On the implications of recent observations of the inner knot in the Crab nebula

Abstract

Recent observations of the Crab nebula (Rudy et al.) have maintained its reputation for highenergy astrophysical enlightenment and its use as a test-bed for theories of the behaviour of magnetized, relativistic plasma. In particular, new observations of the inner knot located 0.65 arcsec SE from the pulsar confirm that it is compact, elongated transversely to the symmetry axis and curved concave towards the pulsar. 60 per cent polarization has been measured along the symmetry axis (Moran et al.). The knot does not appear to be involved in the gamma-ray flares. The new observations both reinforce the interpretation of the knot as dissipation of the pulsar wind at a strong shock and challenge the details of existing models of this process. In particular, it is argued that the compactness, high polarization, and curvature are difficult to reconcile with simple relativistic shock models. Alternative possibilities include deflection of the outflow ahead of the shock and spatial variation in which the knot is interpreted as a caustic. Some future observations are proposed and new theoretical investigations are suggested.