The spectral properties of the bright FRB population

TL;DR

This study analyzes the spectral properties of 23 fast radio bursts, revealing significant spectral modulation likely influenced by propagation effects, and compares their spectral indices to Galactic pulsars.

Contribution

It provides the first detailed spectral analysis of a sizable FRB sample, highlighting the role of propagation effects like scintillation in spectral modulation.

Findings

Mean spectral index close to Galactic pulsars

Most bursts show significant spectral modulation

Spectral modulation amplitude may anti-correlate with DM

Abstract

We examine the spectra of 23 fast radio bursts 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 $\alpha = -1.6_{-0.2}^{+0.3}$ ($F_\nu \propto \nu^\alpha$) 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 $\approx$ to 49 MHz. Most decorrelation bandwidths are an order of magnitude lower than the $\gtrsim 30\,$MHz expected of Galactic interstellar scintillation at the Galactic latitude of the survey, $|b| = 50 \pm 5 \deg$. 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. Moreover, five of six FRBs with a signal-to-noise ratio exceeding 18 are consistent with this prediction at less than 1% confidence. Nonetheless, there is weak evidence (88-95% confidence) that the amplitude of the fine-scale spectral modulation is anti-correlated with dispersion measure (DM) that would suggest it originates as a propagation effect. This effect appears to be 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.