The Origins of Narrow Spectra of Fast Radio Bursts

Observations find that some fast radio bursts (FRBs) have extremely narrow-band spectra, i.e., $\Delta\nu/\nu_0 \ll 1$. We show that when the angular size of the emission region is larger than the Doppler beaming angle, the observed spectral width ($\Delta\nu/\nu_0$) exceeds 0.58 due to the high latitude effects for a source outside the magnetosphere, even when the spectrum in the source's comoving frame is monochromatic. The angular size of the source for magnetospheric models of FRBs can be smaller than the Doppler beaming angle, in which case this geometric effect does not influence the observed bandwidth. We discuss various propagation effects to determine if any could transform a broad-spectrum radio pulse into a narrow-spectrum signal at the observer's location. We find that plasma lensing and scintillation can result in a narrow bandwidth in the observed spectrum. However, the likelihood of these phenomena being responsible for the narrow observed spectra with $\Delta\nu/\nu_0 < 0.58$ in the fairly large observed sample of FRBs is exceedingly small.