IXPE view of the Sco-like source GX 349+2 in the normal branch

TL;DR

This study uses X-ray polarimetry to analyze the accretion geometry and emission components of the Sco-like Z-source GX 349+2, revealing complex energy-dependent polarization and insights into its accretion state and geometry.

Contribution

First detailed spectropolarimetric analysis of GX 349+2, showing energy-dependent polarization and implications for accretion geometry in Sco-like Z-sources.

Findings

Polarization degree of 1.1% in 2-8 keV band

Energy-dependent polarization behavior

Reflection component with ~10% polarization

Abstract

We present a detailed spectropolarimetric study of the Sco-like Z-source GX 349+2, simultaneously observed with the Imaging X-ray Polarimetry Explorer (IXPE) and Nuclear Spectroscopic Telescope Array (NuSTAR). During the observations GX 349+2 was found mainly in the normal branch. A model-independent polarimetric analysis yields a polarisation degree of $1.1\% \pm 0.3\%$ at a polarisation angle of $29{\deg} \pm 7{\deg}$ in the 2--8 keV band, with ${\sim}4.1\sigma$ confidence level significance. No variability of polarisation in time and flux has been observed, while an energy-resolved analysis shows a complex dependence of polarisation on energy, as confirmed by a spectropolarimetric analysis. Spectral modeling reveals a dominant disc blackbody component and a Comptonising emitting region, with evidence of a broad iron line associated with a reflection component. Spectropolarimetric fits suggest differing polarisation properties for the disc and Comptonised components, slightly favouring a spreading layer geometry. The polarisation of the Comptonised component exceeds the theoretical expectations, but is in line with the results for other Z-sources with similar inclination. A study of the reflection's polarisation is also reported, with polarisation degree ranging around 10% depending on the assumptions. Despite GX 349+2's classification as a Sco-like source, these polarimetric results align more closely with the Cyg-like system GX 340+0 of similar inclination. This indicates that polarisation is governed primarily by accretion state and orbital inclination, rather than by the subclass to which the source belongs.