Modelling aperiodic X-ray variability in black hole binaries as propagating mass accretion rate fluctuations: a short review

TL;DR

This paper reviews the propagating mass accretion rate fluctuations model explaining the aperiodic X-ray variability in black hole binaries, highlighting its successes and challenges in matching observations.

Contribution

It provides a comprehensive review of the theoretical basis and observational evidence for the propagating fluctuations model in black hole binary variability.

Findings

Model explains key variability properties in light curves.

Highlights challenges in data comparison and theoretical development.

Summarizes observational support for the propagating fluctuations paradigm.

Abstract

Black hole binary systems can emit very bright and rapidly varying X-ray signals when material from the companion accretes onto the black hole, liberating huge amounts of gravitational potential energy. Central to this process of accretion is turbulence. In the propagating mass accretion rate fluctuations model, turbulence is generated throughout the inner accretion flow, causing fluctuations in the accretion rate. Fluctuations from the outer regions propagate towards the black hole, modulating the fluctuations generated in the inner regions. Here, I present the theoretical motivation behind this picture before reviewing the array of statistical variability properties observed in the light curves of black hole binaries that are naturally explained by the model. I also discuss the remaining challenges for the model, both in terms of comparison to data and in terms of including more sophisticated theoretical considerations.