On the Implications of Recent Observations of the Inner Knot in the Crab Nebula

TL;DR

Recent high-resolution observations of the Crab Nebula's inner knot challenge existing shock models by revealing its compactness, polarization, and curvature, prompting consideration of alternative interpretations and further investigations.

Contribution

This paper analyzes new observational data of the Crab Nebula's inner knot, questioning current shock models and proposing alternative explanations for its properties.

Findings

Knot is compact, elongated, and curved towards the pulsar.

High polarization (60%) observed along the symmetry axis.

Observations challenge simple relativistic shock models.

Abstract

Recent observations of the Crab Nebula (Rudy et al 2015) have maintained its reputation for high energy astrophysical enlightenment and its use as a testbed for theories of the behaviour of magnetized, relativistic plasma. In particular, new observations of the inner knot located 0.65" SE from the pulsar confirm that it is compact, elongated transversely to the symmetry axis and curved concave towards the pulsar. 60 percent polarization has been measured along the symmetry axis (Moran et al 2013). 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.