Coherent Cherenkov Radiation by Bunches in Fast Radio Bursts

TL;DR

This paper proposes a new coherent Cherenkov radiation mechanism from magnetar magnetospheres to explain fast radio bursts, predicting narrow-band spectra, frequency drifting, and high polarization.

Contribution

It introduces a novel coherent Cherenkov radiation model for FRBs involving bunched particles in magnetar magnetospheres, with detailed calculations of emission properties.

Findings

Predicts narrow-band FRB spectra

Explains frequency downward drifting patterns

Achieves near 100% linear polarization

Abstract

Fast radio bursts (FRBs) are extragalactic radio transients with extremely high brightness temperature, which strongly suggests the presence of coherent emission mechanisms. In this study, we introduce a novel radiation mechanism for FRBs involving coherent Cherenkov radiation (ChR) emitted by bunched particles that may originate within the magnetosphere of a magnetar. We assume that some relativistic particles are emitted from the polar cap of a magnetar and move along magnetic field lines through a charge-separated magnetic plasma, emitting coherent ChR along their trajectory. The crucial condition for ChR to occur is that the refractive index of the plasma medium, denoted as $n_r$, must satisfy the condition $n_r^2 > 1$. We conduct comprehensive calculations to determine various characteristics of ChR, including its characteristic frequency, emission power, required parallel electric field, and coherence factor. Notably, our proposed bunched coherent ChR mechanism has the remarkable advantage of generating a narrower-band spectrum. Furthermore, a frequency downward drifting pattern, and $\sim100\%$ linearly polarized emission can be predicted within the framework of this emission mechanism.