Accretion Onto Weakly Magnetized Neutron Stars: Polarization Theory and Its Application to X-Ray Burster GX 13+1

Abstract

Observations show that the x-ray emission of the accreting weakly magnetized neutron stars (WMNSs) is polarized. In this article, we summarize the analytical models for the polarized emission of various components of the WMNSs. We introduce a missing theoretical model, where we assume the emission comes from the spreading layer, the extension of the boundary layer between the accretion disk and the neutron star. We show how these models and the results of the simulations provide new insights into the x-ray polarization from weakly magnetized neutron stars observed with the imaging x-ray polarimetry explorer (IXPE). We specifically focus on the most peculiar case of the X-ray burster GX 13+1.