Speaker
Description
The geometry and physical conditions of black hole accretion discs are expected to change dramatically as a function of mass accretion rate and the resulting X-ray luminosity. Theoretical models predict that these changes become particularly relevant as the luminosity approaches the Eddington limit (LEdd), a regime where the standard thin-disc solution is thought to break down in favour of alternative configurations, many of which predict vertically extended, radiation pressure-supported structures in the inner regions. However, the threshold for this transition, and its observational signatures, remain poorly constrained.
I will present simultaneous X-ray, optical, and near-infrared spectroscopy of a black hole outburst, revealing a puzzling behaviour during its soft-state decay. At lower luminosities, the optical and infrared spectra indicate a strongly irradiated outer disc, while near the outburst peak (~0.2LEdd), the spectrum is unexpectedly cooler and dominated by strong recombination lines of hydrogen and helium. This reversed evolution can be explained by self-shadowing effects, where a vertically extended inner accretion flow shields the outer disc from the high-energy radiation produced close to the black hole. The evolution of the X-ray spectra also fits naturally within this scenario, suggesting that the inner flow geometry changes as the outburst decays. This suggests that the inner accretion flow would deviate from the standard thin-disc structure already at moderate luminosities, providing an observational link between inner flow geometry and outer disc spectral behaviour in stellar-mass black holes. I will discuss how these structural changes could affect our understanding of black hole accretion flows and their observational properties, such as inferred black hole spins and X-ray polarisation measurements, as well as the physical conditions of the outer disc and its associated outflows.