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 – September 22 .
We measure a precise spin period of P = 8.43771963 ( 5 ) s and a spin-down rate of \dot { P } = 2.97 ( 28 ) \times 10 ^ { -13 } , at MJD 55761.0 , corresponding to an inferred surface dipole magnetic field strength of B = 5.1 \times 10 ^ { 13 } G , the second lowest thus far measured for a magnetar , though similar to 1E 2259+586 as well as to several high-magnetic field radio pulsars .
We show that the pulsed X-ray flux decay in the 2 – 10 keV band is best fit by an exponential with a time constant of 16.4 \pm 0.3 days .
After increasing from \sim 35 % during the first week after the onset of the outburst , the pulsed fraction in the 2 – 10 keV band remained constant at \sim 45 % .
We argue that these properties confirm this source to be a new member of the class of objects known as magnetars .