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 $\approx16.6$-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.

💡 Research Summary

The authors report the first detection of radio emission from the Be/X‑ray binary A0538‑66, located in the Large Magellanic Cloud, using the Australian Square Kilometre Array Pathfinder (ASKAP) and a dedicated weekly monitoring campaign with the MeerKAT telescope. ASKAP’s VAST survey identified the source as an outlier in its light‑curve database through a machine‑learning anomaly detection pipeline, revealing several detections between 2022 and 2024 with peak flux densities of ~6 mJy at 888 MHz. Prompted by this discovery, the team initiated a MeerKAT L‑band (centered at 1.284 GHz) monitoring program that obtained 35 epochs between February and October 2025. The source was detected in every epoch, with flux densities ranging from ~0.2 mJy up to a maximum of 8.78 ± 0.02 mJy on 2025‑05‑04, corresponding to a radio luminosity of ≈3 × 10²² erg s⁻¹ Hz⁻¹ – one of the most radio‑luminous neutron‑star X‑ray binaries known.

A phase‑folded analysis using the optical ephemeris (P_orb = 16.64002 d, T₀ = MJD 55673.71) shows that the strongest radio flares occur within ~0.2 orbital phase after the optical maximum, i.e., shortly after periastron passage when the neutron star interacts with the Be star’s decretion disc. Intra‑band spectral indices were measured for the brightest epochs (S/N > 30σ) by fitting a power law (S_ν ∝ ν^α) to 16 sub‑bands; the resulting α values are flat to slightly positive (α ≈ ‑0.1 to +0.2), contrasting with the steeper spectra (α ≈ ‑0.7) typical of synchrotron emission from steady jets in low‑mass X‑ray binaries. No significant linear or circular polarisation was detected: the circular polarisation fraction is constrained to –0.5 % < m_c < +0.4 % (3σ), and the linear polarisation fraction is <1.1 % (3σ). Rotation measure synthesis yields a marginal peak at RM ≈ 1600 rad m⁻² with only 2.3σ significance, suggesting the apparent signal is likely spurious.

The paper discusses four potential origins for the radio emission: (1) a transient, relativistic jet launched during super‑Eddington accretion episodes; (2) a propeller‑transition scenario where the rapidly rotating magnetosphere temporarily allows matter to escape, producing outflows; (3) a shock (or “collision‑shock”) region formed where the neutron star’s wind or magnetosphere collides with the dense Be disc material; and (4) a pulsar‑wind nebula powered by the neutron star’s spin‑down. The flat spectral index, orbital modulation, and lack of strong polarisation favour a combination of a transient jet and propeller‑driven outflow, while the shock model remains viable given the dense circumstellar environment. The pulsar‑wind hypothesis is less supported by the current data, primarily because the measured polarisation limits and RM are inconsistent with typical pulsar wind nebulae in the Magellanic Clouds.

The authors place A0538‑66 in context with other high‑mass X‑ray binaries, noting that its radio luminosity exceeds that of well‑studied systems such as SMC X‑1 and LMC X‑4, and that its rapid 69 ms spin period makes it the fastest‑spinning accretion‑powered neutron star in a high‑mass system. They argue that the extreme spin, high orbital eccentricity (e ≈ 0.72), and a possibly misaligned, precessing decretion disc together create the conditions for episodic, highly luminous radio outbursts.

Future work is outlined: high‑resolution VLBI imaging to resolve any jet structure, broadband radio spectra (0.3–10 GHz) to better constrain the spectral curvature, simultaneous X‑ray monitoring to correlate radio flares with super‑Eddington accretion events, and deeper polarimetric observations to test for low‑level intrinsic polarisation. The authors anticipate that such multi‑wavelength campaigns will decisively discriminate between jet, propeller, and shock scenarios, and will illuminate the physics of extreme accretion and outflow in fast‑spinning neutron‑star binaries.