Modelling of the atomic lines' emission of fast moving pulsar nebulae

TL;DR

This paper introduces Shu, a computational package for modeling non-LTE emission in atomic lines from pulsar bow shocks, revealing how different spectral lines can probe the interstellar medium's density structures.

Contribution

The authors develop a new tool, Shu, for calculating detailed non-LTE emission maps across multiple spectral lines, integrating relativistic hydrodynamics and ionization states.

Findings

OIII, SII, and NII fluxes can rival Hα in slow pulsars.

Non-LTE emission morphology reveals ISM density structures.

Spectral lines serve as sensitive probes of the interstellar medium.

Abstract

Bow shocks generated by pulsars moving through weakly ionized interstellar medium (ISM) produce emission dominated by non-equilibrium atomic transitions. These bow shocks are primarily observed as H$_\alpha$ nebulae. We developed a package, named Shu, that calculates non-LTE intensity maps in more than 150 spectral lines, taking into account geometrical properties of the pulsars' motion and lines of sight. We argue here that atomic (CI, NI, OI) and ionic (SII, NII, OIII, NeIV) transitions can be used as complementary and sensitive probes of ISM. We perform self-consistent 2D relativistic hydrodynamic calculations of the bow shock structure and generate non-LTE emissivity maps, combining global dynamics of relativistic flows, and detailed calculations of the non-equilibrium ionization states. We find that though typically H$_\alpha$ emission is dominant, spectral fluxes in OIII, SII and NII may become comparable for relatively slowly moving pulsars. Overall, morphology of non-LTE emission, especially of the ionic species, is a sensitive probe of the density structures of the ISM.