Images at the observer’s plate of accretion discs tilted with i = 60°, f_col = 1.6, mass and accretion rate of Cygnus X-1 and a uniform magnetic field with λ = 0.1. Left panel: corresponds to the silhouette of a disc formed around a black hole with a = 0.99. Right panel: corresponds to a disc around a naked singularity with a = 1.05. The grey scale represents the logarithm of the temperature scale, gT_col, in Kelvin. Cartesian axes represent the impact parameters on the observer’s plate in gravitational radii, r_g = GM/c².