  We report on the long term X-ray monitoring with Swift, RXTE, Suzaku, Chandra
and XMM-Newton of the outburst of the newly discovered magnetar Swift
J1822.3-1606 (SGR 1822-1606), from the first observations soon after the
detection of the short X-ray bursts which led to its discovery, through the
first stages of its outburst decay (covering the time-span from July 2011,
until end of April 2012). We also report on archival ROSAT observations which
witnessed the source during its likely quiescent state, and on upper limits on
Swift J1822.3-1606's radio-pulsed and optical emission during outburst, with
the Green Bank Telescope (GBT) and the Gran Telescopio Canarias (GTC),
respectively. Our X-ray timing analysis finds the source rotating with a period
of P=8.43772016(2) s and a period derivative \dot{P}=8.3(2)x10^{-14} s s^{-1} ,
which entails an inferred dipolar surface magnetic field of B~2.7x10^{13} G at
the equator. This measurement makes Swift J1822.3-1606 the second lowest
magnetic field magnetar (after SGR 0418+5729; Rea et al. 2010). Following the
flux and spectral evolution from the beginning of the outburst, we find that
the flux decreased by about an order of magnitude, with a subtle softening of
the spectrum, both typical of the outburst decay of magnetars. By modeling the
secular thermal evolution of Swift J1822.3-1606, we find that the observed
timing properties of the source, as well as its quiescent X-ray luminosity, can
be reproduced if it was born with a poloidal and crustal toroidal fields of
B_{p}~1.5x10^{14} G and B_{tor}~7x10^{14} G, respectively, and if its current
age is ~550 kyr.
