1999ApJ...523L..69T

We report here the first measurement of the spin-down rate of the 65 ms X-ray pulsar within the supernova remnant G11.2-0.3. The period derivative is measured to be P{dot}=(4.40^+0.03_–0.04)x10^–14 s.s^–1 using the Advanced Satellite for Cosmology and Astrophysics and BeppoSAX. From the pulsar period and its derivative, the corresponding surface magnetic field (B=1.7x10¹² G) and the characteristic age [P/(2P{dot})=2.4x10⁴ yr] are derived. The physical association of the pulsar and supernova remnant with the historical record of A.D. 386 is discussed in the context of the measured P{dot}. If the pulsar was formed during the historical event, the initial pulse period should have been relatively slow (P₀≃62 ms). This initial period is more than a factor of 3 larger than that of a small number of prototypical objects, suggesting a diversity of initial periods for newly formed neutron stars.