Vela pulsar and its synchrotron nebula

We present high-resolution Chandra X-ray observations of PSR B0833-45, the 89 ms pulsar associated with the Vela supernova remnant. We have acquired t wo observations separated by 1 month to search for changes in the pulsar an d its environment following an extreme glitch in its rotation frequency. We find a well-resolved nebula with a toroidal morphology remarkably similar to that observed in the Crab Nebula, along with an axial Crab-like jet. Bet ween the two observations, taken similar to3 x 10(5) s and similar to3 x 10 (6) s after the glitch, the flux from the pulsar is found to be steady to w ithin 0.75%; the 3 sigma limit on the fractional increase in the pulsar's X -ray flux is less than or similar to 10(-5) of the inferred glitch energy. We use this limit to constrain parameters of glitch models and neutron star structure. We do find a significant increase in the flux of the nebula's o uter arc; if associated with the glitch, the inferred propagation velocity is greater than or similar to0.7c, similar to that seen in the brightening of the Crab Nebula wisps. We propose an explanation for the X-ray structure of the Vela synchrotron nebula based on a model originally developed for t he Crab Nebula. In this model, the bright X-ray arcs are the shocked termin ation of a relativistic equatorial pulsar wind that is contained within the surrounding kidney-bean shaped synchrotron nebula comprising the postshock , but still relativistic, flow. In a departure from the Crab model, the mag netization parameter sigma of the Vela pulsar wind is allowed to be of orde r unity; this is consistent with the simplest MHD transport of magnetic fie ld from the pulsar to the nebula, where B less than or equal to 4x10(-4) G. The inclination angle of the axis of the equatorial torus with respect to the line of sight is identical to that of the rotation axis of the pulsar a s previously measured from the polarization of the radio pulse. The project ion of the rotation axis on the sky may also be close to the direction of p roper motion of the pulsar if previous radio measurements were confused by orthogonal-mode polarized components. We review effects that may enhance th e probability of alignment between the spin axis and space velocity of a pu lsar, and speculate that short-period, slowly moving pulsars are just the o nes best-suited to producing synchrotron nebulae with such aligned structur es. Previous interpretations of the compact Vela nebula as a bow-shock in a very weakly magnetized wind suffered from data of inadequate spatial resol ution and less plausible physical assumptions.