What are black hole coronas? Are they different from accretion disks? If so, how are they formed?
The corona is the layer above/outside the main accretion disk. Just as with the solar corona, this is the region of extremely diffuse, hot material. It is relatively optically thin (transparent), and it is a source of X-rays due to thermal bremsstrahlung. The idea is that the accretion disk isn't just bordered by complete vacuum; some material is stripped from it, and some electron-positron pairs can be produced by the high-energy photons and strong magnetic fields in the region.