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

¹ INAF–Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
² Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
³ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
⁴ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels, (Barcelona), Spain
⁵ INAF–Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate (LC), Italy
⁶ Department of Physics and Astronomy, 20014 University of Turku, Turku, Finland
⁷ Department of Physics and Astronomy, Wayne State University, 666 W. Hancock St., 48201 Detroit (MI), USA
⁸ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China
⁹ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
¹⁰ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Beijing 100101, China
¹¹ School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, 19A Yuquan Road, Beijing 100049, China
¹² Institute for Frontier in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China

^⋆ Corresponding author: fabio.lamonaca@inaf.it

Received: 11 April 2025
Accepted: 16 August 2025

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 of ∼1.4% in the NB and ∼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.