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 from 1991 March to 1997 March. A comparison of the observations reveals that there are significant (>10 sigma) differences in the emission from the nebula at a rate of similar to 2% yr(-1). These differences are confined to rather 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 various regions have different time histories, but they generally show a monotonic change in count rate, some increasing and others decreasing. A possible explanation 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 years of observations.