The Variable Jet of the Vela Pulsar

Observations of the Vela pulsar-wind nebula (PWN) with the Chandra X-Ray Observatory have revealed a complex, variable PWN structure, including inner and outer arcs, a jet in the direction of the pulsar's proper motion, and a counterjet in the opposite direction, embedded in diffuse nebular emission. The jet consists of a bright, 8″ long inner jet, between the pulsar and the outer arc, and a dim, curved outer jet that extends up to ∼100″ in approximately the same direction. From the analysis of 13 Chandra observations spread over ≈ 2.5 yr we found that this outer jet shows particularly strong variability, changing its shape and brightness. We observed bright blobs in the outer jet moving away from the pulsar with apparent speeds (0.3-0.6)c and fading on timescales of days to weeks. If the blobs are carried away by a flow along the jet, the observed variations suggest mildly relativistic flow velocities, about (0.3-0.7)c. The spectrum of the outer jet fits a power-law model with a photon index T = 1.3 ± 0.1. For a distance of 300 pc, the apparent average luminosity of the outer jet in the 1-8 keV band is about 3 × 10³⁰ ergs s^-1, compared to 6 × 10 ³² from the whole PWN within 42″ from the pulsar. The X-ray emission of the outer jet can be interpreted as synchrotron radiation of ultrarelativistic electrons/positrons. This interpretation allows one to estimate the magnetic field, ∼100 μG, maximum energy of X-ray-emitting electrons, ∼2 × 10¹⁴ eV, and energy injection rate, ∼8 × 10³³ ergs s^-1, for the outer jet. In the summed PWN image, we see a faint, strongly bent, extension of the outer jet. This bending could be caused by combined action of a wind within the supernova remnant, with a velocity of a few × 10 km s^-1, along with the ram pressure due to the pulsar's proper motion. The more extreme bends closer to the pulsar, as well as the apparent side motions of the outer jet, can be associated with kink instabilities of a magnetically confined, pinched jet flow. Another feature found in the summed image is a dim, ∼2′ long outer counterjet, which also shows a power-law spectrum with T ≈ 1.2-1.5. Southwest of the jet/counterjet (i.e., approximately perpendicular to the direction of pulsar's proper motion), an extended region of diffuse emission is seen. Relativistic particles responsible for this radiation are apparently supplied by the outer jet.