Data from Fig. Panels as in Fig. The emitted spectrum is the superposition of black bodies of temperatures decreasing from the inner radius to the outer radius of the disc. More detailed attempts to represent the optical-ultraviolet emission of 3C 273 using standard accretion disc models followed. The blue bump is broader than the spectrum of a single black body.
Accretion discs do have a distribution of temperatures.
The accretion disc models can be parametrized in terms of the central mass of the black hole and the accretion rate. It must be noted that these fits depend on the assumptions made for the underlying components. Meanwhile the accretion disc models grew more complete, including optically thin parts and a corona. These models can account for many features of the blue bump emission of quasars and are a natural consequence of accretion of matter with some angular momentum provided that an adequate source of viscosity is available to transport angular momentum towards the outer regions of the nucleus.