Discovery of radio eclipses from 4FGL J1646.5-4406: a new candidate redback pulsar binary

Large widefield surveys make possible the serendipitous discovery of rare subclasses of pulsars. One such class are 'spider'-type pulsar binaries, comprised of a pulsar in a compact orbit with a low-mass (sub)stellar companion. In a search for circularly polarized radio sources in Australian Square Kilometre Array Pathfinder (ASKAP) Pilot Survey observations, we discovered highly variable and circularly polarized emission from a radio source within the error region of the gamma-ray source 4FGL J1646.5-4406. The variability is consistent with the eclipse of a compact, steep-spectrum source behind ablated material from a companion in an similar to 5.3 h binary orbit. Based on the eclipse properties and spatial coincidence with 4FGL J1646.5-4406, we argue that the source is likely a recycled pulsar in a 'redback' binary system. Using properties of the eclipses from ASKAP and Murchison Widefield Array observations, we provide broad constraints on the properties of the eclipse medium. We identified a potential optical/infrared counterpart in archival data consistent with a variable low-mass star. Using the Parkes radio telescope 'Murriyang' and the Meer Karoo Array Telescope (MeerKAT) , we searched extensively for radio pulsations but yielded no viable detections of pulsed emission. We suggest that the non-detection of pulses is due to scattering in the intra-binary material, but scattering from the interstellar medium can also plausibly explain the pulse non-detections if the interstellar dispersion measure exceeds similar to 600 pc cm(-3). Orbital constraints derived from optical observations of the counterpart would be highly valuable for future gamma-ray pulsation searches, which may confirm the source nature as a pulsar.