A1 Refereed original research article in a scientific journal
X-ray spectropolarimetric characterisation of the Z source GX 340+0 in the normal branch
Authors: La Monaca, Fabio; Di Marco, Alessandro; Coti Zelati, Francesco; Bobrikova, Anna; Ludlam, Renee M.; Poutanen, Juri; Marino, Alessio; Li, Songwei; Xie, Fei; Feng, Hua; Jin, Chichuan; Rea, Nanda; Tao, Lian; Yuan, Weimin
Publisher: EDP Sciences
Publication year: 2025
Journal: Astronomy and Astrophysics
Article number: A101
Volume: 702
ISSN: 0004-6361
eISSN: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202555134
Publication's open availability at the time of reporting: Open Access
Publication channel's open availability : Open Access publication channel
Web address : https://doi.org/10.1051/0004-6361/202555134
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/505465874
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.
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Funding information in the publication:
This work is supported by National Key R&D Program of China (grant No. 2023YFE0117200), and National Natural Science Foundation of China (grant No. 12373041 and No. 12422306), and Bagui Scholars Program (XF).
This work is based on data obtained with the Einstein Probe, a space mission supported by the Strategic Priority Program on Space Science of the Chinese Academy of Sciences, in collaboration with ESA, MPE and CNES (Grant No. XDA15310000, No. XDA15052100).
FLM and ADM are partially supported by MAECI with grant CN24GR08 “GRBAXP: Guangxi-Rome Bilateral Agreement for X-ray Polarimetry in Astrophysics”. AB is supported by the Finnish Cultural Foundation grant No. 00240328. RML and SL acknowledge support by NASA under grant No. 80NSSC23K0498.
