IXPE view of the Sco-like source GX 349+2 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 Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
⁴ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
⁵ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
⁶ INAF–Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate (LC), Italy
⁷ INAF–Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
⁸ Nordita, KTH Royal Institute of Technology and Stockholm University, Hannes Alfvéns väg 12, SE-10691 Stockholm, Sweden
⁹ Department of Physics, University of Helsinki PO Box 64 00014 Helsinki, Finland
¹⁰ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China

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

Received: 23 May 2025
Accepted: 9 July 2025

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 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° ±7° in the 2–8 keV band, with a ∼4.1σ 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.