Triaxial Magnetars as Sources of Fast Radio Bursts

TL;DR

This paper tackles three enigmatic FRB properties — large disparities in observed duty factors, aperiodic activity in repeaters, and the absence of a detected rotational period — by positing that FRBs originate from magnetars with a dynamically triaxial moment of inertia, arising from frozen-in magnetic and elastic stresses. It develops a quantitative framework using the rigid-body Euler equations to show that the reorientation timescale satisfies $\tau \sim (I/\Delta I)(1/\omega)$ with $\Delta I/I \sim 10^{-6} B_{15}^2$; for a young magnetar ($\omega \sim 10\,\mathrm{s^{-1}}$) this yields $\tau \sim 1$ day, and with a narrow beam width ($\approx 0.003$ rad) and $\gamma \sim 300$, the apparent recurrence time of non-repeaters can be of order a few years. The model links observer geometry and beam wandering to the observed duty factors, predicting aperiodic activity and explaining the lack of rotational timing signatures in repeaters. If validated, this framework provides a concrete magnetar-based mechanism for FRB emission along open field lines and suggests that younger, higher-field magnetars could disproportionately drive the most extreme FRB activity, with possible implications for gravitational-wave visibility in related neutron-star systems.

Abstract

I suggest that some of the mysterious temporal properties of Fast Radio Bursts (FRB) may be explained if they are produced by dynamically triaxial magnetars. If the bursts are narrowly collimated along open field lines, then observed repeating FRB may be those in which the moment of inertia tensor is only slightly triaxial and the rotation axis, open field lines and radiation point nearly to the observer. Apparently non-repeating FRB may be triaxial with the direction of open field lines and radiation wandering across the sky, reducing their duty factors by several orders of magnitude. A slightly triaxial moment tensor in repeaters moves the line of sight into or out of the radiation pattern or within it, explaining periods of greater or lesser (or absent) activity, and making the probability of detecting a burst vary aperiodically. The dynamics of triaxial bodies may also thwart the coherent integration of gravitational signals from fast-rotating accreting neutron stars.