Magnetar birth: rotation rates and gravitational-wave emission

TL;DR

This paper models the coupled evolution of a magnetar's magnetic and rotational axes, revealing how internal and external forces influence its spin and magnetic alignment, and predicting associated gravitational and electromagnetic signals.

Contribution

It introduces a comprehensive model including proto-magnetar winds and buoyancy effects, providing new insights into magnetar birth properties and their observational signatures.

Findings

Magnetar magnetic axis tends to align with the rotation axis over hundreds of years.

Magnetars are born spinning faster than 100-300 Hz.

Predictions made for gravitational and electromagnetic signals from newborn magnetars.

Abstract

Understanding the evolution of the angle $\chi$ between a magnetar's rotation and magnetic axes sheds light on the star's birth properties. This evolution is coupled with that of the stellar rotation $\Omega$, and depends on the competing effects of internal viscous dissipation and external torques. We study this coupled evolution for a model magnetar with a strong internal toroidal field, extending previous work by modelling -- for the first time in this context -- the strong proto-magnetar wind acting shortly after birth. We also account for the effect of buoyancy forces on viscous dissipation at late times. Typically we find that $\chi\to 90^\circ$ shortly after birth, then decreases towards $0^\circ$ over hundreds of years. From observational indications that magnetars typically have small $\chi$, we infer that these stars are subject to a stronger average exterior torque than radio pulsars, and that they were born spinning faster than $\sim 100-300$ Hz. Our results allow us to make quantitative predictions for the gravitational and electromagnetic signals from a newborn rotating magnetar. We also comment briefly on the possible connection with periodic Fast Radio Burst sources.