Luminous hot accretion discs

We find a new two-temperature hot branch of equilibrium solutions for stationary accretion discs around black holes. In units of Eddington accretion rate defined as 10L(Edd)/c(2), the accretion rates to which these solutions correspond are within the range (m) over dot (1) less than or similar to (m) over dot less than or similar to1, where (m) over dot (1) is the critical rate of advection-dominated accretion flow (ADAF). In these solutions, the energy loss rate of the ions by Coulomb energy transfer between the ions and electrons is larger than the viscously heating rate and it is the advective heating together with the viscous dissipation that balances the Coulomb cooling of ions. When (m) over dot (1) less than or similar to (m) over dot less than or similar to (m) over dot (2), where (m) over dot (2) similar to 5 (m) over dot (1) < 1, the accretion flow remains hot throughout the disc. When (m) over dot (2) <less than or similar to> (m) over dot less than or similar to 1, Coulomb interaction will cool the inner region of the disc within a certain radius (r(tr) similar to several tens of Schwarzschild radii or larger depending on the accretion rate and the outer boundary condition) and the disc will collapse on to the equatorial plane and form an optically thick cold annulus. Compared with ADAF, these hot solutions are much more luminous because of the high accretion rate and efficiency; therefore, we call them luminous hot accretion discs.