First Polarimetric View of GX 349+2 with the Imaging X-Ray Polarimetry Explorer

Abstract

We conducted a spectropolarimetric study of the bright Z source GX 349+2 using the Imaging X-ray Polarimetry Explorer (IXPE) observation. Our findings reveal a significant polarization degree (PD) of 1.1% ± 0.3% in the 2.0–8.0 keV energy range. Spectropolarimetric analysis was performed by modeling the source spectra with an absorbed multicolor disk component and a blackbody. This allowed us to constrain the polarization contributions from the disk and boundary/spreading layer. The results indicate that the observed polarization signal primarily originates from the disk and the spreading layer at the neutron star’s surface, rather than the boundary layer. Additionally, we detect an excess polarization component, which we attribute to either an outflow or reflection processes within the system, indicating the presence of a third component, albeit not observed in the IXPE spectra. Furthermore, energy-resolved polarization analysis in the 2.0–4.0 and 4.0–8.0 keV energy ranges hinted at a marginal increase of PD with energy and rotation of polarization angle (PA). This also pointed to an energy-dependent dominance of emission and indicated that the variation in PA with energy (∼17∘ in the 2.0–4.0 keV energy range and ∼48∘ in the 4.0–8.0 keV energy range) is likely associated with the different nonorthogonal PAs of the disk and spreading layer components, which peak at different energies.