On the Nature of the Bar-Shaped X-ray Feature in the Lee Jet of the Vela Pulsar Wind Nebula

X-ray morphology of pulsar wind nebulae (PWNe) reflects the structure of underlying plasma outflows. This study aims to reveal the nature of one fine X-ray feature in images of the Vela nebula. The feature is shaped like a narrow bar, oriented across the south-eastern jet of the nebula. It is located in the jet at the beginning of its bright middle section (stretching from 5''.4 to 10''.5 from the pulsar) and has an angular size of 4''.5× 1''.5 . Within the framework of relativistic MHD modeling, we show that such a bar may be a characteristic feature of Vela-like objects-transonic PWNe with a double-torus X-ray structure seen from the leeward side. Our modeling indicates the shock-wave origin for the bar. The shock is formed in the jet due to its dynamical interaction with two regular toroidal vortices in the lee hemisphere of the nebula. Namely, with large-scale circulation at middle latitudes, and with smaller-scale recirculation within the polar funnel of the pulsar wind termination shock. The former vortex almost dams the lee jet in its far downstream (so the bright middle section of the jet is limited in length by the axial size of the vortex) and the latter chokes the jet at its very beginning. The shock underlying the bar feature occurs immediately after the breakthrough of the supersonic jet through the recirculation. The shock is strong, so it propagates from the jet’s body into the surrounding plasma, and it is quasi-stationary–it can disappear and reappear again at the same place.