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

TL;DR

This work reports the first radio detection of the Be/X-ray binary A0538-66 in the LMC using ASKAP, followed by a weekly MeerKAT monitoring campaign that reveals persistent, bright radio emission with signs of orbital modulation (peaking near periastron). The emission is interpreted through several synchrotron scenarios—jet ejecta, intra-binary shocks, or propeller-driven outflows—with energetic constraints indicating a substantial non-thermal component and a likely complex dependence on accretion state. By placing A0538-66 in the L_R–L_X parameter space alongside Cir X-1 and various gamma-ray binaries, the study highlights how an extragalactic Be/XRB can reach extreme radio luminosities at modest X-ray luminosities, challenging standard Be/XRB radio expectations. The findings motivate dense, multi-wavelength follow-up to unravel the emission mechanism and its link to the system’s fast spin, eccentric orbit, and disc geometry, with implications for accretion physics in high-mass X-ray binaries.

Abstract

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 $\approx16.6$-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.

Figures (4)

• Figure 1: 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 3$\sigma_\text{rms}$ upper limits (where $\sigma_\text{rms}$ is the image root-mean-square noise), and error bars show the 1$\sigma$ statistical uncertainties.
• Figure 2: (a) 1.28 GHz MeerKAT radio core flux densities, and (b) intra-band spectral indices. (c) ATLAS filter o (560–820 nm) and c (420–650 nm) flux densities. (d) Swift/XRT unabsorbed 1–10 keV fluxes. Grey lines indicate optical maxima, and error bars show 1$\sigma$ statistical uncertainties.
• Figure 3: Phase-folded 1.28 GHz MeerKAT (a) flux densities and (b) intra-band spectral indices, and (c) ATLAS filter o (560–820 nm) data, over the date range MJDs = [60710, 60961] (covering multiple orbital cycles), where phase zero is optical maximum. The light blue shading in (a) shows average data in $\sim$0.8-day bins. Phase-folded Swift/XRT data are not shown, as they do not appear to be orbitally modulated.
• Figure 4: A schematic showing regions in radio:X-ray parameter space occupied by different types of systems (see the text for caveats). Orange and blue respectively indicate the hard/quiescent-state black hole and neutron star low-mass X-ray binaries. Purple and red show subsets of the Galactic Be/X-ray binary and $\gamma$-ray binary classes, respectively, while yellow indicates the extra-Galactic $\gamma$-ray binary LMC P3. A0538–66 and Cir X–1 are shown in green and grey, respectively.