Quasi-periodic sub-pulse structure as a unifying feature for radio-emitting neutron stars

Magnetars are highly-magnetised rotating neutron stars that are predominantly observed as high-energy sources. Six of this class of neutron star are known to also emit radio emission, and magnetars are, thus, a favoured model for the origin for at least some of the Fast Radio Bursts (FRBs). If magnetars, or neutron stars in general, are indeed responsible, sharp empirical constraints on the mechanism producing radio emission are required. Here we report on the detection of polarised quasi-periodic sub-structure in the emission of all well-studied radio-detected magnetars. A correlation previously seen, relating sub-structure in pulsed emission of radio emitting neutron stars to their rotational period, is extended, and shown to now span more than six of orders of magnitude in pulse period. This behaviour is not only seen in magnetars but in members of all classes of radio-emitting rotating neutron stars, regardless of their evolutionary history, their power source or their inferred magnetic field strength. If magnetars are responsible for FRBs, it supports the idea of being able to infer underlying periods from sub-burst timescales in FRBs.