Empirical Assessment of Aperiodic and Periodic Radio Bursts from Young Precessing Magnetars

TL;DR

This paper investigates the periodic and aperiodic radio bursts from young magnetars, proposing a precessing neutron star model with phase jitter and variable emission altitudes to explain observed phenomena and challenges in detecting fast periodicities.

Contribution

It introduces a detailed model of precessing magnetars with phase jitter and emission variability, explaining observed periodicities and the difficulty in detecting fast spin periods.

Findings

Young, precessing magnetars can produce observed burst periodicities.

Phase jitter and emission altitude variability explain the absence of fast periodicity.

Detection of fast spin periods is hindered by decoherence and spin noise.

Abstract

We analyze the slow periodicities identified in burst sequences from FRB 121102 and FRB 180916 with periods of about 16 and 160 d, respectively, while also addressing the absence of any fast periodicity that might be associated with the spin of an underlying compact object. Both phenomena can be accounted for by a young, highly magnetized, precessing neutron star that emits beamed radiation with significant imposed phase jitter. Sporadic narrow-beam emission into an overall wide solid angle can account for the necessary phase jitter, but the slow periodicities with 25 to 55 % duty cycles constrain beam traversals to be significantly smaller. Instead, phase jitter may result from variable emission altitudes that yield large retardation and aberration delays. A detailed arrival-time analysis for triaxial precession includes wobble of the radio beam and the likely larger, cyclical torque resulting from the changes in the spin-magnetic moment angle. These effects will confound identification of the fast periodicity in sparse data sets longer than about a quarter of a precession cycle unless fitted for and removed as with orbital fitting. Stochastic spin noise, likely to be much larger than in radio pulsars, may hinder detection of any fast-periodicity in data spans longer than a few days. These decoherence effects will dissipate as FRB sources age, so they may evolve into objects with properties similar to Galactic magnetars.