THE STRUCTURE AND MOTION OF THE CRAB-NEBULA JET

A high-resolution [O III]lambda5007 image of the northern 'jet' feature of the Crab nebula shows it to be a delicate structure containing numerous knots and filaments. Comparison of this image with one taken five years earlier yields proper motions of 0.19-0.34 arcsec yr-1 for 14 features located in the northern half of the jet. Adopting 0.26 arcsec yr-1 as a representative value, the motion of the jet corresponds to a transverse velocity of 2500x(d/2 kpc) km s-1. This is consistent with an age and postsupernova acceleration comparable to the rest of the nebula, suggesting an origin coeval with the AD 1054 supernova event. The measurements also suggest a radial-like expansion away from the centre of the remnant, despite the highly non-radial appearance of the jet. Radial motions imply that the non-radial structure of the jet is not kinematically driven by peculiar filament motions along the northern periphery of the remnant. Slit spectroscopy along the major axis shows no radial velocity decrease near the northern end of the jet to indicate any filament closure. Possible origins of the jet are briefly discussed.