solar Wind, and our Galaxy, as well as the impact on other experiments involving pulsars, such as Pulsar Timing Arrays. In this article, we review the state-of-the-art in plasma research using pulsars, the aspects that should be taken into consideration for optimal plasma studies, and we provide future perspectives on improvements enabled by the SKA.

This review summarises recent progress in pulsar timing array research and the current status of nanohertz gravitational wave astronomy. I outline the techniques enabling decade-long, sub-microsecond-precision timing, present results from PTA collaborations between 2023–2025, and discuss their implications for supermassive black-hole binaries, cosmological sources, and beyond-Standard-Model physics. I also highlight complimentary efforts probing the nanohertz regime.