HST WFPC-2 Observations of the Inner Synchrotron Component of the Crab Nebula

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Session 56 -- Supernova Remnants Observation
Display presentation, Thursday, 2, 1994, 9:20-6:30

[56.07] HST WFPC-2 Observations of the Inner Synchrotron Component of the Crab Nebula

P.A. Scowen, J.J. Hester (ASU), J. Gallagher (U. Wisconsin), R. Lynds, E.J. O'Neil, Jr. (KPNO), D.G. Currie (U. Maryland), WF/PC IDT, WFPC-2 IDT

Using the improved resolution and limiting surface brightness of the WFPC-2 we have resolved the inner Crab nebula revealing new structure and clarifying known structures.

We report the discovery of a cap-like knot of synchrotron emission located 0.7" to the SE of the pulsar (0.7" = 1400 AU at a distance of 2 kpc). This knot is aligned with the X-ray jet, opposite to the direction to Scargle's wisps. We tentatively suggest this knot may be a shock in the polar Poynting-flux-driven wind from the pulsar. A second sharp knot 0.2" $\times$ 0.6" in extent is located $\sim$ 4" from the pulsar along a line with the first knot. No such compact features are seen to the NW of the pulsar.

Scargle's wisps are typically ~0.5" wide, but show structure down to the 0.1" resolution of the camera. We have compared the morphology of the new images with that apparent in the superb ground based data of van den Bergh and Prichet (1989), and find very significant changes in appearance over the intervening 5.5 years.

The extent of the changes over such a short temporal baseline, together with the fact that wisp-like features have been observed in the same general area over much longer timescales, suggests that wisps are constantly forming and dissipating in the region to the NW of the pulsar. This fact could greatly complicate attempts to use longer baseline observations to infer proper motions for the wisps.

Together, the two data sets suggest that the inner wisps lie on a ring. If the ring is assumed to be circular, its aspect ratio suggests an inclination of 26$^\circ$ with respect to the line of sight. In this interpretation, the inner wisps are not simply shocks in a polar outflow.

These data is presented, together with conjecture on how the inferred geometry of the system relates to models of the pulsar and its wind.

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