X-ray binaries (XRBs) are bright X-ray sources in which a compact object – either a neutron star (NS) or black hole (BH) – accretes matter from a stellar binary companion (donor star). A0538–66 is a Be/XRB in the Large Magellanic Cloud, a near-neighbour to our own Galaxy, that was discovered in 1977 when the Ariel V satellite detected two Xray outbursts separated by ∼17 days. Crook-Mansour et al. present the discovery of radio emission from the Be/X-ray binary A0538–66 with ASKAP, and results from a subsequent weekly monitoring campaign with the MeerKAT radio telescope. A0538–66 hosts a neutron star with a short spin period, 69 milli-seconds, in a highly eccentric 16.6-day orbit around a B-type donor star. 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. The MeerKAT data reveal that it is also one of the most radio-luminous neutron star X-ray binaries observed to date, with radio emission that appears to be orbitally modulated.

Radio emission was discovered from A0538–66 using data from the ASKAP Variables and Slow Transients (VAST) Survey.  VAST initially observed 40 fields in the Galactic plane and two in the Magellanic Clouds at roughly fortnightly cadence, achieving typical sensitivities of ∼250 micro-jansky per beam across the full survey footprint.  The VAST pipeline used the Observatory-provided data products and source catalogues to perform source detection, association, and forced photometry, producing a set of light curves.  with A0538–66 detected as its light curve was highly anomalous.  The plot above shows the 888 MHz flux densities (top) and phase-folded light curve (bottom) from the VAST survey on ASKAP. Grey lines mark the dates of the optical maxima, arrows denote 3𝜎 upper limits (where 𝜎 is the image root-mean-square noise), and error bars show the 1𝜎 statistical uncertainties.