A new scenario for magnetar formation: Tayler-Spruit dynamo in a proto-neutron star spun up by fallback

TL;DR

This paper proposes a novel magnetar formation scenario where fallback-induced spin-up of a proto-neutron star triggers the Tayler-Spruit dynamo, amplifying magnetic fields to magnetar levels within seconds, even from slow rotators.

Contribution

It introduces a new magnetar formation model involving fallback-induced spin-up and dynamo action, expanding the understanding of magnetar origins beyond fast-rotating progenitors.

Findings

Magnetic fields reach 10^{14}-10^{15} G within 20-40 seconds after core bounce.

Magnetar-like fields can form from slow rotators with fallback masses above certain thresholds.

Predicted initial rotation periods are consistent with recent supernova simulations.

Abstract

Magnetars are isolated young neutron stars characterized by the most intense magnetic fields known in the universe. The origin of their magnetic field is still a challenging question. In situ magnetic field amplification by dynamo action is a promising process to generate ultra-strong magnetic fields in fast-rotating progenitors. However, it is unclear whether the fraction of progenitors harboring fast core rotation is sufficient to explain the entire magnetar population. To address this point, we propose a new scenario for magnetar formation, in which a slow-rotating proto-neutron star is spun up by the supernova fallback. We argue that this can trigger the development of the Tayler-Spruit dynamo while other dynamo processes are disfavored. Using previous works done on this dynamo and simulations to characterize the fallback, we derive equations modelling the coupled evolution of the proto-neutron star rotation and magnetic field. Their time integration for different fallback masses is successfully compared with analytical estimates of the amplification timescales and saturation value of the magnetic field. We find that the magnetic field is amplified within $20$ to $40$s after the core bounce, and that the radial magnetic field saturates at intensities $10^{14}-10^{15}$G, therefore spanning the full range of magnetar's dipolar magnetic fields. We also compare predictions of two proposed saturation mechanisms showing that magnetar-like magnetic fields can be generated for neutron star spun up to rotation periods $\lesssim8$ms and $\lesssim28$ms, corresponding to fallback masses $\gtrsim4\times10^{-2}{\rm M}_{\odot}$ and $\gtrsim10^{-2}{\rm M}_{\odot}$. Thus, our results suggest that magnetars can be formed from slow-rotating progenitors for fallback masses compatible with recent supernova simulations and leading to plausible initial rotation periods of the proto-neutron star.