Spin-down in the Anomalous X-Ray Pulsars

Abstract

The anomalous X-ray pulsars (AXPs) are currently explained in terms of either magnetars—neutron stars with superstrong magnetic field strengths (~1014-1015 G)—or isolated, accreting neutron stars with normal surface magnetic field strengths (~1012 G). For the latter, AXPs were recently suggested to be neutron stars accreting from a surrounding disk formed by the supernova fallback matter. We have critically examined the spin evolution of neutron stars in the accretion model and found that it is unable to account for the distributions of the spin period, age, and X-ray luminosity of AXPs. A more promising explanation for such objects is the magnetar model.