Kavli Affiliate: Roger Romani
| First 5 Authors: Ajay Ratheesh, Michal Dovčiak, Henric Krawczynski, Jakub Podgorný, Lorenzo Marra
| Summary:
Large, energy-dependent X-ray polarisation is observed in 4U 1630-47, a black
hole in an X-ray binary, in the high-soft emission state. In this state, X-ray
emission is believed to be dominated by a thermal, geometrically thin,
optically thick accretion disc. However, the observations with the Imaging
X-ray Polarimetry Explorer (IXPE) reveal an unexpectedly high polarisation
degree, rising from 6% at 2 keV to 10% at 8 keV, which cannot be reconciled
with standard models of thin accretion discs. We argue that an accretion disc
with an only partially ionised atmosphere flowing away from the disc at mildly
relativistic velocities can explain the observations.
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