1999ApJ...520..271L

The nature of the 5-12 s ``anomalous'' X-ray pulsars (AXPs) remains a mystery. Among the models that have been proposed to explain the properties of AXPs, the most likely are (1) isolated accreting neutron stars evolved from the Thorne-{Zdot}ytkow objects (T{Zdot}Os) due to complete spiral-in during the common envelope (CE) evolution of high-mass X-ray binaries (HMXBs), and (2) magnetars, which are neutron stars with ultrahigh (∼10¹⁴-10¹⁵ G) surface magnetic fields. We have critically examined the predicted change of a neutron star's spin in the accretion model, and found that it is unable to account for the steady spin-down observed in AXPs. A simple analysis also shows that any accretion disk around an isolated neutron star has an extremely limited lifetime. A more promising explanation for such objects is the magnetar model.