On the evolution of anomalous X-ray pulsars and soft gamma ray repeaters with fallback disks

TL;DR

This paper proposes a model where fallback disks with high initial angular momentum explain the observed properties and period clustering of anomalous X-ray pulsars and soft gamma-ray repeaters, including their luminosities and ages.

Contribution

It introduces a disk evolution model with specific torque dependence and passive transition criteria that accounts for the observed clustering and transient behavior of AXPs and SGRs.

Findings

Fallback disks can explain period clustering of AXPs and SGRs.

Transient AXPs and SGRs are likely older than persistent ones.

Disk activity persists at large radii, influencing observable properties.

Abstract

We show that the period clustering of anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs), their X-ray luminosities, ages and statistics can be explained with fallback disks with large initial specific angular momentum. The disk evolution models are developed by comparison to self-similar analytical models. The initial disk mass and angular momentum set the viscous timescale. An efficient torque, with (1 - w^2) dependence on the fastness parameter w leads to period clustering in the observed AXP-SGR period range under a wide range of initial conditions. The timescale t_0 for the early evolution of the fallback disk, and the final stages of fallback disk evolution, when the disk becomes passive, are the crucial determinants of the evolution. The disk becomes passive at temperatures around 100 K, which provides a natural cutoff for the X-ray luminosity and defines the end of evolution in the observable AXP and SGR phase. This low value for the minimum temperature for active disk turbulence indicates that the fallback disks are active up to a large radius greater than ~10^{12} cm. We find that transient AXPs and SGRs are likely to be older than their persistent cousins. A fallback disk with mass transfer rates corresponding to the low quiescent X-ray luminosities of the transient sources in early evolutionary phases would have a relatively lower initial mass, such that the mass-flow rate in the disk is not sufficient for the inner disk to penetrate into the light cylinder of the young neutron star, making mass accretion onto the neutron star impossible. The transient AXP phase therefore must start later. The model results imply that the transient AXP/SGRs, although older, are likely to be similar in number to persistent sources (abridged).