This work was partly supported by a grant from ASI and a grant from the Italian Ministry of University and Research (Cofin-99-02-02). We are indebted to Dr. M.P. Véron-Cetty and Dr. M. F. Bietenholz for putting at our disposal the optical and radio images of the Crab Nebula, respectively. One of us (M. S.) acknowledges the hospitality of the Department of Astronomy of the University of California at Berkeley, where part of this work was done.
© European Southern Observatory (ESO) 2000
Online publication: July 13, 2000