Can the anomalous X-ray pulsars be powered by accretion?

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 ones are: (1) isolated accreting neutron stars evolved from the Thorne-Żytkow objects (TŻOs ) 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 ultra-high (∼ 1014− 10 G) surface magnetic fields. We have critically examined the predicted change of 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 extremely limited lifetime. A more promising explanation for such objects is the magnetar model. Subject headings: accretion, accretion disks binaries: close pulsars: general stars: neutron X-ray: stars