Radio Study of Vela X Cocoon

TL;DR

This study uses high-resolution radio observations to analyze the complex filamentary structure and magnetic field properties of the Vela X Cocoon, revealing insights into its evolution and interaction with the surrounding environment.

Contribution

It provides new detailed radio imaging and spectral analysis of the Vela X Cocoon, including a simple 3D model of a spiral filament and polarization data, advancing understanding of PWN structures.

Findings

Identification of a complex filamentary structure with ordered magnetic fields.

Anti-correlation between rotation measure, polarization fraction, and intensity.

Large-scale features are stable over time, distinct from X-ray features.

Abstract

The evolution of pulsar Wind Nebulae (PWNe) influences how high energy particles in the vicinity are generated and transport. The Vela PWN (only $\sim300$\,pc away), provides a rather rare case between young and well-evolved systems. We therefore performed new 6 and 16\,cm high-resolution observations of the Vela X Cocoon region with the Australia Telescope Compact Array (ATCA). The observations reveal a complex region with a $\sim0.5^\circ$ major curved filament extending to far south from the pulsar, as well as other intersecting filaments and wisps. Our spectral analysis hints its connection with the PWN. Our results also found strongly linearly polarized emission, ordered and tangential $B$-field to the filaments. We find the rotation measure (RM) and polarization fraction (PF) along the filament are anti-correlated with the total intensity. We develop a simple 3D model of a spiral filament to explain these, while the PF distribution requires external interpretations such as interaction with the reverse shock. Comparison with archival data suggests that large scale features like the major filament are generally stable and large motions near the X-ray filament, all these confirm the distinction between radio and X-ray features.