Spectro-Polarimetric Study of Weakly Magnetized Neutron Star X-ray binary GX 349+2

Abstract

We report the first spectro-polarimetric investigation of the bright Z-type source GX 349+2 using simultaneous observations of \textit{IXPE}, \textit{NuSTAR} and \textit{Swift/XRT}. The source exhibited significant polarization in the 2-8 keV energy range during the flaring branch (FB) and normal branch (NB). The estimated polarization degree (PD) and polarization angle (PA) for FB are $1.74\pm 0.52\%$ ($3.3\ddot{I}ƒ$) and $19.4\pm 8.9^{\circ }$, respectively; while for NB, PD and PA are $0.8\pm 0.22\%$ ($3.6\ddot{I}ƒ$) and $35.4\pm 7.9^{\circ }$, respectively. The energy-resolved polarization for NB revealed an increase in PD from $0.78\pm 0.2\%$ ($3.6\ddot{I}ƒ$) to $1.32\pm 0.40\%$ ($3.3\ddot{I}ƒ$) and a change in PA from $17.9\pm 8.1^{\circ }$, and $53.2\pm 8.6^{\circ }$, in the energy range of 2-4 and 4-8 keV, respectively. Using the simultaneous observations of \textit{Swift/XRT} and \textit{NuSTAR}, we investigated the spectral properties of the source during NB, and the Western model well explained it. The spectra also depicted a strong and broad Fe K$\hat{I}\pm$ line. However, spectro-polarimetric analyses carried out by \textit{IXPE} align closely with model-independent polarimetric results. We discuss results obtained from the polarimetric studies in the context of various coronal geometries, and we confirm the slab-like geometry in the NB for GX 349+2.

8 pages, 3 figures