We present moderate resolution [ O iii ] \lambda \lambda 4959,5007 line emission spectra of the Crab Nebula ’ s northern ejecta jet .
These data along with an [ O iii ] image of the Crab Nebula were used to build 3-dimensional kinematic maps of the jet and adjacent remnant nebulosity to better understand the jet ’ s properties and thus its likely origin .
We find the jet ’ s systemic velocity to be +170 \pm 15 km s ^ { -1 } with radial velocities ranging from -190 to +480 km s ^ { -1 } .
Our data indicate that the jet consists of thin filamentary walls ( V _ { exp } \simeq 40 – 75 km s ^ { -1 } ) , is virtually hollow in [ O iii ] emission , and elliptical and funnel-like in shape rather than a straight cylindrical tube as previously thought .
Examination of the Crab ’ s 3D filamentary structure along the jet ’ s base reveals a large and nearly emission-free opening in the remnant ’ s thick outer ejecta shell .
The jet ’ s blueshifted and redshifted sides are surprisingly well defined and , like the jet ’ s sharp western limb , appear radially aligned with the remnant ’ s center of expansion .
These alignments , along with the opening in the nebula at the jet ’ s base and proper motions indicating an expansion age in line with the 1054 supernova event , suggest a direct connection between the jet ’ s formation and the Crab ’ s radial expansion .
While our analysis supports the scenario that the jet may simply represent the highest velocity material of the remnant ’ s N-S bipolar expansion , the nature of this expansion asymmetry remains unclear .