  In previous papers, we presented the discovery of a 12-s X-ray pulsar in the
supernova remnant Kes 73, providing the first direct evidence for an
ultramagnetized neutron star, a magnetar, with an equivalent dipole field of
nearly twenty times the quantum critical magnetic field. Our conclusions were
based on two epochs of measurement of the spin, along with an age estimate of
the host supernova remnant. Herein, we present a spin chronology of the pulsar
using additional GINGA, ASCA, XTE, & SAX datasets spanning over a decade.
Timing and spectral analysis confirms our initial results and severely limit an
accretion origin for the observed flux. Over the 10 year baseline, the pulsar
is found to undergo a rapid, constant spindown, while maintaining a steady flux
and an invariant pulse profile. Within the measurement uncertainties, no
systematic departures from a linear spin-down are found - departures as in the
case of glitches or simply stochastic fluctuations in the pulse
times-of-arrival (e.g. red timing noise). We suggest that this pulsar is akin
to the soft gamma-ray repeaters, however, it is remarkably stable and has yet
to display similar outbursts; future gamma-ray activity from this object is
likely.
