Jet break revealed in the transitional millisecond pulsar candidate 4FGL J0427.8−6704

ABSTRACT Understanding the formation and properties of relativistic jets from accreting compact objects has far-reaching implications in astrophysics. Transitional millisecond pulsars (tMSPs) – a class of neutron stars transitioning between radio pulsar and accretion states – offer a unique opportunity to study jet behaviour within a low-level accretion regime around fast-spinning, magnetized neutron stars. We analysed archival spectral energy distributions (SEDs) for both confirmed and candidate tMSPs from literature and various data bases, aiming to identify jet spectra and determine physical conditions within these jets. For the tMSP candidate 4FGL J0427.8–6704, a high-inclination system that displays eclipses in optical, X-ray, and $\gamma$-ray wavelengths, we derived a jet break frequency at $\nu _{\rm br} \approx 10^{11}$ Hz and determined properties of the jet base using a conical jet model (opening angle of $\phi < 32^\circ$, magnetic field strength of $B_{0}\sim 100$ G, and radius of $R_{0}\sim 10^{10}$ cm). Observations from the Atacama Large Millimeter/submillimeter Array reveal an average flux density of 0.4 mJy, with flares reaching up to 2 mJy on short (seconds) time-scales. No eclipses were detected in the millimeter light curves, suggesting the jet base is farther from the central source than in other X-ray binaries ($z_{0}>7\times 10^{10}$ cm). We also investigated SEDs of other confirmed and candidate tMSPs but did not find well-defined jet spectral breaks. However, a mid-infrared flux excess in tMSP XSS J12270–4859 suggests that the compact jet emission may extend into near-infrared or optical wavelengths. These results provide new insights into jet formation in tMSPs, highlighting the need for further multiwavelength observations to fully characterize jet behaviour in similar low-accretion systems.