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Session 75 - The Crab Nebula.
Display session, Thursday, June 13
Great Hall,
The Hubble Space Telescope WFPC2 has been used to monitor the temporal evolution of the Crab Nebula. The inner region of the nebula is extremely dynamic, with significant variation in the structure of the nebula on time scales of one week. The sharp (\approx 0".2 wide) striations in the nebula to the NW of the pulsar, seen in previous HST images of the Crab (Hester et al. 1995 ApJ, 448, 240), are observed to be moving away from the pulsar at apparent velocities that are typically about 0.4 -- 0.5c, but can be as high as \approx .7c. The observed velocities are higher than the true velocity, owing to the same ``arrival time'' effect responsible for superluminal motions in galactic radio jets. The slower motions of wisp-like features to the south of the pulsar are consistent with the opposite effect in material moving away from us on the far side of the torus. The sharp striations appear to slow down with distance away from the pulsar as they ``pile up'' against the body of the nebula. The wisps making up the ``halo'' reported by Hester et al. do not show pronounced motions away from the pulsar, which supports the view that these features are distinct from the main body of the equatorial torus. Faint diffuse emission, similar in character to that seen in the ``anvil'' region to the south of the pulsar, is seen to come and go in the central part of the halo as well. "Knot 2" from Hester et al. is extremely dynamic, and diffuse features in the anvil region are seen to move away from the pulsar along the axis of the system, confirming the earlier interpretation of this as emission from a polar jet. The new data also show that the knot 0".6 from the pulsar seen previously in the HST images is a persistent feature which shifts slightly in position relative to the pulsar. The knot and jet will be discussed in more detail in accompanying papers.
