Detection of an Extremely Luminous Radio Counterpart to the Be/X-ray Binary A0538-66

We present the discovery of radio emission from the Be/X-ray binary A0538-66 with the Australian Square Kilometre Array Pathfinder (ASKAP), and results from a subsequent weekly monitoring campaign with the MeerKAT radio telescope. A0538-66, located in the Large Magellanic Cloud, hosts a neutron star with a short spin period ($P\approx 69$ ms) in a highly eccentric $\approx 16.6$-day orbit. Its rare episodes of super-Eddington accretion, rapid optical and X-ray flares, and other peculiar properties make it an interesting system among high-mass X-ray binaries. Our MeerKAT data reveal that it is also one of the most radio-luminous neutron star X-ray binaries observed to date, reaching $\approx 3\times {10}^{22}\text{ erg }{\text{s}}^{-1}{\text{Hz}}^{-1}$, with radio emission that appears to be orbitally modulated. We consider several possible mechanisms for the radio emission, and place A0538-66 in context by comparing it to similar systems.