Temporal Variability of the X-ray Emission of the Crab Nebula Torus
  We have analyzed five ROSAT HRI images of the Crab Nebula spanning the years 1991 to 1997 and have found significant changes in the emission structure of the X-ray torus surrounding the pulsar. Certain regions increase in brightness by about 20% over the six years, while others show decreases in surface brightness. The origin of these changes is unclear, but a possible explanation is that the bulk velocity of the synchrotron radiating electrons has decreased on the order of 20% as well. 
C. Greiveldinger and B. Aschenbach