Parkes observations for project P1413 semester 2026APRS_05

We request 75 hours of Parkes observing time for dedicated radio follow-up observations of PSR J1635-5134, a promising candidate pulsar-subdwarf B (sdB) star binary system. PSR J1635-5134 is a non-recycled 340 ms pulsar in a nearly circular 18-hour orbit with a minimum companion mass of 0.26 solar mass. It lies in a sparsely populated region of binary-pulsars parameter space and may represent a short-lived evolutionary stage immediately following inefficient spin-up, thereby providing a missing link between canonical binaries and the millisecond pulsar-WD population. The system exhibits peculiar, orbital-phase-dependent dispersion-measure (DM) variability, with DM variations exceeding 30 pc/cc, indicating substantial ionised material plausibly associated with an outflow from the companion. Moreover, during prior monitoring with Parkes, the DM variations shifted from being predominantly modest to being persistently large, implying an evolving plasma environment that cannot be characterised with sparse or irregular observations. Determining whether these high-DM variations are intermittent or long-lived, and mapping how the excess material varies with orbital phase and epoch, are essential for constraining the origin and geometry of the plasma and for testing the hypothesis that an sdB wind supplies the required electron column. Through this proposal, we aim to (i) extend the timing baseline to improve astrometric precision to sub-arcsecond accuracy, enabling secure identification of the optical counterpart in a crowded field, and (ii) perform phase-targeted observations during eclipse/DM variation phases to understand the temporal behavior of excess DM.