Temporal variability of the X-ray emission of the Crab Nebula torus

We present ROSAT HRI observations of the Crab Nebula spanning the 6 years f rom 1991 March to 1997 March. A comparison of the observations reveals that there are significant (>10 sigma) differences in the emission from the neb ula at a rate of similar to 2% yr(-1). These differences are confined to ra ther large (greater than or similar to 25 " x 25 "), well-defined regions. One region is coincident with the end of the southern polar outflow, while the others are coincident with the torus surrounding the pulsar. The variou s regions have different time histories, but they generally show a monotoni c change in count rate, some increasing and others decreasing. A possible e xplanation for the observed behavior is that the radial bulk motion of the relativistic wind after shock passage has slowed by 10%-20% over the 6 year s of observations.