Rotational evolution of magnetars in the presence of a fallback disk

TL;DR

This paper models how fallback disks influence magnetar spin evolution, explaining different observed magnetar types and their rotational periods through a unified framework considering disk mass and magnetic field strength.

Contribution

It introduces a self-similar fallback disk model to explain the diverse rotational behaviors of magnetars based on disk mass and magnetic field strength.

Findings

Low disk mass leaves magnetars unaffected, resulting in normal magnetars.

High disk mass with moderate magnetic field yields passive fallback disks.

High magnetic field with large disk mass causes rapid spin-down to super-slow periods.

Abstract

Magnetars may have strong surface dipole field. Observationally, two magnetars may have passive fallback disks. In the presence of a fallback disk, the rotational evolution of magnetars may be changed. In the self-similar fallback disk model, it is found that: (1) When the disk mass is significantly smaller than $10^{-6} \,\rm M_{\odot}$, the magnetar is unaffected by the fallback disk and it will be a normal magnetar. (2) When the disk mass is large, but the magnetar's surface dipole field is about or below $10^{14} \,\rm G$, the magnetar will also be a normal magnetar. A magnetar plus a passive fallback disk system is expected. This may correspond to the observations of magnetars 4U 0142$+$61, and 1E 2259$+$586. (3) When the disk mass is large, and the magnetar's surface dipole field is as high as $4\times 10^{15} \,\rm G$, the magnetar will evolve from the ejector phase to the propeller phase, and then enter rotational equilibrium. The magnetar will be slowed down quickly in the propeller phase. The final rotational period can be as high $2\times 10^4 \,\rm s$. This may correspond to the super-slow magnetar in the supernova remnant RCW 103. Therefore, the three kinds of magnetars can be understood in a unified way.