A Toy Model for the Time-Frequency Structure of Fast Radio Bursts: Implications for the CHIME Burst Dichotomy

Abstract

We introduce a toy model for the time-frequency structure of fast radio bursts (FRB), in which the observed emission is produced as a narrowly-peaked intrinsic spectral energy distribution sweeps down in frequency across the instrumental bandpass as a power-law in time. Though originally motivated by emission models which invoke a relativistic shock, the model could in principle apply to a wider range of emission scenarios. We quantify the burst's detectability using the frequency bandwidth over which most of its signal-to-noise ratio (SNR) is accumulated. We demonstrate that by varying just a single parameter of the toy model-the power-law index βof the frequency drift rate-one can transform a long (and hence preferentially time-resolved) burst with a narrow time-integrated spectrum into a shorter burst with a broad power-law time-integrated spectrum. We suggest that burst-to-burst diversity in the value of βcould generate the dichotomy between burst duration and frequency-width recently found by CHIME. In shock models, the value of βis related to the radial density profile of external medium, which in light of the preferentially longer duration of bursts from repeating sources may point to diversity in the external environments surrounding repeating versus one-off FRB sources.

11 pages, 7 figures, to be submitted to ApJ, comments appreciated

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• The Physics of Fast Radio Bursts
• The FRB 20121102A November rain in 2018 observed with the Arecibo Telescope
• Can a Single Population Account for the Discriminant Properties in Fast Radio Bursts?
• Spectropolarimetric variability in the repeating fast radio burst source FRB 20180301A
• A highly magnetized long-period radio transient exhibiting unusual emission features
• Fast radio bursts at the dawn of the 2020s
• A Practical Framework for Estimating the Repetition Likelihood of Fast Radio Bursts from Spectral Morphology