X-ray spectra of Cygnus X-1. Crosses represent observational data from Gou et al. (2011). Continuous (dot-dashed) lines correspond to black holes (naked singularities) with a = 0.998 (a = 1.05). Black (grey) lines represent uniformly (non-) magnetised accretion discs with λ = 0.1 (λ = 0). The hydrogen column density is set to N_H = 0.25 × 10²² cm^-2. As an illustration, the dashed line represents unabsorbed thermal spectra for N_H = 0.