Magnetar birth: rotation rates and gravitational-wave emission

Understanding the evolution of the angle χ 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 ω, 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 protomagnetar wind acting shortly after birth. We also account for the effect of buoyancy forces on viscous dissipation at late times. Typically, we find that χ → 90° shortly after birth, then decreases towards 0° over hundreds of years. From observational indications that magnetars typically have small χ, we infer that these stars are subject to a stronger average exterior torque than radio pulsars, and that they were born spinning faster than ∼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.