X-Ray Polarization Study of the Neutron Star Low-mass X-Ray Binary GX 349+2

Abstract We report the first X-ray polarimetric results of the neutron star (NS) low-mass X-ray binary Z-source GX 349+2 using the Imaging X-ray Polarimetry Explorer (IXPE). We discovered that the X-ray source was polarized at a polarization degree of PD = 1.1% ± 0.3% (1 σ errors) with a polarization angle of PA = 32° ± 6° (1 σ errors). Simultaneous Nuclear Spectroscopic Telescope Array observations show that the source transitioned through the normal branch, flaring branch, and soft apex of the Z-track during our IXPE observations. The X-ray spectropolarimetry results suggest a source geometry comprising an accretion disk component, a blackbody representing the emission from the NS surface, and a Comptonized component. We discuss the accretion geometry of the Z-source in light of the spectropolarimetric results.