Lijun Gou coauthored an estimate of black-hole spin in Cygnus X-1 using the thermal spectrum of its accretion disk. The analysis combines independent constraints on mass, distance and inclination with a relativistic thin-disk model to infer an inner emitting radius and hence a spin parameter.[ref]
The Extreme Spin of the Black Hole in Cygnus X-1 by Lijun Gou and others. Abstract: continuum fitting and external parameter uncertainties
This is a model-based inference rather than a direct image of the event horizon. Disk geometry, calibration and the relation between the spin and orbital axes are part of the interpretation.
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