The spin of Cygnus X-1 is measured by fitting reflection models to Suzaku data covering the energy band 0.9–400 keV .
The inner radius of the accretion disc is found to lie within 2 gravitational radii ( r _ { g } = GM / c ^ { 2 } ) and a value for the dimensionless black hole spin is obtained of 0.97 ^ { +0.014 } _ { -0.02 } .
This agrees with recent measurements using the continuum fitting method by Gou et al .
and of the broad iron line by Duro et al .
The disc inclination is measured at 23.7 ^ { +6.7 } _ { -5.4 } deg , which is consistent with the recent optical measurement of the binary system inclination by Orosz et al of 27 \pm 0.8 deg .
We pay special attention to the emissivity profile caused by irradiation of the inner disc by the hard power-law source .
The X-ray observations and simulations show that the index q of that profile deviates from the commonly used , Newtonian , value of 3 within 3 r _ { g } , steepening considerably within 2 r _ { g } , as expected in the strong gravity regime .