Period clustering of the anomalous X-ray pulsars

TL;DR

This paper proposes that the period clustering of anomalous X-ray pulsars and soft gamma-ray repeaters results from their evolution as remnants of high-mass X-ray binaries, with residual disks influencing their spin periods.

Contribution

It introduces a model where AXPs and SGRs are former HMXB neutron stars with residual magnetically-levitating disks affecting their period clustering.

Findings

Residual disks can sustain accretion for thousands of years.

The period clustering is explained by the disk lifetime matching the spin-down timescale.

Neutron stars in this scenario exhibit parameters similar to observed AXPs and SGRs.

Abstract

In this paper we address the question of why the observed periods of the Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are clustered in the range 2-12 s. We explore a possibility to answer this question assuming that AXPs and SGRs are the descendants of High Mass X-ray Binaries (HMXBs) which have been disintegrated in the core-collapse supernova explosion. The spin period of neutron stars in HMXBs evolves towards the equilibrium period, averaging around a few seconds. After the explosion of its massive companion, the neutron star turns out to be embedded into a dense gaseous envelope, the accretion from which leads to the formation of a residual magnetically-levitating (ML) disk. We show that the expected mass of a disk in this case is 10^-7 - 10^-8 M_sun which is sufficient to maintain the process of accretion at the rate 10^14 - 10^15 g/s over a time span of a few thousand years. During this period the star manifests itself as an isolated X-ray pulsar with a number of parameters resembling those of AXPs and SGRs. Period clustering of such pulsars can be provided if the lifetime of the residual disk does not exceed the spin-down timescale of the neutron star.