arXiv subjects

O. A. Johnson

Publications and source records attributed to O. A. Johnson.

6 recordsLinked to original sources

arXiv2026

A Square Kilometre Array Pulsar Census

Most of the pulsar science case with the Square Kilometre Array (SKA) depends on long-term precision timing of a large number of pulsars, as well as their astrometric measurements using very long baseline interferometry (VLBI). However, before we can time them, or VLBI them, we must first find them. Here, we describe the considerations and strategies needed when planning an all-sky blind pulsar survey using the SKA. Based on our understanding of the pulsar population, the performance of the now-under-construction SKA elements, and practical constraints such as evading radio frequency interference, we project pulsar survey yields; this is done using two complementary methods for a number of illustrative survey designs, combining SKA-Low and SKA-Mid Bands 1 and 2 in a variety of ways. A composite survey using both SKA-Mid and SKA-Low is optimal, with Mid Band 2 focused in the plane. We find that, given its much higher effective area and survey speed, the best strategy is to use SKA-Low to cover as much sky as possible, ideally also overlapping with the areas covered by Mid. We find that an all-sky blind survey with Phase 1 of the SKA with the AA* array assembly will detect $\sim10,000$ slow pulsars and $\sim 800$ millisecond pulsars (MSPs) if SKA-Mid covers the region within $5°$ of the plane, while higher latitudes will be covered with SKA-Low. For the same survey region the yield with AA4 is $\sim 20\%$ higher, but this increases considerably by broadening the range covered by SKA-Mid Bands 1 and 2. In particular one could expect a yield of $\sim 1300$ MSPs with AA4. The pulsar census will enable us to set new constraints on the uncertain physical properties of the entire neutron star population. This will be crucial for addressing major SKA science questions including the dense-matter equation of state, strong-field gravity tests, and gravitational wave astronomy.

astro-ph.HE↗

arXiv2025

A Square Kilometre Array Pulsar Census

Most of the pulsar science case with the Square Kilometre Array (SKA) depends on long-term precision pulsar timing of a large number of pulsars, as well as astrometric measurements of these using very long baseline interferometry (VLBI). But before we can time them, or VLBI them, we must first find them. Here, we describe the considerations and strategies one needs to account for when planning an all-sky blind pulsar survey using the SKA. Based on our understanding of the pulsar population, the performance of the now-under-construction SKA elements, and practical constraints such as evading radio frequency interference, we project pulsar survey yields using two complementary methods for a number of illustrative survey designs, combining SKA1-Low and SKA1-Mid Bands 1 and 2 in a variety of ways. A composite survey using both Mid and Low is optimal, with Mid Band 2 focused in the plane. We find that, given its much higher effective area and survey speed, the best strategy is to use SKA1-Low to cover as much sky as possible, ideally also overlapping with the areas covered by Mid. In our most realistic scenario, we find that an all-sky blind survey with Phase 1 of the SKA with the AA* array assembly will detect $\sim10,000$ slow pulsars and $\sim 800$ millisecond pulsars (MSPs) if SKA1-Mid covers the region within $5°$ of the plane, while higher latitudes will be covered with SKA1-Low. The yield with AA4 is $\sim 20\%$ higher. One could increase these numbers by increasing the range covered by SKA1-Mid Bands 1 and 2, at the cost of a considerably longer survey. The pulsar census will enable us to set new constraints on the uncertain physical properties of the entire neutron star population. This will be crucial for addressing major SKA science questions including the dense-matter equation of state, strong-field gravity tests, and gravitational wave astronomy.

astro-ph.HE↗

arXiv2025

Radio Observations of a Candidate Redback Millisecond Pulsar: 1FGL J0523.5-2529

Redback pulsars are a subclass of millisecond pulsar system with a low-mass non-degenerate companion star being ablated by the pulsar. They are of interest due to the insights they can provide for late-stage pulsar evolution during the recycling process. J0523.5-2529 is one such candidate where redback-like emission has been seen at multiple wavelengths except radio. It is a system with a binary orbit of 16.5 hours and a low-mass non-degenerate companion of approximately 0.8 solar masses. The aim of this work was to conduct follow-up radio observations to search for any exhibited radio pulsar emission from J0523.5-2529. This work employs a periodicity and single burst search across 74 percent of the system's orbital phase using a total of 34.5 hours of observations. Observations were carried out using the Murriyang Telescope at Parkes and the Robert C. Byrd Green Bank Telescope (GBT). Despite extensive orbital phase coverage, no periodic or single-pulse radio emission was detected above a signal-to-noise threshold of 7. A comprehensive search for radio pulsations from J0523.5-2529 using Parkes and GBT yielded no significant emission, likely due to intrinsic faintness, scattering, or eclipses by the companion's outflow. The results demonstrate the elusiveness of the pulsar component in some redback systems and highlight the need for multi-wavelength follow-up and higher-frequency radio observations to constrain the source nature and binary dynamics.

astro-ph.HE↗

arXiv2025

RRAT-like behaviour of PSR B0656+14 observed with I-LOFAR

Single pulse studies offer vital insights into the emission physics of pulsars, particularly in the case of young, nearby sources where intrinsic variability is often pronounced. PSR~B0656+14, known for its sporadic and sometimes intense pulses, provides an excellent opportunity to investigate such behaviour at low radio frequencies. This study aims to characterize the single pulse behaviour of PSR~B0656+14 using low-frequency observations at 110-190 MHz from the Irish LOFAR station. Single-pulse extraction is performed, and individual pulse DMs are estimated to probe pulse-to-pulse dispersion variability. We also perform a wait-time analysis to understand the statistical nature of pulse occurrence, and estimate the spectral index from frequency-resolved flux density measurements. The pulse energy distribution is modelled using a combination of log-normal and power-law components. A total of 41 pulses were detected in a 5-hour observation, allowing a wait-time distribution analysis which is well-modelled by an exponential function, indicative of a Poisson process. Profile stability analysis indicates that a significant number of pulses are required to reach a stable average profile, unusual compared to many other pulsars. The single-pulse spectral index varies significantly from pulse to pulse, with a mean value of $α= -0.5$ and a standard deviation of $Δα=1.3$. The pulse energy distribution shows a hybrid behaviour, consistent of a log-normal distribution and a power-law tail. Our results confirm that PSR~B0656+14 exhibits highly variable, memory-less emission at low frequencies, with characteristics that resemble those seen in some rotating radio transients (RRATs). If such variability proves to be widespread among pulsars, population synthesis models and survey yield predictions would need to incorporate this currently overlooked feature to ensure accuracy.

astro-ph.HE↗

arXiv2025

GReX: An Instrument Overview and New Upper Limits on the Galactic FRB Population

We present the instrument design and initial results for the Galactic Radio Explorer (GReX), an all-sky monitor for exceptionally bright transients in the radio sky. This instrument builds on the success of STARE2 to search for fast radio bursts (FRBs) from the Milky Way and its satellites. This instrument has deployments across the globe, with wide sky coverage and searching down to $32\,μ\text{s}$ time resolution, enabling the discovery of new super giant pulses. Presented here are the details of the hardware and software design of the instrument, performance in sensitivity and other key metrics, and experience in building a global-scale, low-cost experiment. We follow this discussion with experimental results on validation of the sensitivity via hydrogen-line measurements. We then update the rate of Galactic FRBs based on non-detection in the time since FRB 200428. Our results suggest FRB-like events are even rarer than initially implied by the detection of a MJy burst from SGR J1935+2154 in April 2020.

astro-ph.HE↗

arXiv2025

Long-term Timing Results of Ecliptic Pulsars Observed with I-LOFAR

Pulsar timing at low frequencies offers a powerful tool for studying the interstellar medium. Additionally, pulsar observations in the ecliptic enables us to study the effects of the solar wind which becomes much more prominent at low radio frequencies. The Irish station of the LOw Frequency ARray (I-LOFAR) is a sensitive low-frequency radio telescope, capable of delivering high-precision data for pulsar studies. We present a comprehensive dataset of times-of-arrival, timing solutions and dispersion measure (DM) time series for seven ecliptic pulsars observed over two-to-three years with I-LOFAR. The primary objectives are to investigate time-dependent dispersion effects and provide high-precision timing data for pulsar timing experiments. We measure DM variations through pulsar timing and analysed these across different ecliptic latitudes to assess the impact of the solar wind on each pulsar. We model the intrinsic pulse-profile variability as a function of frequency. The high-precision DM time series for all seven pulsars exhibit clear variations dependent on their ecliptic latitudes, revealing the impact of the solar wind. Some pulsars show significant changes in their pulse widths across the frequency band, while others remain stable. We examine and quantify the pulse-nulling present in PSR J0826+2637, we report evidence for DM chromaticity in PSR J1645-0317, and we describe how PSR J2145-0750's DM precision is such that it could resolve the ionospheric DM contribution. This makes it a target of interest for telescopes in areas of the globe where the ionospheric electron density is higher, e.g. the Murchison Radio Observatory in Australia. This data release underscores the potential of I-LOFAR, or any standalone international LOFAR station, for advancing low-frequency pulsar studies, particularly in analyses of dispersion in the interstellar medium, the solar wind and the ionosphere.

astro-ph.HE↗