Post-outburst X-ray flux and timing evolution of Swift J1822.3-1606

Abstract

Swift J1822.3-1606 was discovered on 2011 July 14 by the Swift Burst Alert Telescope following the detection of several bursts. The source was found to have a period of 8.4377 s and was identified as a magnetar. Here we present a phase-connected timing analysis and the evolution of the flux and spectral properties using RXTE, Swift, and Chandra observations. We measure a spin frequency of 0.1185154343(8) s$^{-1}$ and a frequency derivative of $-4.3\pm0.3\times10^{-15}$ at MJD 55761.0, in a timing analysis that include significant non-zero second and third frequency derivatives that we attribute to timing noise. This corresponds to an estimated spin-down inferred dipole magnetic field of $B\sim5\times10^{13}$ G, consistent with previous estimates though still possibly affected by unmodelled noise. We find that the post-outburst 1--10 keV flux evolution can be characterized by a double-exponential decay with decay timescales of $15.5\pm0.5$ and $177\pm14$ days. We also fit the light curve with a crustal cooling model which suggests that the cooling results from heat injection into the outer crust. We find that the hardness-flux correlation observed in magnetar outbursts also characterizes the outburst of Swift J1822.3-1606. We compare the properties of Swift J1822.3-1606 with those of other magnetars and their outbursts.