Quasars are incredibly luminous objects powered by supermassive black holes at the centers of distant galaxies. The gravitational energy of quasars comes from the immense gravitational forces exerted by their supermassive black holes, which typically have masses ranging from millions to billions of times that of the Sun. As matter is pulled towards the black hole, it forms an accretion disc due to the conservation of angular momentum.
As the matter in the accretion disc spirals inward, it accelerates and heats up due to viscous forces and gravitational energy conversion, reaching temperatures of millions of degrees. This process releases enormous amounts of electromagnetic radiation across the spectrum, particularly in the X-ray and ultraviolet regions, resulting in the extreme luminosity observed in quasars.
For the second part of the question, a smaller black hole doesn’t directly lead to a larger accretion disc. The size of the accretion disc is more dependent on the angular momentum of the infalling material and the intensity of the gravitational forces acting upon it. A black hole’s mass dictates the innermost stable circular orbit (ISCO) and the gravitational pull at the event horizon, whereas the actual size of the disc is determined by how matter is fed into it and the distribution of angular momentum within the infalling material. While a smaller black hole might have a gravitational potential that allows for a more extended disc relative to its event horizon, the accretion rate and disc size are also influenced by other factors such as the environment of the black hole, the presence of a companion star, and the initial rotation speed of the infalling gas.