Several long-period radio transients have recently been discovered, with strongly polarised coherent radio pulses appearing on timescales between tens to thousands of seconds1,2. In some cases, the radio pulses have been interpreted as coming from rotating neutron stars with extremely strong magnetic fields, known as magnetars; the origin of other, occasionally periodic and less-well-sampled radio transients is still debated3. Coherent periodic radio emission is usually explained by rotating dipolar magnetic fields and pair-production mechanisms, but such models do not easily predict radio emission from such slowly rotating neutron stars and maintain it for extended times. On the other hand, highly magnetic isolated white dwarfs would be expected to have long spin periodicities, but periodic coherent radio emission has not yet been directly detected from these sources. Here Hurley-Walker et al. report observations of a long-period (21 min) radio transient, which we have labelled GPM J1839-10. The pulses vary in brightness by two orders of magnitude, last between 30 and 300s and have quasiperiodic substructure. The observations prompted a search of radio archives and we found that the source has been repeating since at least 1988. The archival data enabled constraint of the period derivative to <3.6 × 10-13 s s-1, which is at the very limit of any classical theoretical model that predicts dipolar radio emission from an isolated neutron star.
The figure above shows polarisation properties of the brightest pulse observed with MeerKAT, in the observation starting at UTC 2022-07-20 19:12:33. Panel (a) shows the Stokes I flux density, circular polarised intensity Stokes V, and linear polarised intensity P; panel (b) shows their ratio, i.e. the fractional polarisation; and panel (c) shows the polarisation position angle, derived from the PTUSE data at 65 ms resolution. For clarity, in the bottom two panels, only data where the errors are less than 100 % are shown. In the third panel, the grey line shows the typical (arbitrary) position angle of the polarisation, while the red dotted line shows orthogonal (±90◦) angles.