Accretion within the innermost stable circular orbit: analytical thermodynamic solutions in the adiabatic limit

Abstract

We present analytical solutions for the thermodynamic (temperature, pressure, density, etc.) properties of thin accretion flows in the region within the innermost stable circular orbit (ISCO) of a Kerr black hole, the first analytical solutions of their kind. These solutions are constructed in the adiabatic limit and neglect radiative losses, an idealisation valid for a restricted region of parameter space. We highlight a number of remarkable properties of these solutions, including that these solutions cool for radii $r_I/2 < r < r_I$, before increasing in temperature for $0 < r < r_I/2$, independent of black hole spin and assumptions regarding the equation of state of the accretion flow. The radiative temperature of these solutions can, for some values of the free parameters of the theory, peak within the ISCO and not in the main body of the disc. These solutions represent a fundamentally new class of analytical accretion solutions, which are both non-circular and non-radial in character.