Bumpy spin-down of anomalous X-ray pulsars: The link with magnetars

Abstract

The two anomalous X-ray pulsars (AXPs) with well-sampled timing histories, 1E 1048.1-5937 and 1E 2259+586, are known to spin down irregularly, with "bumps" superposed on an overall linear trend. Here we show that if AXPs are nonaccreting magnetars, i.e., isolated neutron stars with surface magnetic fields B0 ≳ 1010 T, then they spin down electromagnetically in exactly the manner observed, because of an effect called "radiative precession." Internal hydromagnetic stresses deform the star, creating a fractional difference ε = (I3 - I1)/I1 ∼ 10-8 between the principal moments of inertia I1 and I3; the resulting Eulerian precession couples to an oscillating component of the electromagnetic torqu..