Australia Telescope Compact Array observations for project C3772 semester 2025OCTS

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Radio pulsars in orbit around high-mass stellar companions offer a rich source of extreme physics for both observational astronomers and theorists alike. Despite being discovered 25 years ago, PSR J1638-4725 remains a veritable mystery compared to similar pulsar binary systems. Long-term monitoring has revealed extended 'eclipses' where the pulsar is undetectable for up to year around periastron, along with intense variations in the observed pulsar dispersion measure, scatter broadening and flux density throughout the entire orbit. Preliminary spectrospcopic characterisation of the companion star suggests it may be an evolved giant or supergiant, a first for such pulsar systems and a key missing observational link in the evolutionary cycle of massive stellar binaries. This raises the unique opportunity to answer two fundamental questions about this unique system: What is the structure and origin of the stellar winds of high-mass stars? And do collisions between the pulsar and stellar winds of such evolved stars lead to shock-induced synchrotron emission like that seen among the population of pulsar/Be-star binaries? In this proposal we will conduct an ambitious monitoring campaign of PSR J1638-4725 with the 4-cm receiver system on ATCA. Combined with X-ray and gamma-ray monitoring, these observations will play a pivotal role in a broader multi-wavelength campaign to fully capture the upcoming 2026 periastron passage. This will allow us to map both the density and temperature distribution of the stellar wind from the companion star before and after the pulsar swings through periastron, and thereby answer the above questions.