Mass-loading of bow shock pulsar wind nebulae

TL;DR

This paper studies how neutral hydrogen atoms entering pulsar wind nebulae cause mass loading, significantly altering the nebulae's flow dynamics and shapes, with implications for observed Hα and X-ray emissions.

Contribution

It introduces a quasi 1-D hydrodynamic model showing that even low densities of neutral hydrogen can greatly impact pulsar wind tail flow and morphology.

Findings

Mass loading causes rapid expansion of the pulsar wind tail.

Flow becomes non-stationary due to mass loading effects.

Predicted shapes align with observed nebulae in Hα and X-rays.

Abstract

We investigate the dynamics of bow shock nebulae created by pulsars moving supersonically through a partially ionized interstellar medium. A fraction of interstellar neutral hydrogen atoms penetrating into the tail region of a pulsar wind will undergo photo-ionization due to the UV light emitted by the nebula, with the resulting mass loading dramatically changing the flow dynamics of the light leptonic pulsar wind. Using a quasi 1-D hydrodynamic model of both non-relativistic and relativistic flow, and focusing on scales much larger than the stand-off distance, we find that if a relatively small density of neutral hydrogen, as low as $10^{-4}\,\text{cm}^{-3}$, penetrate inside the pulsar wind, this is sufficient to strongly affect the tail flow. Mass loading leads to the fast expansion of the pulsar wind tail, making the tail flow intrinsically non-stationary. The shapes predicted for the bow shock nebulae compare well with observations, both in H$\alpha$ and X-rays.