A1 Vertaisarvioitu alkuperäisartikkeli tieteellisessä lehdessä
IXPE view of the Sco-like source GX 349+2 in the normal branch
Tekijät: La Monaca, Fabio; Bobrikova, Anna; Poutanen, Juri; Zelati, Francesco Coti; Pilia, Maura; Veledina, Alexandra; Bachetti, Matteo; Loktev, Vladislav; Xie, Fei
Kustantaja: EDP Sciences
Julkaisuvuosi: 2025
Lehti:: Astronomy and Astrophysics
Artikkelin numero: A40
Vuosikerta: 702
ISSN: 0004-6361
eISSN: 1432-0746
DOI: https://doi.org/10.1051/0004-6361/202555647
Verkko-osoite: https://doi.org/10.1051/0004-6361/202555647
Rinnakkaistallenteen osoite: https://research.utu.fi/converis/portal/detail/Publication/504926291
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 mainly found in the normal branch. A model-independent polarimetric analysis yields a polarisation degree of 1.1%+/- 0.3% at a polarisation angle of 29 degrees +/- 7 degrees in the 2-8 keV band, with a similar to 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 modelling 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, and slightly favour 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 inclinations. A study of the reflection's polarisation is also reported, with the 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 primarily governed by accretion state and orbital inclination, rather than by the subclass to which the source belongs.
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FLM is supported by the Italian Space Agency (Agenzia Spaziale Italiana, ASI) through contract ASI-INAF-2022-19-HH.0, by the Istituto Nazionale di Astrofisica (INAF) in Italy, and partially 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. AV acknowledges the Academy of Finland grant 355672. Nordita is supported in part by NordForsk. FX 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).
