Radio emission from beyond the light cylinder in millisecond pulsars

ABSTRACT A striking aspect of the radio profiles of many millisecond pulsars (MSPs) is that they consist of components separated from each other by regions lacking in emission. We devise a technique for determining ‘disjoint’ from ‘contiguous’ components and show that 39 per cent of MSPs have disjoint components as opposed to only 3 per cent of the slow pulsar population. We surmize that the pulsars with these disjoint components show evidence for both emission above the polar cap and from the current sheet beyond the light cylinder (LC), colocated with gamma-ray emission. For some of the radio MSPs only the LC emission is being observed. It is our contention that almost all of the current population of gamma-ray MSPs show evidence for colocated radio emission. A simple geometric explanation allows the presence (or not) of LC emission and the relationship (or not) between the gamma-ray and radio profiles to be determined. The LC components have frequently very high polarization and typically flat position-angle traverses thus helping to explain the difficulties in determining the geometry of MSPs. In cases where the geometry can be determined the values broadly align with expectations. In this picture, the number of potentially detectable radio MSPs is higher than previously thought, although the actual detectability of LC components depends on their luminosity function. A mechanism is required to produce coherent radio emission far from the stellar surface. These ideas have implications for our understanding of the populations of radio-loud and radio-quiet rotation-powered millisecond pulsars, and may have implications for the long-term timing stability of some of these sources.