Tracking the energetics of the non-thermal disc-corona-jet in the very high state GX 339-4

TL;DR

This paper develops a detailed physical model of the accretion flow in black hole binary GX 339-4 during the very high state, incorporating relativistic effects and hybrid electron distributions, to explain spectral features and energetics.

Contribution

It introduces a comprehensive accretion flow model with relativistic emissivity and hybrid Comptonisation, fitting the very high state spectrum and challenging previous spin constraints.

Findings

Hybrid electron distribution explains spectral curvature.

Inner flow is likely radiatively inefficient during VHS.

Inner flow truncation may occur outside the innermost stable orbit.

Abstract

The dramatic hard-soft spectral transition in Black Hole Binaries is important as it is associated with the collapse of the jet and with the strongest low frequency QPOs. These transition spectra (intermediate and very high state: VHS) are complex, with soft but distinctly non-thermal Comptonisation which merges smoothly into the disc emission. Here we develop a physical model for the accretion flow which can accommodate all these features, with an outer standard disc, which can make a transition to an energetically coupled disc-corona region, and make a further transition to a hot inner flow which can be radiatively inefficient if required. The code explicitly uses fully relativistic emissivity (Novikov-Thorne), and all Comptonisation is calculated with a hybrid (thermal and non-thermal) electron distribution. We fit this to a VHS spectrum from GX 339-4. We show that the complex continuum curvature produced by a hybrid electron distribution is enough to remove the strong constraint on black hole spin derived from reflection using simpler Comptonisation models. More fundamentally, we show that the VHS cannot be fit with the same Novikov-Thorne emissivity which can fit the disc dominated spectrum but instead requires that the inner flow is somewhat radiatively inefficient. This is consistent with an accretion powered jet, but simultaneous radio data show that the jet has already collapsed at the time of our data. Instead, it could point to truncation of the inner flow at radii larger than the innermost stable circular orbit, as predicted by the Lense-Thirring QPO models.