Evolution of relativistic thin discs with a finite ISCO stress: II. Late time behaviour

Abstract

We present solutions to the relativistic thin disc evolutionary equation using a modified description of the mean fluid flow within the disc. The model takes into account the effects of sub-circular velocities in the innermost disc regions, and resolves otherwise unsustainable behaviour present in simple finite ISCO stress disc models. We show that the behaviour of a relativistic thin disc evolving with a finite ISCO stress is comprised of three distinct stages which join the ordinarily distinct finite and vanishing ISCO stress solutions into a fully continuous model parameterisation. The most important prediction of our model is the existence of an intermediate stage of "stalled accretion", controlled by a single dimensionless parameter. The hallmarks of this evolutionary phase appear to have been seen in GRMHD simulations as well as in the late time X-ray observations of tidal disruption events, but dedicated simulations and extended observations are needed for a deeper understanding.