On the spin history of the X-ray pulsar in Kes 73: Further evidence for anultramagnetized neutron star

In previous papers, we presented the discovery of a 12 s X-ray pulsar in th e supernova remnant Kes 73, providing the first direct evidence for an ultr amagnetized neutron star, a magnetar, with an equivalent dipole field of ne arly 20 times the quantum critical magnetic field (m(e)(2)c(3)/e(h) over ba r). Our conclusions were based on two epochs of the measurement of the spin , along with an age estimate of the host supernova remnant. Herein, we pres ent a spin chronology of the pulsar using additional Ginga, ASCA, RXTE, and BeppoSAX data sets spanning over a decade. The timing and spectral analyse s confirm our initial results and severely Limit an accretion origin for th e observed flux. Over the 10 yr baseline, the pulsar is found to undergo a rapid, constant spin-down while maintaining a steady flux and an invariant pulse profile. Within the measurement uncertainties, no systematic departur es 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 re peaters; however, it is remarkably stable and has yet to display similar ou tbursts. Future gamma-ray activity from this object is likely.