Pulsar Wind Nebulae with Thick Toroidal Structure

TL;DR

This paper studies pulsar wind nebulae with thick toroidal structures, proposing that differences in observed morphologies are due to Rayleigh-Taylor instability effects during their evolution.

Contribution

It introduces a model explaining the formation of thick toroidal structures in pulsar wind nebulae and contrasts their features with more chaotic nebulae like the Crab.

Findings

Thick toroidal nebulae are associated with a transient phase involving reverse shocks.

Rayleigh-Taylor instability influences the nebula morphology.

Different structures result from the presence or absence of this instability.

Abstract

We investigate a class of pulsar wind nebulae that show synchrotron emission from a thick toroidal structure. The best studied such object is the small radio and X-ray nebula around the Vela pulsar, which can be interpreted as the result of interaction of a mildly supersonic inward flow with the recent pulsar wind. Such a flow near the center of a supernova remnant can be produced in a transient phase when the reverse shock reaches the center of the remnant. Other nebulae with a thick toroidal structure are G106.6+2.9 and G76.9+1.0. Their structure contrasts with young pulsar nebulae like the Crab Nebula and 3C 38, which show a more chaotic, filamentary structure in the synchrotron emission. In both situations, a torus-jet structure is present where the pulsar wind passes through a termination shock, indicating the flow is initially toroidal. We suggest that the difference is due to the Rayleigh-Taylor instability that operates when the outer boundary of the nebula is accelerating into freely expanding supernova ejecta. The instability gives rise to mixing in the Crab and related objects, but is not present in the nebulae with thick toroidal regions.