The process of accretion onto compact objects and its role in the production of relativistic, collimated jets remains a major open question in astronomy. Transitional millisecond pulsars (tMSPs) provide a rare opportunity to study the formation of relativistic jets in neutron stars that experience low-level accretion yet possess strong magnetic fields. By analyzing archival multiwavelength data from the Atacama Large Millimeter/submillimeter Array (ALMA), we identified a so-called jet break at ~100 GHz in the tMSP candidate 4FGL J0427.9-6704. This discovery offers new insights into the jet structure of the source and, more broadly, into jet formation around strongly magnetized neutron stars. The observations also revealed rapid millimeter flaring and, interestingly, showed no eclipses, despite the source exhibiting a high, nearly edge-on inclination. This suggests that the jet base extends farther from the neutron star than in other X-ray binaries.
Despite differences in jet structure, the millimeter luminosity and estimated jet power of 4FGL J0427.9-6704 are consistent with those of other accreting neutron star and black hole X-ray binaries in the low-accretion regime, challenging the predictions of typical theoretical models of jet formation. A similarly stable, low-frequency jet break has been inferred in only one other neutron star X-ray binary, Aql X-1, suggesting possible similarities in the jet bases of these two sources. This hints that the jets may be, at least in part, powered by the relatively strong magnetic fields of the neutron stars in these systems. Overall, we demonstrate that tMSP jets offer a unique window into understanding jets at low mass accretion rates and the impact of strong magnetic fields on their properties.
