The spin-down of Swift J1822.3-1606: A new galactic magnetar

Abstract

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 , at MJD 55761.0, corresponding to an inferred surface dipole magnetic field of B = 4.7(2) × 10 13G, 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 1-10keV band is best fit by a double exponential with timescales of 9 1 and 55 9days. The pulsed count rate decay in the 2-10keV band, by contrast, is better fit by a single exponential decay with timescale 15.9 0.2days. After increasing from 35% for 20days after the onset of the outburst, the pulsed fraction in the 2-10keV band remained constant at 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 2-12s range, while the latter appears peaked in the 10 14-10 15G range. © 2011. The American Astronomical Society. All rights reserved.