Why Does an Accretion Disk Become So Hot?

Where does the disk's radiation come from before matter reaches the horizon?

You followed matter inward and angular momentum outward, connected magnetic turbulence to released orbital energy, read the inward temperature trend, and kept all escaping radiation outside the event horizon.

An artistic reconstruction of a thin black-hole accretion disk changing from dim red at its outer edge to white-blue near its inner edge outside the dark central shadow. — Vastward artistic reconstruction. The colour gradient communicates an inward rise in representative disk temperature; it is not a direct photograph, a fixed colour map, or light escaping from inside the event horizon.

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