X-ray spectropolarimetric characterization of the Z-source GX 340+0 in the normal branch

TL;DR

This paper uses X-ray spectropolarimetry to analyze the accretion processes and geometry of the Z-source GX 340+0, revealing state-dependent polarization and contributions from extended corona or wind.

Contribution

It provides the first detailed spectropolarimetric comparison of GX 340+0 in different accretion states, highlighting the role of extended structures in polarization.

Findings

Polarization decreases from HB to NB.

Comptonised component shows higher polarization than theoretical models.

Extended corona or wind likely contribute to observed polarization.

Abstract

This study presents an X-ray spectropolarimetric characterisation of the Z-source GX 340+0 during the normal branch (NB) and compares it with that obtained for the horizontal branch (HB), using IXPE, NICER and NuSTAR observations. The analysis reveals significant polarisation, with polarisation degrees (PD) of ${\sim}1.4$\% in the NB and ${\sim}3.7$\% in the HB, indicating a notable decrease in polarisation when transitioning from the HB to the NB. The polarisation angles show a consistent trend across the states. Spectropolarimetric analysis favours a dependence of the polarisation on the energy. The Comptonised component shows similar polarisation in both the HB and NB and is higher than the theoretical expectation for a boundary or spreading layer. This suggests a contribution from the wind or the presence of an extended accretion disc corona (ADC) to enhance the polarisation. The results obtained here highlight the importance of using polarimetric data to better understand the accretion mechanisms and the geometry of this class of sources, providing insights into the nature of the accretion flow and the interplay between different spectral components. Overall, the findings advance our understanding of the physical processes governing accretion in low-mass X-ray binaries.