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

TL;DR

This paper presents a simple model for FRB time-frequency structure, showing how variations in a key parameter can explain observed diversity and the CHIME burst dichotomy, with implications for the external environments of different FRB types.

Contribution

It introduces a toy model linking spectral energy distribution evolution to burst diversity and the CHIME dichotomy, suggesting environmental differences influence FRB properties.

Findings

Varying the power-law index  of frequency drift transforms burst durations.

The model explains the CHIME burst dichotomy through parameter variation.

External medium density profiles relate to burst duration differences.

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 \beta of 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 \beta could generate the dichotomy between burst duration and frequency-width recently found by CHIME. In shock models, the value of \beta is 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.