Jets and the Accretion Flow in Low Luminosity Black Holes

TL;DR

This paper investigates the origin of X-ray emission in low luminosity black holes, supporting hot flow models over jet transition models by analyzing spectral index changes and radio-X-ray correlations.

Contribution

It introduces a combined truncated disc, hot inner flow, and conical jet model to explain X-ray spectral behavior and challenges jet transition explanations based on radio-X-ray correlation observations.

Findings

X-ray spectral index change explained by seed photon switch in hot flow

Radio-X-ray correlation supports hot flow dominance over jet transition

Cooling effects in jets do not match observed radio-X-ray correlation

Abstract

The X-ray spectra of black hole binaries in the low/hard state first harden as the flux decreases, then soften. This change in behaviour has been variously attributed to either the X-rays switching from being produced in the flow to being dominated by the jet, or to the flow switching seed photons from the disc to self generated seed photons from cyclo-synchrotron. Here we build a simple truncated disc, hot inner flow, plus standard conical synchrotron jet model to explore what this predicts for the X-ray emission mechanism as a function of mass accretion rate. We find that the change in X-ray spectral index can be quantitatively (not just qualitatively) explained by the seed photon switch in the hot flow i.e. this supports models where the X-rays are always produced by the hot flow. By contrast, standard conical jet models are as radiatively inefficient as the hot flow so there is no transition in X-ray production mechanism with $\dot{m}$. Including the effects of electron cooling allows the jet X-rays to drop more slowly with accretion rate and hence overtake the X-rays from the hot flow, however this produces a corresponding change in the radio-X-ray correlation, which is not observed. We argue that the unbroken radio-X-ray correlation down to quiescence rules out the jet transition model as an explanation for the trend in X-ray spectral index. Our favoured model is then a truncated disc with an inner, hot, radiatively inefficient flow which always dominates the hard X-rays, coupled to a conical synchrotron jet which produces the radio emission. However, even this has issues at low $\dot{m}$ as the low optical depth and high temperature of the flow means that the Compton spectrum is not well approximated by a power law. This shows the need for a more sophisticated model for the electron distribution in the hot flow.