The role of outflows in black-hole X-ray binaries

TL;DR

This paper presents a model of outflowing corona in black-hole X-ray binaries that explains multiple observed correlations in X-ray timing and spectral data with only two key parameters, challenging the necessity of a narrow jet.

Contribution

The authors develop a quantitative model of an outflowing corona that reproduces several observed correlations without requiring a compact jet, highlighting the importance of corona shape and outflow properties.

Findings

Successfully reproduces observed timing and spectral correlations.

Shows outflowing corona can explain radio-X-ray correlation.

Model does not depend on a narrow relativistic jet.

Abstract

The hot inner flow in black-hole X-ray binaries is not just a static corona rotating around the black hole, but it must be partially outflowing. We have developed a model, in which Comptonization takes place in this outflowing corona. We are able to reproduce quantitatively five observed correlations: a) the time lag as a function of Fourier frequency, b) the time lag as a function of photon energy, c) the time lag as a function of $\Gamma$, d) the time lag as a function of the cut-off energy in the spectrum, and e) the long-standing radio - X-ray correlation. All of them with only two parameters (optical depth and width of the outflow), which vary in the same ranges for all the correlations. Our model does not require a compact, narrow relativistic jet, although its presence does not affect the results. The essential ingredient of our model is the parabolic shape of the Comptonizing corona. The outflow speed also plays a minor role. Furthermore, the bottom of the outflow, in the hard state, looks like a ``slab'' to the incoming soft photons from the disk, and this can explain the observed X-ray polarization, which is along the outflow.