SIMBAD references

We present four new, high-resolution VLA radio images of the Crab Nebula, taken between 2001 February 25 and April 17. The radio images show systematic variability in the Crab's radio emission throughout the region near the pulsar. The principal geometry of the variable features is that of elliptical ripples very similar to the optical wisps. The radio wisps are seen to move systematically outward with projected speeds of up to 0.3c. Comparing the new radio images with our earlier ones from 1998 and 2000, we show that there are also more slowly moving features somewhat farther away from the pulsar. In particular, there is a prominent moving feature to the northwest of the pulsar that has a projected speed of the order of 10⁴ km/s. Striation is seen throughout the nebula, suggesting the presence of wavelike disturbances propagating through the synchrotron bubble. The radio images were taken simultaneously with HST optical observations as part of a unique observing campaign to obtain simultaneous, time-resolved, high-resolution images of the Crab in different wavebands. Comparing the radio to the optical images, we find that the radio wisps are sometimes displaced from the optical ones or have no optical counterparts. We also find that some optical wisps in particular, the brightest optical wisps near the pulsar, do not seem to have radio counterparts. In the exterior of the nebula, by contrast, there is generally a good correspondence between the radio and optical features.