The long-term post-outburst spin down and flux relaxation of magnetar Swift J1822.3$-$1606

Abstract

The magnetar Swift J1822.3$-$1606 entered an outburst phase in 2011 July. Previous X-ray studies of its post-outburst rotational evolution yielded inconsistent measurements of the spin-inferred magnetic field. Here we present the timing behavior and flux relaxation from over two years of Swift, XTE, and Chandra observations following the outburst. We find that the ambiguity in previous timing solutions was due to enhanced spin down that resembles an exponential recovery following a glitch at the outburst onset. After fitting out the effects of the recovery, we measure a long-term spin-down rate of $\dot\nu=(-3.0\pm0.3)\times10^{-8}$ s$^{-2}$ which implies a dipolar magnetic field of $1.35\times10^{13}$ G, lower than all previous estimates for this source. We also consider the post-outburst flux evolution, and fit it with both empirical and crustal cooling models. We discuss the flux relaxation in the context of both crustal cooling and magnetospheric relaxation models.