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Radio Study of Vela X Cocoon

Yihan Liu, Yu Zhang, C. -Y. Ng, Zijian Qiu, Sujie Lin, Lili Yang

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.

Radio Study of Vela X Cocoon

Abstract

The evolution of pulsar Wind Nebulae (PWNe) influences how high energy particles in the vicinity are generated and transport. The Vela PWN (only \,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 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 -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.
Paper Structure (13 sections, 2 equations, 8 figures, 1 table)

This paper contains 13 sections, 2 equations, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. Observations and Data Reduction
  3. Results
  4. Morphology
  5. Polarization Map
  6. Rotation Measure
  7. Magnetic Field Structure
  8. Spectral Index Map
  9. Discussions
  10. Nature of the Radio "Cocoon"
  11. Dynamics
  12. Modeling of Vela X Cocoon PL and RM Distribution
  13. Conclusion

Figures (8)

  • Figure 1: ATCA total intensity maps of Vela X Cocoon at 6 and 16 cm, with the beams in the bottom left. The gray-scale color bars on the right show the intensity with a unit of Jy beam$^{-1}$.
  • Figure 2: Polarization intensity map of the Vela X Cocoon region at 6 cm (left) and 13 cm(right). The contours are the total intensity map at levels of 2.25, 4.5, 6.75 mJy beam$^{-1}$ at both 6 and 13 cm. The color bar shows the intensity of polarized emission of the Vela X cocoon region.
  • Figure 3: Map of rotation measure in the Vela X Cocoon region. The color bar shows the rotation measure values in the map from $-$150 to $+$100, in units of rad m$^{-2}$. The contours show the 6 cm full-band radio image of cocoon at intensities of 2.25, 4.50, and 6.75 mJy beam$^{-1}$.
  • Figure 4: 6 cm ATCA image with $B$-field directions. The vectors show the $B$-field directions in the Vela Cocoon region, the bar at bottom left indicates a polarized intensity scale of 0.5 mJy beam$^{-1}$, which is shown as lengths of $B$-vectors.
  • Figure 5: Map of radio spectral index ($\alpha$) in the Vela X Cocoon region from -2 to 0, the black contours show the S-band intensities at levels of 7.5, 11.0, 15.0 mJy beam$^{-1}$.
  • ...and 3 more figures