What does FRB light-curve variability tell us about the emission mechanism?

TL;DR

This paper investigates how the rapid variability in FRB light-curves constrains emission mechanisms, explores models explaining large radii emission, and predicts observable features to distinguish between different scenarios, especially favoring magnetospheric origins if microsecond variability is confirmed.

Contribution

It introduces models that explain fast flux modulations at large emission radii and predicts spectral-temporal features that can be tested observationally.

Findings

Light-curve variability constrains source compactness and emission mechanism.

High-latitude emission predicts specific flux correlations across frequencies.

Detection of microsecond variability would support magnetospheric models.

Abstract

A few fast radio bursts' (FRBs) light-curves have exhibited large intrinsic modulations of their flux on extremely short ($t_{\rm r}\sim 10\mu$s) time scales, compared to pulse durations ($t_{\rm FRB}\sim1$ms). Light-curve variability timescales, the small ratio of rise time of the flux to pulse duration, and the spectro-temporal correlations in the data constrain the compactness of the source and the mechanism responsible for the powerful radio emission. The constraints are strongest when radiation is produced far ($\gtrsim 10^{10}$cm) from the compact object. We describe different physical set-ups that can account for the observed $t_{\rm r}/t_{\rm FRB}\ll 1$ despite having large emission radii. The result is either a significant reduction in the radio production efficiency or distinct light-curves features that could be searched for in observed data. For the same class of models, we also show that due to high-latitude emission, if a flux $f_1(\nu_1)$ is observed at $t_1$ then at a lower frequency $\nu_2<\nu_1$ the flux should be at least $(\nu_2/\nu_1)^2f_1$ at a slightly later time ($t_2=t_1\nu_1/\nu_2$) independent of the duration and spectrum of the emission in the comoving frame. These features can be tested, once light-curve modulations due to scintillation are accounted for. We provide the timescales and coherence bandwidths of the latter for a range of possibilities regarding the physical screens and the scintillation regime. Finally, if future highly resolved FRB light-curves are shown to have intrinsic variability extending down to $\sim \mu$s timescales, this will provide strong evidence in favor of magnetospheric models.