X-ray spectro-polarimetric characterization of GX 340+0 in the horizontal branch: a highly inclined source?

TL;DR

This paper reports the first X-ray polarization detection of GX 340+0 in the horizontal branch, revealing a polarization degree of 4.3%, and discusses implications for the source's inclination and geometry.

Contribution

It provides the first polarization measurement of GX 340+0 in the horizontal branch and constrains the polarization of its spectral components using broad-band data.

Findings

Polarization degree of 4.3% detected in GX 340+0

Polarization angle differs by ~40° between components

Inclination angle likely >60° based on polarization models

Abstract

We report the first detection of X-ray polarization in the horizontal branch for GX 340+0 as obtained by Imaging X-ray Polarimetry Explorer (IXPE). A polarization degree of 4.3%$\pm$0.3% is obtained. This value is in agreement with the previous polarization measurements of Z-sources in the horizontal branch. Spectro-polarimetric analysis, performed using a broad-band spectral model obtained by NICER and NuSTAR quasi-simultaneous observations, allowed us to constrain the polarization for the soft and hard spectral components typical to these sources. The polarization angle for the two components differs by ${\sim}40{\deg}$. This result could be explained by a misalignment of the NS rotations axis with respect to the accretion disk axis. We provide a comparison of the results with polarization expected in different models. Theoretical expectations for the polarization of the disk and the Comptonization components favor an orbital inclination for GX 340+0 higher than 60{\deg}, as expected for Cyg-like sources, in contrast with results we report for the reflection component using broad-band spectrum.