  Large energy-dependent X-ray polarization degree is detected by the Imaging
X-ray Polarimetry Explorer ({IXPE}) in the high-soft emission state of the
black hole X-ray binary 4U 1630--47. The highly significant detection (at
$\approx50\sigma$ confidence level) of an unexpectedly high polarization,
rising from $\sim6\%$ at $2$ keV to $\sim10\%$ at $8$ keV, cannot be easily
reconciled with standard models of thin accretion discs. In this work we
compare the predictions of different theoretical models with the {IXPE} data
and conclude that the observed polarization properties are compatible with a
scenario in which matter accretes onto the black hole through a thin disc,
covered by a partially-ionized atmosphere flowing away at mildly relativistic
velocities.
