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The Imaging X-ray Polarimetry Explorer (IXPE) observations of the X-raybinary 4U 1630-47 in the high soft state revealed high linear polarizationdegrees (PDs) rising from 6% at 2 keV to 10% at 8 keV. We discuss in thisletter three different mechanisms that impact the polarization of the observedX-rays: the reflection of gravitationally lensed emission by the accretiondisk, reprocessing of the emission in outflowing plasma, and electron and ionanisotropies in the accretion disk atmosphere. We conducted detailed raytracingstudies to evaluate the impact of the reflection of strongly gravitationallylensed emission on the PDs. Although the reflected emission can produce highPDs in the high-energy tail of the thermal emission component, we do not findmodels that describe the PDs and are consistent with independent estimates ofthe source distance. We discuss the energetics of another proposed mechanism:the emission or scattering of the X-rays in mildly relativistically movingplasma outflows. We argue that these models are disfavored as they requirelarge mechanical luminosities on the order of, or even exceeding the EddingtonLuminosity. We investigated the impact of electron and ion anisotropies, butfind that their impact on the observed PDs are likely negligible. We concludewith a discussion of all three effects and avenues for future research.

Evaluation of several explanations of the strong X-ray polarization of the black hole X-ray binary 4U 1630-47