In Section 3.4, UV-to-optical continuum flux ratio, Gaskell et al. (2023) Estimating reddening of the continuum and broad-line region of active galactic nuclei: the mean reddening of NGC 5548 and the size of the accretion disc states:
...since the heating of the surface of the accretion disc comes from external illumination, geometric effects alone flatten the rise in temperature with decreasing radius as one approaches the disc centre. (Section 3.4)
I thought the accretion disc was the part of the quasar which was emitting most of the radiation. What's causing the "external illumination" in this case? Is it from the narrow-line or broad-line regions? Or something else?