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?

  • $\begingroup$ Probably the blackhole's corona $\endgroup$
    – user47732
    Commented Feb 1, 2023 at 7:15

1 Answer 1


Ok, let's understand it this way:

The central engine of an Active Galactic Nucleus (AGN) is a supermassive black hole, which accretes matter from its surroundings. This accretion process forms a disk of matter around the black hole, known as the accretion disk. As the matter spirals inward towards the black hole, it loses gravitational potential energy, which is converted into kinetic and thermal energy. This energy conversion process heats up the innermost regions of the disk to extremely high temperatures, causing them to emit X-rays. The X-ray emission from the accretion disk is primarily due to two processes: thermal emission and non-thermal emission. Thermal emission, also known as blackbody radiation, occurs when the hot gas in the inner accretion disk emits X-rays directly. Non-thermal emission, on the other hand, occurs when high-energy particles in the disk collide with photons, boosting their energy up to the X-ray range. This process is known as inverse Compton scattering.

Above the accretion disk is a region of even hotter, less dense gas known as the corona. The corona is thought to be responsible for a significant amount of the X-ray emission observed from AGN. The X-rays from the corona can irradiate the accretion disk, causing it to fluoresce and emit its own X-rays. This process is known as Compton reflection or coronal upscattering. In particular, the inverse Compton scattering of photons from the geometrically thin disk by hot electrons in the corona quickly cools the coronal gas. The reflection component in the X-ray spectra of AGN arises from the interaction of the X-ray photons with the accretion disk. I think this is the 'external illumination' being taught about here.

You may have a look at the Figure 1 in this paper for the model of AGN inner regions that I described above.


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