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Session 75 - The Crab Nebula.
Display session, Thursday, June 13
The Crab Nebula continues to provide astronomers with a plethora of puzzles. One of these is a knot, first seen in an image taken by the HST-WFPC2 in March 1994 (Hester et al, 1995, ApJ 448,240). The knot lies approximately 1500 AU south-east of the Crab pulsar along the symmetry axis of the Crab system, and is the brightest feature in the synchrotron nebula. New observations of the Crab Nebula at seven epochs over a period of six months (August 1995 to February 1996), presented in an accompanying poster, show the knot to be a persistent feature which may have moved slightly closer to the pulsar over the course of the observations. Polarization observations show that the knot is polarized with the direction of polarization being along the symmetry axis of the system. This is consistent with the expected helical magnetic field of a high latitude pulsar wind. The knot itself does not move outward but is an approximately stationary disturbance in the overall flow. Images taken through different filters (300W, 547M and 814W) allow us to estimate the spectral index of the emission from the knot, which is somewhat redder than the rest of the nebula. The data from all epochs allows us to put stronger constraints on the possible existence of a counterknot on the other side (north-west) of the pulsar. The knot is especially puzzling given the existence of a shock in the polar jet from the pulsar about 1.5 \times 10^17 cm further along to the south-east. A conjecture is that the knot is actually a ring around the jet and maybe is the result of an interaction between the jet and the surrounding high latitude wind.
Program listing for Thursday