Detailed successful self-consistent models of the continuum source are still lacking.

However, it is also widely acknowledged that the thin-disk model is too natïve the accretion-disk itself is probably immersed in a hot corona that inverse-Compton upscatters UV photons into X-rays ( Section III.A.2), which in turn play a role in heating the disk and modifying its temperature structure.

The observed behavior is consistent with a thin-disk structure in which the variations are driven by radiation from an on-axis source (i.e., so-called “reprocessing models”): Variations are seen first at the shortest wavelengths, which arise in the inner, hotter regions of the disk, and later at longer wavelengths, which arise further out. On the other hand, in the case of NGC 7469, one of the best-studied variable Seyfert 1 galaxies across the UV/optical spectrum, brightness variations at longer wavelengths follow those at shorter wavelengths. Also, the shape of the continuum in the UV/optical is a poor match to the simple prediction that the specific luminosity per unit frequency should follow the thin-disk prediction, L ν ∝ ν 1/3. Expected signatures such as Lyman edges (due to onset of hydrogen opacity shortward of 912 Å) and continuum polarization are not observed. But the observational details do not agree well with this simplest of accretion-disk theories. Is there evidence that accretion disks do indeed exist in AGNs? Certainly, the spectral energy distributions of AGNs peak somewhere in the UV to soft X-ray range, as expected for their masses and mass accretion rates. For a 10 8- M ⊙ black hole, the radiation from the inner accretion disk will thus be in the far-UV part of the spectrum ( Section III.C). Accretion disks are expected to form as a mechanism for dissipating angular momentum, and are known to exist around stellar-mass compact objects.

It is generally assumed that the radiation is emitted from an “accretion disk” that forms around the black hole. We lack, however, a detailed understanding of how this process actually occurs. Vestergaard, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 II.B Accretion Disksįundamentally the luminosity of AGNs seems to derive from conversion of gravitational energy into radiation.

Luminosity