The impact of electron anisotropy on the polarization of the X-ray emission from black hole accretion disks and implications for the black hole X-ray binary 4U 1630-47

The Imaging X-ray Polarimetry Explorer (IXPE) observations of the X-ray binary 4U 1630-47 in the high soft state revealed linear polarization degrees (PDs) rising from 6% at 2 keV to 10% at 8 keV. Explaining the results in the framework of the standard optically thick, geometrically thin accretion disk scenario requires careful fine-tuning of the relevant model parameters. We argue here that the emission of polarized Bremsstrahlung by anisotropic electrons in the accretion disk atmosphere can account for the overall high PDs and the increase of the PDs with energy. We discuss plasma and accretion effects that can generate electron anisotropies at a level required by the 4U 1630-47 results. We conclude by emphasizing that X-ray polarimetry affords us the opportunity to obtain information about the magnetization of the accretion disk atmosphere.