We report the discovery of pulsed X-ray emission from the compact source 1E 1841 - 045 , using data obtained with the Advanced Satellite for Cosmology and Astrophysics . The X-ray source is located in the center of the small-diameter supernova remnant ( SNR ) Kes 73 and is very likely to be the compact stellar-remnant of the supernova which formed Kes 73 . The X-rays are pulsed with a period of \simeq 11.8 s , and a sinusoidal modulation of roughly 30 % . We interpret this modulation to be the rotation period of an embedded neutron star , and as such would be the longest spin period for an isolated neutron star to-date . This is especially remarkable since the surrounding SNR is very young , at \sim 2000 yr old . We suggest that the observed characteristics of this object are best understood within the framework of a neutron star with an enormous dipolar magnetic field , B \simeq 8 \times 10 ^ { 14 } G .