A physical model for radio and X-ray correlation in black hole X-ray binaries

TL;DR

This paper presents a physical model linking magnetic fields, accretion flows, and jet launching in black hole X-ray binaries, successfully reproducing observed radio and X-ray correlations.

Contribution

It introduces a model where magnetic field advection in hot accretion flows explains the radio/X-ray correlation in BHXRBs, including the steepening at higher luminosities.

Findings

Model reproduces observed radio/X-ray correlation quantitatively.

Steeper correlations at higher brightness explained by disc pressure transition.

Magnetic field strength varies with accretion rate and disc pressure regime.

Abstract

A tight correlation between the radio and X-ray emission in the hard state of black hole X-ray binaries (BHXRBs) indicates an intrinsic disc-jet connection in stellar black hole accretion systems, though the detailed physics processes at work are still quite unclear. A hot accretion flow is suggested to match the outer cold thin disc at a certain radius in the hard state, which may vary with the accretion rate. In this work, we assume that the magnetic field generated in the outer thin disc is advected inwards by the inner hot accretion flow, which is substantially enhanced near the BH. Such a strong field threading the horizon of a spinning BH is responsible for launching relativistic jets in BHXRBs via the Blandford-Znajek mechanism. Thus, both the jet power and the X-ray emission increase with the mass accretion rate, and we find that our model calculations are able to reproduce the observed radio/X-ray correlation quantitatively. For some individual BHXRBs, the slopes of the radio/X-ray correlations become steeper when the sources are brighter. Our model calculations show that this feature is due to the transition of the outer disc with gas pressure dominated to radiation pressure dominated, which leads to different accretion rate dependence of the field strength in the outer disc.