Implications of magnetar non-precession

Abstract

Abstract The objects known as anomalous X-ray pulsars and soft gamma repeaters are commonly identified with magnetars, neutron stars with ultrastrong magnetic fields. The rotational history of these objects has, so far, revealed no evidence of free precession. At the same time, these objects do not generally appear to have magnetic axes nearly parallel or orthogonal to their spin axes. In this Letter we show that the combination of these two observations, together with simple rigid-body dynamics, leads to non-trivial predictions about the interior properties of magnetars: either (i) elastic stresses in magnetar crusts are close to the theoretical upper limit above which the crustal matter yields or (ii) there is a ‘pinned’ superfluid component in the magnetar interior. As a potentially observable consequence of these ideas we point out that in the case of no pinned superfluidity, magnetars of stronger magnetic field strength than those currently observed would have to be nearly aligned/orthogonal rotators.