# Repeating Fast Radio Bursts from Magnetars with Low Magnetospheric Twist

## TL;DR

This paper proposes a magnetar-based model for repeating fast radio bursts, linking their statistics to magnetar bursts, and explains their properties through a pulsar-like emission mechanism in low-twist magnetospheres.

## Contribution

It introduces a novel magnetar magnetosphere model for FRBs, connecting burst statistics to magnetar activity and explaining FRB properties with a pulsar-like emission process.

## Key findings

- FRB 121102 pulse statistics resemble magnetar high-energy bursts.
- The proposed mechanism accounts for FRB energetics with realistic efficiency.
- The model explains the absence of FRBs from known magnetars and predicts polarization properties.

## Abstract

We analyze the statistics of pulse arrival times in fast radio burst (FRB) 121102 and demonstrate that they are remarkably similar to statistics of magnetar high-energy short bursts. Motivated by this correspondence, we propose that repeating FRBs are generated during short bursts in the closed field line zone of magnetar magnetospheres via a pulsar-like emission mechanism. Crustal slippage events dislocate field line foot points, initiating intense particle acceleration and pair production, giving rise to coherent radio emission similar to that generated near pulsar polar caps. We argue that the energetics of FRB 121102 can be readily accounted for if the efficiency of the conversion of Poynting flux into coherent radio emission is $\sim10^{-4}-10^{-2}$, values consistent with empirical efficiencies of radio emission in pulsars and radio-loud magnetars. Such a mechanism could operate only in magnetars with preexisting low twist of the magnetosphere, so that the charge density in the closed zone is initially insufficient to screen the electric field provoked by the wiggling of magnetic field lines and is low enough to let $\sim 1$ GHz radio emission escape the magnetosphere, which can explain the absence of FRBs from known magnetars. The pair cascades crowd the closed flux tubes with plasma, screening the accelerating electric field, thus limiting the radio pulse duration to $\sim1$ ms. Within the framework of our model, the current dataset of the polarization angle variation in FRB 121102 suggests a magnetic obliquity $\alpha\lesssim40^\circ$ and viewing angle $\zeta$ with respect to the spin axis $\alpha<\zeta<180^\circ-\alpha$.