The detection of high X-ray polarization from an accretion disc corona source and its modelling via Monte Carlo radiation transfer simulation

We report a time averaged 2--8 keV X-ray polarization degree (PD) of $8.5\pm 1.6\%$ ($>3 σ$ detection) from the accretion-disc-corona (ADC) neutron-star system 2S 0921-630 (=V395 Car) observed with the Imaging X-ray Polarimetry Explorer (IXPE). As the observation includes an eclipse, we analyze eclipse and out-of-eclipse intervals separately. The eclipse PD is $15\pm3\%$, compared to $5.9\pm1.9\%$ out of eclipse, with no clear evidence for an associated change in polarization position angle (PA). We also search time averaged, eclipse and non-eclipse spectra and find marginal evidence ($2 σ$) for a change in PA with energy, and even weaker evidence for an increase in PD with energy. We use a Monte-Carlo spectropolarimetric radiation transfer simulation to model the polarization produced from a disc accreting neutron star, combining boundary-layer emission, its disc reflection, and the disc continuum, each with its intrinsic polarization. The model then also includes scattering of this composite spectrum in the column density distribution produced by a thermal-radiative wind launched by X-ray irradiation of the outer disc. At high inclination angles, where the observed flux is seen only via scattering in the wind, this model can reproduce both the observed PD and its (very weakly significant) increase with energy. However, it does not predict the stronger (but still only marginally significant) change in PA with energy. If this is a real effect then it points to a more complex, non-axisymmetric scattering geometry than that assumed in our model.