3D dynamics and morphology of bow-shock Pulsar Wind Nebulae

TL;DR

This paper uses 3D simulations and analytical models to explore how geometrical factors influence the diverse shapes of bow-shock pulsar wind nebulae, revealing the roles of magnetic stresses and environmental inhomogeneities.

Contribution

It introduces a classification of PWN morphologies based on geometry and provides detailed 3D RMHD simulations to reproduce observed features and variations.

Findings

Magnetic stresses produce diverse nebula shapes like whiskers and mushroom structures.

Morphology is mainly influenced by geometrical orientation, with environmental factors having mild effects.

Results suggest many PWNe do not have aligned spin axes and velocities.

Abstract

Bow-shock pulsar wind nebulae (PWNe) show a variety of morphological shapes. We attribute this diversity to the geometrical factors: relative orientations of the pulsar rotation axis, proper velocity, and the line of sight (magnetic inclination angle may also have a certain influence on the morphology). We identify three basic types of bow-shock nebulae: (i) a "Rifle Bullet" (pulsar spin axis and proper velocity are aligned); (ii) a "Frisbee" (pulsar spin axis and proper velocity are orthogonal with the spin axis lying in the plane of the sky), and (iii) a Cart Wheel" (like frisbee but the spin axis is perpendicular to the plane of the sky). Using 3D RMHD simulations, as well as analytical calculations, we reproduce the key morphological features of the bow-shock PWNe, as well as variations, are seen across different systems. magnetic stresses within the shocked pulsar wind affect the overall structure strongly, producing "whiskers", "tails", "filled-in" and "mushroom" shapes, as well as non-symmetric morphologies. On the other hand, the interstellar medium inhomogeneities and the anisotropy of the energy flux in the pulsar wind have only a mild impact of the PWN morphology. In a few cases, when we clearly identify specific morphological structures, our results do not favor alignment of the pulsar spin axis and proper velocity. Our calculations of the underlying emission processes explain the low synchrotron X-ray efficiency (in terms of the spin-down luminosity) and imply an energetical subdominant contribution of the inverse Compton process.