Spin-down of the 65 millisecond X-ray pulsar in the supernova remnant G11.2-0.3

We report here the first measurement of the spin-down rate of the 65 ms X-r ay pulsar within the supernova remnant G11.2-0.3. The period derivative is measured to be (P)over dot = (4.40(-0.04)(+0.03)) x 10(-14) s s(-1) using t he Advanced Satellite for Cosmology and Astrophysics and BeppoSAX. From the pulsar period and its derivative, the corresponding surface magnetic field (B = 1.7 +/- 10(12) G) and the characteristic age [P/(2(P)over dot) = 2.4 x 10(4) yr] are derived. The physical association of the pulsar and superno va remnant with the historical record of A.D. 386 is discussed in the conte xt of the measured (P)over dot. If the pulsar was formed during the histori cal event, the initial pulse period should have been relatively slow (P-0 s imilar or equal to 62 ms). This initial period is more than a factor of 3 l arger than that of a small number of prototypical objects, suggesting a div ersity of initial periods for newly formed neutron stars.