The inner knot of the Crab nebula

TL;DR

This paper models the Crab Nebula's inner knot as synchrotron emission from a mildly relativistic, low-magnetization shock in the pulsar wind, explaining its observed properties and implications for particle acceleration and gamma-ray flares.

Contribution

The study provides a detailed MHD shock model that reproduces the knot's features and constrains the pulsar wind's magnetization and geometry, offering insights into particle acceleration sites.

Findings

The knot's properties are consistent with a low magnetization (σ ≤ 1) shock.

The striped wind zone is likely wide, with an inclination angle ≥ 45°.

The shock is a site of efficient particle acceleration but unlikely for gamma-ray flares.

Abstract

We model the inner knot of the Crab Nebula as a synchrotron emission coming from the non-spherical MHD termination shock of relativistic pulsar wind. The post-shock flow is mildly relativistic; as a result the Doppler-beaming has a strong impact on the shock appearance. The model can reproduce the knot location, size, elongation, brightness distribution, luminosity and polarization provided the effective magnetization of the section of the pulsar wind producing the knot is low, $\sigma \leq 1$. In the striped wind model, this implies that the striped zone is rather wide, with the magnetic inclination angle of the Crab pulsar $\ge 45^\circ$; this agrees with the previous model-dependent estimate based on the gamma-ray emission of the pulsar. We conclude that the tiny knot is indeed a bright spot on the surface of a quasi-stationary magnetic relativistic shock and that this shock is a site of efficient particle acceleration. On the other hand, the deduced low magnetization of the knot plasma implies that this is an unlikely site for the Crab's gamma-ray flares, if they are related to the fast relativistic magnetic reconnection events.