TRIShUL: Technique for Reconstructing magnetic Interstellar Structure Using starLight polarization

TL;DR

This paper introduces a new, prior-free method to reconstruct 3D interstellar magnetic structures by decomposing stellar polarization data, effectively identifying dust layers and their properties in various Galactic regions.

Contribution

The novel technique decomposes line-of-sight stellar polarization into discrete dust layers without requiring priors, improving efficiency and applicability for large-scale polarization surveys.

Findings

Successfully recovers dust cloud distances and properties with polarization >0.1%

Reliable in samples with >10% background-star fraction

Agrees well with existing tomographic solutions in test cases

Abstract

We present a novel technique to decompose line-of-sight (LOS) stellar polarization as a function of distance, aimed at reconstructing three dimensional (3D) plane-of-sky (POS) magnetic structures in the interstellar medium (ISM). The method assumes that the observed polarization arises from discrete, thin dust layers located at varying distances along the LOS. Using a simple frequentist framework, it identifies structural changes in the distance-sorted cumulative Mahalanobis distance of Stokes parameters (q and u) to detect the locations of dust layers and estimate their associated physical properties (parallax and Stokes parameters) necessary to construct 3D maps. We benchmark the method using mock datasets representative of high-Galactic-latitude regions, incorporating realistic Gaia parallax uncertainties and polarization expected from the upcoming Pasiphae survey. Tests show that the method reliably recovers dust cloud distances and polarization properties when the polarization exceeds 0.1%, and the effective background-star fraction is greater than 10% in samples of about 345 stars. The dependence on background fraction decreases as the intrinsic polarization amplitude of the dust field increases. We apply our method to existing polarization data from two illustrative sightlines, one at intermediate-high Galactic latitude and one near the Galactic plane, with known tomographic solutions, finding excellent agreement with the literature and demonstrating its accuracy across both regions. Comparing with the BISP-1 approach, both methods effectively recover dust cloud properties, but our approach is prior-free and computationally more efficient in determining the optimal number of clouds along the LOS. These advantages make it flexible and broadly applicable for multi-layer dust cloud reconstruction for the upcoming era of large-scale stellar polarization surveys.