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Session 75 - The Crab Nebula.
Display session, Thursday, June 13
The remarkable temporal variations observed within the Crab Nebula (see poster paper "A") allow us to interpret some otherwise seemingly minor features around the central pulsar as being a shock in an outflowing polar jet. This interpretation is consistent with a generic model of the pulsar wind as being composed of two polar jets and a disk-like outflow, as suggested by models in which pair-production discharges provide for both the coherent radio emission and the current closure as well.
Identification of "shock" features is not easy in the Crab nebula owing to a plethora of quasi-stationary features (the so-called wisps being a subset of these). Each of these could be identified as a shock, as well as the knot very close to the pulsar (paper "C"). Regardless of whether other features might be shock-like, the feature called knot 2 in Hester, et al. (1995) has very suggestive behavior. For one, it is not a previously "named" feature although it is found just before the "anvil" along the line thought to be the spin axis of the pulsar and rough symmetry axis of the nebula (ibid). HST images only weeks apart show this feature to have changed grossly in shape and position, apparently "bouncing" around in the same fixed vicinity. This motion is very suggestive of a high-speed stream encountering a slower moving medium. Given only a couple of short interval observations, we cannot accurately characterize whether this motion is chaotic or possibly systematic in some way.
Animation is very suggestive of the anvil being the slower moving post-shock flow, showing broad features being convected away from the pulsar. This behavior is difficult to show in fixed frames, but the change in shape and position of knot 2 is obvious. Moreover, the thickness of knot 2, which often appears as a filament aligned orthogonal to the apparent stream, is very narrow and suggests (when viewing angle considerations are included) that the shock is not resolved (consistent with what would be expected for the usual estimates for nebular magnetic fields and particle energies). Another clue that we are observing an unstable shock is the fact that the knot is significantly less polarized as the surrounding features, presumably do to small scale structure and a turbulent surface superimposing emission from magnetic fields that have been twisted in different orientations because the plasma pressure is now comparable to the magnetic pressure. On the larger scale of the nebula, one can see that this flow becomes an optical/x-ray jet curving away from the pulsar. Although there is evidence of a large-scale jet in the opposite direction (ibid), it has proven difficult to find matching features in the opposite direction from the pulsar, and this apparent asymmetry is not presently understood.
Hester, J. J., et al. 1995, Ap. J. 448, 240.
Program listing for Thursday