  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 2.97(28)E-13, at MJD 55761.0, corresponding to an
inferred surface dipole magnetic field strength of B=5.1E13 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+/-0.3 days. After increasing from ~35% during the first week
after the onset of the outburst, the pulsed fraction in the 2--10 keV 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.
