THE SPIN-DOWN OF SWIFT J1822.3-1606: A NEW GALACTIC MAGNETAR

On 2011 July 14, a new magnetar candidate, Swift J1822.3-1606, was identified via a rate trigger on the Swift/Burst Alert Telescope. Here we present an initial analysis of the X-ray properties of the source, using data from the Rossi X-ray Timing Explorer, Swift, and the Chandra X-ray Observatory, spanning 2011 July 16-October 8. We measure a precise spin period of P = 8.43771968(6) s and a spin-down rate of (P)over dot = 2.54(22) x 10(-13), at MJD 55761.0, corresponding to an inferred surface dipole magnetic field of B = 4.7(2) x 10(13) G, the second lowest thus far measured for a magnetar, though similar to those of 1E 2259+586 and several high-magnetic field radio pulsars. We show that the flux decay in the 1-10 keV band is best fit by a double exponential with timescales of 9 +/- 1 and 55 +/- 9 days. The pulsed count rate decay in the 2-10 keV band, by contrast, is better fit by a single exponential decay with timescale 15.9 +/- 0.2 days. After increasing from similar to 35% for similar to 20 days after the onset of the outburst, the pulsed fraction in the 2-10 keV band remained constant at similar to 45%. We argue that these properties confirm this source to be a new member of the class of objects known as magnetars. We consider the distribution of magnetar periods and inferred dipole magnetic field strengths, showing that the former appears flat in the 2-12 s range, while the latter appears peaked in the 10(14)-10(15) G range.