IXPE view of the Sco-like source GX 349+2 in the normal branch
/ Authors
/ Abstract
We present a detailed spectropolarimetric study of the Sco-like Z-source simultaneously observed with the Imaging X-ray Polarimetry Explorer ( and Nuclear Spectroscopic Telescope Array (νstar). During the observations 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.
Journal: Astronomy & Astrophysics