Tracing Magnetic Fields by Atomic Alignment in Extended Radiation Fields

TL;DR

This paper extends the application of Ground State Alignment (GSA) to detect weak magnetic fields in various astrophysical environments with extended radiation fields, providing a versatile tool for magnetic field mapping.

Contribution

It generalizes GSA analysis to extended radiation fields, including multipole components, and demonstrates its effectiveness in diverse astrophysical contexts.

Findings

Polarization from GSA aligns with magnetic field projection in the sky.

Dipole and quadrupole components dominate the radiation field contributions.

GSA effectively detects magnetic field directions in diffuse media.

Abstract

Tracing magnetic fields is crucial as magnetic fields play an important role in many astrophysical processes. Earlier studies have demonstrated that Ground State Alignment (GSA) is a unique way to detect weak magnetic fields (1G> B> 1exp(-15)G) in diffuse media, they consider the situation when the pumping source is a point source, which applies when the star is very far away from the diffuse media. In this paper, we explore the GSA in the presence of extended radiation fields. For the radiation fields with a clear geometric structure, we consider the alignment in circumstellar medium, binary systems, disc, and Local Interstellar Medium (LISM). For the radiation fields with unidentified pumping sources, we apply the method of multipole expansion and discuss the GSA induced by each component. We demonstrate that for general radiation fields, it is adequate to consider the contribution from dipole and quadrupole radiation components. We find that in general polarization of absorption arizing from GSA coincides with the projection of magnetic field in the 2D sky with 90 degree degeneracy. We conclude that the GSA is a unique tool to detect the direction of weak magnetic field, and it can be applied to diffuse media in any radiation field.