A quantitative explanation of the radio--X-ray correlation in black-hole X-ray binaries

TL;DR

This paper provides a quantitative explanation for the radio--X-ray correlation in black-hole X-ray binaries by modeling the outflowing corona as the source of hard X-rays and demonstrating its consistency with observed data.

Contribution

It introduces a model where the outflowing corona explains the radio--X-ray correlation and predicts specific behaviors during state transitions in black-hole X-ray binaries.

Findings

The outflowing corona model quantitatively matches observed radio--X-ray correlations.

Predicted bell-shaped radio flux as a function of spectral index Gamma.

Forecasts a breakdown of the correlation during state transitions with specific flux behaviors.

Abstract

The observed correlation between the radio and X-ray fluxes in the hard state of black-hole X-ray binaries (BHXRBs) has been around for more than two decades. It is currently accepted that the hard X-rays in BHXRBs come from Comptonization in the corona and the radio emission from the relativistic jet (Lorentz $\gamma >> 1$), which is a narrow structure of a few $R_g=GM/c^2$ at its base. The relativistic jet and the corona, however, are separate entities with hardly any communication between them, apart from the fact that both are fed from the accreting matter. It is also widely accepted that the accretion flow around black holes in BHXRBs consists of an outer thin disk and an inner hot flow. From this hot inner flow, an outflow emanates in the hard and hard-intermediate states of the source. By considering Compton up-scattering of soft disk photons in the outflow (i.e., in the outflowing corona, which is a wider structure, tens to hundreds of $R_g$ at its base, with low Lorentz gamma) as the mechanism that produces the hard X-ray spectrum, we have been able to explain quantitatively a number of observed correlations. Here, we demonstrate that this outflowing corona can also explain quantitatively the observed radio - X-ray correlation. In addition, we make the following theoretical predictions for GX 339-4: 1) the radio flux in the hard and hard-intermediate states should be a bell-shaped curve as a function of the photon-number spectral index Gamma, 2) the radio - X-ray correlation should break down when the source moves from the hard to the hard-intermediate state and instead the radio flux should first increase sharply in the hard-intermediate state and then decrease also sharply, in a very narrow range of the X-ray flux, and 3) the X-ray polarization will be parallel to the outflow in the hard state and perpendicular to it in the hard-intermediate one.