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 counter-jet in the opposite direction , embedded in diffuse nebular emission .
The jet consists of a bright , 8 ^ { \prime \prime } -long inner jet , between the pulsar and the outer arc , and a dim , curved outer jet that extends up to \sim 100 ^ { \prime \prime } in approximately the same direction .
From the analysis of thirteen Chandra observations spread over \approx 2.5 years 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 time-scales 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 \Gamma = 1.3 \pm 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 \times 10 ^ { 30 } erg s ^ { -1 } , compared to 6 \times 10 ^ { 32 } from the whole PWN within 42 ^ { \prime \prime } 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 , \sim 100 \mu G , maximum energy of X-ray emitting electrons , \sim 2 \times 10 ^ { 14 } eV , and energy injection rate , \sim 8 \times 10 ^ { 33 } { erg 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 \times 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 , \sim 2 ^ { \prime } -long outer counter-jet , which also shows a power-law spectrum with \Gamma \approx 1.2 –1.5 .
Southwest of the jet/counter-jet ( 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 .