Axion-mediated Transport of Fast Radio Bursts Originating in Inner Magnetospheres of Magnetars

Abstract: Among magnetar models of Fast Radio Bursts (FRBs), there is ongoing debate about whether the site of coherent radio emission lies within or beyond the light cylinder. We propose a mechanism by which FRBs produced near the magnetar surface are transported out of the magnetosphere by axions, which couple to photons. If the emission site hosts strong accelerating electric fields, a considerable fraction of the FRB energy budget is converted to an axion burst. Once produced, the axion burst free-streams out of the magnetosphere due to the rapidly-decreasing magnetic field. The burst may escape through either the open or closed magnetosphere, while retaining the temporal signature of the original FRB. In the wind region, axions resonantly excite ordinary (O) modes that escape as the plasma density decreases. The radio efficiency of this mechanism satisfies energetics constraints from FRB 121102 for axion-photon coupling strengths that have not been excluded by other astrophysical probes.