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 $\alpha =-{1.5}_{-0.3}^{+0.2}$ (Fν ∝ να) 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 ≈1 to 49 MHz. Most decorrelation bandwidths are an order of magnitude lower than the ≳30 MHz expected of Galactic interstellar scintillation at the Galactic latitude of the survey, $| b| =50^\circ \pm 5^\circ $. However, these bandwidths are consistent with the ∼ν4 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.