Pulsars, transients and relativistic astrophysics

Abstract:

Accurate modelling of the effective point spread function (ePSF) is essential for high-precision photometry and astrometry, particularly in undersampled imaging regimes. In this work, we build on a well-established ePSF modelling framework and its commonly used open-source Python implementation and demonstrate that several simple but effective modifications to existing ePSF modelling routines can significantly improve model accuracy. We use synthetic ePSFs to generate simulated data sets of stellar images, allowing us to evaluate the accuracy of ePSF models and determine the scale of the pixel-phase errors in resulting flux and position measurements. We systematically investigate how specific modelling choices affect ePSF accuracy, and evaluate the influence of oversampling, interpolation, gridpoint estimation, smoothing, star-sample distribution and dithering on photometric precision. We apply our refined ePSF modelling routine to images from the Global Jet Watch observatories, demonstrating its improved ability to recover an accurate ePSF for real astronomical images. Our findings highlight the importance of tailoring the modelling approach to the specific characteristics of the instrument and detector, as well as to the nature of the available imaging data used to construct the ePSF model. These results provide practical guidance for optimising ePSF construction, thereby improving the reliability of photometric and astrometric measurements.

Abstract:

On 2025 August 18, the LIGO–Virgo–KAGRA collaboration reported gravitational waves from a subthreshold binary neutron star merger. If astrophysical, this event would have a surprisingly low chirp mass, suggesting that at least one neutron star was below a solar mass. The Zwicky Transient Facility mapped the coarse localization and discovered a transient, ZTF 25abjmnps (AT2025ulz), which was spatially and temporally coincident with the gravitational-wave trigger. The first week of follow-up suggested properties reminiscent of a GW170817-like kilonova. Subsequent follow-up suggests properties most similar to a young, stripped-envelope, Type IIb supernova. Although we cannot statistically rule out chance coincidence, we undertake due diligence analysis to explore the possible association between ZTF 25abjmnps and S250818k. Theoretical models have been proposed wherein subsolar neutron star(s) may form (and subsequently merge) via accretion-disk fragmentation or core fission inside a core-collapse supernova—i.e., a “superkilonova.” Here, we qualitatively discuss our multiwavelength dataset in the context of the superkilonova picture. Future higher-significance gravitational-wave detections of subsolar neutron star mergers with extensive electromagnetic follow-up would conclusively resolve this tantalizing multimessenger association.

Abstract:

The large instantaneous sensitivity, a wide frequency coverage and flexible observation modes with large number of beams in the sky are the main features of the SKA observatory’s two telescopes, the SKA-Low and the SKA-Mid, which are located on two different continents. Owing to these capabilities, the SKAO telescopes are going to be a game-changer for radio astronomy in general and pulsar astronomy in particular. The eleven articles in this special issue on pulsar science with the SKA Observatory describe its impact on different areas of pulsar science. In this lead article, a brief description of the two telescopes highlighting the relevant features for pulsar science is presented followed by an overview of each accompanying article, exploring the inter-relationship between different pulsar science use cases.

Abstract:

Produced by the interaction between the “pulsar wind’’ powered by the rotational energy of a neutron star and its surroundings, the study of pulsar wind nebulae (PWNe) provides vital insight into the physics of neutron star magnetospheres and ultra-relativistic outflows. Spatially-resolved studies of the continuum and polarized radio emission of these sources are vital for understanding the production of $e^{\pm }$ in the magnetospheres of neutron stars, the acceleration of these particles to energies, and the propagation of these particles within the PWN as well as the surrounding interstellar medium. The significant improvements in sensitivity, dynamic range, timing capabilities offered by the Square Kilometer Array have the potential to significantly improve our understanding of the origin of some of the highest energy particles produced in the Milky Way.

Abstract:

The SKA telescopes will bring unparalleled sensitivity across a broad radio band, a wide field of view across the Southern sky, and the capacity for sub-arraying, all of which make it the ideal instrument for studying the pulsar magnetosphere. This paper describes the advances that have been made in pulsar magnetosphere physics over the last decade, and details how these have been made possible through the advances of modern radio telescopes, particularly SKA precursors and pathfinders. It explains how the SKA telescopes would transform the field of pulsar magnetosphere physics through a combination of large-scale monitoring surveys and in-depth follow-up observations of unique sources and new discoveries. Finally, it describes how the specific observing opportunities available with the AA* and AA4 configurations will achieve the advances necessary to solve the problem of pulsar radio emission physics in the coming years.