Spin-down in the anomalous X-ray pulsars

The anomalous X-ray pulsars (AXPs) are currently explained in terms of eith er magnetars-neutron stars with superstrong magnetic field strengths (simil ar to 10(14)-10(15) G)-or isolated, accreting neutron stars with normal sur face magnetic field strengths (similar to 10(12) G). For the latter, AXPs w ere recently suggested to be neutron stars accreting from a surrounding dis k 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 m agnetar model.