  Markwardt and Oegelman (1995) used ROSAT to reveal a 12 by 45 arcmin
structure in 1 keV X rays around the Vela pulsar, which they interpret as a jet
emanating from the pulsar. We here present an alternative view of the nature of
this feature, namely that it consists of material from very deep inside the
exploding star, close to the mass cut between material that became part of the
neutron star and ejected material. The initial radial velocity of the inner
material was lower than the bulk of the ejecta, and formed a bubble of slow
material that started expanding again due to heating by the young pulsar's
spindown energy. The expansion is mainly in one direction, and to explain this
we speculate that the pre-supernova system was a binary. The explosion caused
the binary to unbind, and the pulsar's former companion carved a lower-density
channel into the main ejecta. The resulting puncture of the bubble's edge
greatly facilitated expansion along its path relative to other directions. If
this is the case, we can estimate the current speed of the former binary
companion and from this reconstruct the presupernova binary orbit. It follows
that the exploding star was a helium star, hence that the supernova was of type
Ib. Since the most likely binary companion is another neutron star, the
evolution of the Vela remnant and its surroundings has been rather more
complicated than the simple expansion of one supernova blast wave into
unperturbed interstellar material.
