The turbulent variability of accretion discs observed at high energies

Abstract

ABSTRACT We use numerical stochastic-viscosity hydrodynamic simulations and new analytical results from thin disc theory to probe the turbulent variability of accretion flows, as observed at high energies. We show that the act of observing accretion discs in the Wien tail exponentially enhances small-scale temperature variability in the flow, which in a real disc will be driven by magnetohydrodynamic turbulence, to large-amplitude luminosity fluctuations (as predicted analytically). In particular, we demonstrate that discs with more spatially coherent turbulence (as might be expected of thicker discs), and relativistic discs observed at larger inclinations, show significant enhancement in their Wien tail variability. We believe that this is the first analysis of relativistic viewing angle effects on turbulent variability in the literature. Using these results, we argue that tidal disruption events represent particularly interesting systems with which to study accretion flow variability, and may in fact be the best astrophysical probes of small-scale disc turbulence. This is a result of a typical tidal disruption event disc being naturally observed in the Wien tail and likely having a somewhat thicker disc and cleaner X-ray spectrum than other sources. We argue for dedicated X-ray observational campaigns of tidal disruption events, with the aim of studying accretion flow variability.