THE VELA PULSAR JET - A COMPANION-PUNCTURED BUBBLE OF FALLBACK MATERIAL

Markwardt & Ogelman used ROSAT to reveal a 12 by 45 arcmin(2) structur e in 1-keV X-rays around the Vela pulsar, which they interpret as a je t emanating from the pulsar. Here we present an alternative view of th e nature of this feature, namely that it consists of material from ver y deep inside the exploding star, close to the mass cut between materi al that became part of the neutron star and material that was ejected. The initial radial velocity of the inner material was lower than the bulk velocity of the ejecta, and this formed a bubble of slow material that started expanding again owing to heating by the spindown energy of the young pulsar. The expansion is mainly in one direction, and to explain this we speculate that the presupernova system was a binary. T he explosion caused the binary to unbind, and the former companion of the pulsar carved a lower density channel into the main ejecta. The re sulting puncture of the edge of the bubble greatly facilitated expansi on along its path relative to other directions. If this is the case, w e can estimate the current speed of the former binary companion and fr om this reconstruct the presupernova binary orbit. It follows that the exploding star was a helium star and hence that the supernova was of Type Ib. Since the most likely binary companion is another neutron sta r, the evolution of the Vela remnant and its surroundings has been rat her more complicated than the simple expansion of one supernova blast wave into unperturbed interstellar material.