Long-term evolution of anomalous X-ray pulsars and soft gamma repeaters

TL;DR

This study models the long-term evolution of anomalous X-ray pulsars and soft gamma repeaters using the fallback disc model, showing their properties can be explained with similar disc parameters and highlighting differences in their evolutionary phases and magnetic fields.

Contribution

It provides a comprehensive analysis of AXP/SGRs evolution within the fallback disc model, comparing their properties with XDINs and identifying their evolutionary phases and magnetic field ranges.

Findings

AXP/SGRs can be explained with similar disc parameters as XDINs.

Most AXP/SGRs are currently in the accretion phase, while two are in the propeller phase.

AXP/SGRs have stronger dipole magnetic fields ($1-6 imes 10^{12}$ G) compared to XDINs ($10^{11}-10^{12}$ G).

Abstract

We have investigated the long-term evolution of individual anomalous X-ray pulsars (AXPs) and soft gamma repeaters (SGRs) with relatively well constrained X-ray luminosity and rotational properties. In the frame of the fallback disc model, we have obtained the ranges of disc mass and dipole field strength that can produce the observed source properties. We have compared our results with those obtained earlier for dim isolated neutron stars (XDINs). Our results show that (1) the X-ray luminosity, period and period derivative of the individual AXP/SGR sources can be produced self-consistently in the fallback disc model with very similar basic disc parameters to those used earlier in the same model to explain the long-term evolution of XDINs, (2) except two sources, AXP/SGRs are evolving in the accretion phase; these two exceptional sources, like XDINs, completed their accretion phase in the past and are now evolving in the final propeller phase and still slowing down with the disc torques, (3) the dipole field strength (at the poles) of XDINs are in the $10^{11}-10^{12}$ G range, while AXP/SGRs have relatively strong dipole fields between $1-6 \times 10^{12}$ G, and (4) the source properties can be obtained with large ranges of disc masses which do not allow a clear test of correlation between disc masses and the magnetic dipole fields for the whole AXP/SGRs and XDIN population.