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 $1.74 \pm 0.52\%$ ($3.3\sigma$) and $19.4 \pm 8.9^\circ$, respectively; while for NB, PD and PA are $0.8 \pm 0.22\%$ ($3.6\sigma$) and $35.4 \pm 7.9^\circ$, respectively. The energy-resolved polarization for NB revealed an increase in PD from $0.78\pm0.2\%$ ($3.6\sigma$) to $1.32\pm0.40\%$ ($3.3\sigma$) and a change in PA from $17.9\pm8.1^\circ$, and $53.2\pm8.6^\circ$, 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$\alpha$ 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.