On the Magnetospheric Origin of Repeating Fast Radio Bursts

A bright radio burst was newly discovered in SGR 1935+2154, which exhibits some fast radio burst (FRB)-like temporal and frequency properties, suggesting a neutron star (NS)/magnetar magnetospheric origin of FRBs. We propose an explanation of the temporal and frequency properties of sub-pulses of repeating FRBs based on the generic geometry within the framework of charged-bunching coherent curvature radiation in the magnetosphere. The sub-pulses in a radio burst come from bunches of charged particles moving along different magnetic field lines. Their radiation beams sweep across the line of sight at different times, and those radiating at the more curved part tend to be seen earlier and at higher frequency. However, by considering bunches generated at slightly different times, we find there is also a small probability that the emission from the less curved part can be seen earlier. We simulate the time-frequency structures by deriving various forms of the electric acceleration field in the magnetosphere. This structure of sub-pulses is a natural consequence of coherent curvature radiation from an NS/magnetar magnetosphere with suddenly and violently triggered sparks. We apply this model to explain the time-frequency structure within a specific dipolar configuration by invoking the transient pulsar-like sparking from the inner gap of a slowly rotating NS, and we have also applied it to more generic configurations.