On the radiation mechanism of repeating fast radio bursts

Recent observations show that fast radio bursts (FRBs) are energetic but probably non-catastrophic events occurring at cosmological distances. The properties of their progenitors are largely unknown in spite of many attempts to determine them using the event rate, duration and energetics. Understanding the radiation mechanism for FRBs should provide the missing insights regarding their progenitors, which is investigated in this paper. The high brightness temperatures (>10^{35} K) of FRBs mean that the emission process must be coherent. Two general classes of coherent radiation mechanisms are considered --- maser and the antenna mechanism. We use the observed properties of the repeater FRB 121102 to constrain the plasma conditions needed for these two mechanisms. We have looked into a wide variety of maser mechanisms operating in either vacuum or plasma and find that none of them can explain the high luminosity of FRBs without invoking unrealistic or fine-tuned plasma conditions. The most favorable mechanism is antenna curvature emission by coherent charge bunches where the burst is powered by magnetic reconnection near the surface of a magnetar (B > 10^{14} G). We show that the plasma in the twisted magnetosphere of a magnetar may be clumpy due to two-stream instability. When magnetic reconnection occurs, the pre-existing density clumps may provide charge bunches for the antenna mechanism to operate. This model should be applicable to all FRBs that have multiple outbursts like FRB 121102.