Evidence for a Slim Accretion Disc in GX 17+2: A Polarimetric Study of Super-Eddington Flows

Abstract This study presents a time-resolved spectro-polarimetric analysis of GX 17+2 using IXPE, NICER and NuSTAR observations. Spectral modelling with $\mathtt {constant}$×$\mathtt {edge}$×$\mathtt {tbabs}(\mathtt {nthcomp}$+$\mathtt {diskbb}$+$\mathtt {relxillNS}$) indicates that the source was in a soft spectral state, and accreting at super-Eddington luminosity. Time-resolved PCUBE analysis shows that the polarization degree (PD) and polarization angle (PA) remain nearly constant across the 2.0−8.0 keV energy range, suggesting an aligned geometry between the spin and the binary orbital axes of the neutron star. Energy-resolved PCUBE analysis further indicates that the blackbody emission component most likely originates from the boundary layer. The spectro-polarimetric analysis confirms that the reflected emission contributes a PD < 20percnt, consistent with theoretical expectations. In contrast, the Comptonizing medium exhibits a PD of ~0.8 −1.4percnt, while the accretion disc shows a PD of ~1.4 −3.2percnt in the 2.0−8.0 keV energy range, exceeding theoretical predictions. Energy-resolved spectro-polarimetric analysis hints at excess disc polarization, possibly associated with the formation of a slim disc and the presence of disc wind under super-Eddington accretion conditions. Both time- and energy-resolved polarization measurements reveal a rotation of the PA for the $\mathtt {diskbb}$ and $\mathtt {nthcomp}$ components in the 3.5−8.0 keV energy range, suggesting a coupling between the soft and hard polarized emission components. Our results indicate that while the geometry of GX 17+2 is aligned, the emergent hard X-ray emission is significantly influenced by inner disc outflows. The nearly constant PD across energy and time further points to a complex interplay among the accretion disc, Comptonization, and reflection components.