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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 (rg=GM/c2), and a value of ? is obtained for the dimensionless black hole spin. 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 ?, which is consistent with the recent optical measurement of the binary system inclination by Orosz et al. of 27°± 0?8. 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 3rg, steepening considerably within 2rg, as expected in the strong gravity regime.


Cosmology, Relativity, and Gravity | Stars, Interstellar Medium and the Galaxy


NOTICE IN COMPLIANCE WITH PUBLISHER POLICY: This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©2012 The Authors. Published by Oxford University Press on behalf of The Royal Astronomical Society. All rights reserved. Available at doi:10.1111/j.1365-2966.2012.21185.x