The evolution of Kerr discs and late-time tidal disruption event light curves

TL;DR

This paper models the evolution of debris discs from tidal disruption events around black holes, showing that general relativistic effects produce light curve declines matching observed TDEs better than previous models.

Contribution

It introduces a general relativistic model for TDE disc evolution that accurately predicts late-time light curve decay indices consistent with observations.

Findings

Power law decline indices align with observed TDEs

Relativistic effects improve model accuracy

Finite stress at the innermost stable orbit supported by data

Abstract

An encounter between a passing star and a massive black hole at the centre of a galaxy, a so-called tidal disruption event or TDE, may leave a debris disc that subsequently accretes onto the hole. We solve for the time evolution of such a TDE disc, making use of an evolutionary equation valid for both the Newtonian and Kerr regimes. The late time luminosity emergent from such a disc is of interest as a model diagnostic, as it tends to follow a power law decline. The original simple ballistic fallback model, with equal mass in equal energy intervals, produces a -5/3 power law, while standard viscous disc descriptions yield a somewhat more shallow decline, with an index closer to -1.2. Of four recent, well-observed tidal disruption event candidates however, all had fall-off power law indices smaller than 1 in magnitude. In this work, we revisit the problem of thin disc evolution, solving this reduced problem in full general relativity. Our solutions produce power law indices that are in much better accord with observations. The late time observational data from many TDEs are generally supportive, not only of disc accretion models, but of finite stress persisting down to the innermost stable circular orbit.