The Spectral Properties of the Bright Fast Radio Burst Population

We examine the spectra of 23 fast radio bursts (FRBs) detected in a fly's-eye survey with the Australian SKA Pathfinder, including those of three bursts not previously reported. The mean spectral index of alpha = -1.5(-0.3)(+0.2) (F-nu alpha nu(alpha)) is close to that of the Galactic pulsar population. The sample is dominated by bursts exhibiting a large degree of spectral modulation: 17 exhibit fine-scale spectral modulation with an rms exceeding 50% of the mean, with decorrelation bandwidths (half-maximum) ranging from approximate to 1 to 49 MHz. Most decorrelation bandwidths are an order of magnitude lower than the greater than or similar to 30 MHz expected of Galactic interstellar scintillation at the Galactic latitude of the survey, vertical bar b vertical bar = 50 degrees +/- 5 degrees. However, these bandwidths are consistent with the similar to nu(4) scaling expected of diffractive scintillation when compared against the spectral structure observed in bright UTMOST FRBs detected at 843 MHz. A test of the amplitude distribution of the spectral fluctuations reveals only 12 bursts consistent at better than a 5% confidence level with the prediction of 100%-modulated diffractive scintillation. Five of six FRBs with a signal-to-noise ratio exceeding 20 are only consistent with this prediction at less than 1% confidence. Nonetheless, there is weak evidence (92%-94% confidence) of an anti-correlation between the amplitude of the spectral modulation and dispersion measure (DM), which suggests that it originates as a propagation effect. This effect is corroborated by the smoothness of the higher-DM Parkes FRBs, and could arise due to quenching of diffractive scintillation (e.g., in the interstellar medium of the host galaxy) by angular broadening in the intergalactic medium.