Coherent inverse Compton scattering by bunches in fast radio bursts

The extremely high brightness temperature of fast radio bursts (FRBs) requires that their emission mechanism must be "coherent", either through concerted particle emission by bunches or through an exponential growth of a plasma wave mode or radiation amplitude via certain maser mechanisms. The bunching mechanism has been mostly discussed within the context of curvature radiation or cyclotron/synchrotron radiation. Here we propose a family of model invoking coherent inverse Compton scattering (ICS) of bunched particles that may operate within or just outside of the magnetosphere of a flaring magnetar. Crustal oscillations during the flaring event may excite low-frequency electromagnetic waves near the magnetar surface. The X-mode of these waves could penetrate through the magnetosphere. Bunched relativistic particles in the charge starved region inside the magnetosphere or in the current sheet outside of the magnetosphere would upscatter these low-frequency waves to produce GHz emission to power FRBs. The ICS mechanism has a much larger emission power for individual electrons than curvature radiation. This greatly reduces the required degree of coherence in bunches, alleviating several criticisms to the bunching mechanism raised in the context of curvature radiation. The emission is $\sim 100\%$ linearly polarized (with the possibility of developing circular polarization) with a constant or varying polarization angle across each burst. The mechanism can account for a narrow-band spectrum and a frequency downdrifting pattern, as commonly observed in repeating FRBs.