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Large, energy-dependent X-ray polarisation is observed in 4U 1630-47, a blackhole in an X-ray binary, in the high-soft emission state. In this state, X-rayemission is believed to be dominated by a thermal, geometrically thin,optically thick accretion disc. However, the observations with the ImagingX-ray Polarimetry Explorer (IXPE) reveal an unexpectedly high polarisationdegree, rising from 6% at 2 keV to 10% at 8 keV, which cannot be reconciledwith standard models of thin accretion discs. We argue that an accretion discwith an only partially ionised atmosphere flowing away from the disc at mildlyrelativistic velocities can explain the observations.

The high polarisation of the X-rays from the Black Hole X-ray Binary 4U 1630-47 challenges standard thin accretion disc scenario