Coherent plasma-curvature radiation in FRB

Curvature radiation is a natural candidate for the emission mechanism of Fast Radio Burs (FRB). However, FRB spectra have structure with Δν/ν ∼ 0.03-0.2, inconsistent with the very smooth spectrum of curvature radiation. Although this spectral structure might be attributed to chromatic scintillation or lensing, in four FRB high spectral resolution data indicate scintillation decorrelation bandwidths much narrower than the observed ∼30-300 MHz spectral structure. Some of the observed structures may be intrinsic to the radiation mechanism. I suggest that the observed spectral structure reflects the spatial structure of a clumpy radiating charge distribution, and that the characteristic curvature radiation frequency may be higher than the observed frequencies. In this coupled plasma-curvature radiation process the radiated spectra are the product of the spectra of the plasma wave and that of incoherent curvature radiation. The argument applies to all coherent radiation processes, including those that produce pulsar nanoshots. The implied FRB `clump' charges are large, and produce electrostatic potentials that suggest electron Lorentz factors ≳10². The result applies generally to coherently radiating sources.