Does the Crab Have a Shell?

We present deep images of a region around the Crab nebula made with the VLA,utilizing new imaging and deconvolution algorithms in a search for a faintradio shell. The existence of a high-velocity, hydrogen-rich envelope has beenpredicted to account for the low total mass and kinetic energy of the observednebula. No radio emission was detected from an extended source outside the Crabnebula. Our limits on surface brightness are sufficiently low to rule out theexistence of a shell around the Crab whose brightness is at least two orders ofmagnitude below SN\thinspace{1006}, the faintest historical shell-typesupernova remnant. We consider models for the progenitor star and thepre-supernova environment and conclude that if a fast, outer shock exists thenit has a sharply reduced efficiency at accelerating relativistic particles fromthe kinetic energy of the blast wave. We also looked for a steepening of thespectral index along the boundary of the Crab nebula itself, the signature ofan outer shock. However, contrary to previous claims, no such steepening wasfound. The absence of any evidence at radio wavelengths that either the Crabnebula or a hypothetical shell is interacting with the ambient medium leads toan interpretation that the supernova of 1054 AD was a peculiar low energyevent.Comment: 11 pages, uuencoded, Z-compressed fil