The Variable Jet of the Vela Pulsar

Observations of the Vela pulsar-wind nebula (PWN) with the Chandra X-rayObservatory have revealed a complex, variable PWN structure, including innerand outer arcs, a jet in the direction of the pulsar's proper motion, and acounter-jet in the opposite direction, embedded in diffuse nebular emission. From the analysis of thirteen Chandra observations spread over about 2.5years we found that this outer jet shows particularly strong variability,changing its shape and brightness. We observed bright blobs in the outer jetmoving away from the pulsar with apparent speeds (0.3-0.6)c and fading ontime-scales of days to weeks. The spectrum of the outer jet fits a power-lawmodel with a photon index of 1.3. The X-ray emission of the outer jet can beinterpreted as synchrotron radiation of ultrarelativistic electrons/positrons.This interpretation allows one to estimate the magnetic field, ~100 microGauss,maximum energy of X-ray emitting electrons, $2 \times 10^{14}$ eV, and energyinjection rate, $8\times 10^{33}$ erg/s, for the outer jet. In the summed PWNimage, we see a faint, strongly bent, extension of the outer jet. The moreextreme bends closer to the pulsar, as well as the apparent side motions of theouter jet, can be associated with kink instabilities of a magneticallyconfined, pinched jet flow. Another feature found in the summed image is a dim,2'-long outer counter-jet, which also shows a power-law spectrum with photonindex of 1.2-1.5. Southwest of the jet/counter-jet (i.e., approximatelyperpendicular to the direction of pulsar's proper motion), an extended regionof diffuse emission is seen. Relativistic particles responsible for thisradiation are apparently supplied by the outer jet.