MWA tied-array processing V: Super-resolved localisation via amplitude-only maximum likelihood direction finding

Abstract

Interferometric localisation of transients and pulsars via tied-array beam processing is challenging and can be limited by the native spatial resolution achievable by the instrument, especially at low frequencies and for compact interferometers. Knowledge of the telescope primary and tied-array beam patterns allows the exploitation of the beam structures and the relationship to measured quantities, such as signal-to-noise ratio, through radio direction finding techniques. The additional information provides a "super-resolved" localisation (i.e., where the precision is much better than the native spatial resolution) of a source when there are multiple detections in adjacent tied-array beams. We demonstrate this approach using the Murchison Widefield Array (MWA) and its voltage capture and tied-array processing capabilities, with a specific focus on how it benefits the on-going Southern-sky MWA Rapid Two-metre pulsar survey as it starts producing more candidates requiring follow-up. Examples of localisations with previously discovered MWA pulsars which were subsequently localised via imaging with higher spatial resolution interferometers are used to validate the process, along with localisations of a sample of known pulsars to demonstrate the robustness of the method and its uncertainty estimation.