On the Origin of Knots in the Vela Nebula

Pulsar wind nebulae (PWNe) show complex multiscale X-ray morphology with a wide variety of structures in form of tori, jets, rings, arcs, and knots. The global torus-jet morphology of PWNe is well described in terms of modern relativistic MHD models. Meanwhile, the nature of some of the finer features remains to be elucidated. These include the chain of knots in the ‘‘inner ring’’ in the Crab PWN and the chain of knots around the southeastern jet in the Vela PWN. Recently, the knots in Crab have been interpreted as giant plasmoids. These arise due to magnetic reconnection in the equatorial current sheet just after the shock wave terminating the pulsar wind and responsible for the appearance of the ‘‘inner ring’’ feature. In this work, we aim to find out whether the knots in Vela could have been formed in the same way. Within the frame of numerical relativistic MHD model, we show that in Vela-type nebulae—double-torus objects in transonic motion relative to their surroundings—conditions for strong reconnection and hence for giant plasmoids are formed at the outskirts of the X-ray nebula, where compact regions of high magnetization occur regularly on both sides of the equatorial belt. In the projection onto the celestial plane, these regions line up along two narrow coaxial ‘‘outer rings,’’ of which the windward is larger than the leeward. Due to the difference in size of the rings, their far halves must be very close to each other if the nebula is seen from its leeward side at the same angle as Vela. In this case, giant plasmoids from the far side of the nebula should be concentrated in the vicinity a narrow arcuate stripe bounded by these halves.