Abstract
Curvature radiation is a natural candidate for the emission mechanism of FRB. However, FRB spectra have structure with $\Delta \nu/\nu \sim \text{0.03--0.2}$, 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 $\sim \text{30--300}$ MHz spectral structure. Some of the observed structure 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 much 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 $\gtrsim 10^2$. The result applies generally to coherently radiating sources.
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