[Paper Review] A Scheme to Unify Low-Power Accreting Black Holes - Jet-Dominated Accretion Flows and the Radio/X-Ray Correlation

H. Falcke, Elmar Körding+1 more|ArXiv.org|May 19, 2003

Astrophysical Phenomena and Observations52 references427 citations

TL;DR

This paper proposes a unifying framework for low-power accreting black holes—spanning X-ray binaries, Sgr A*, LINERs, FR I radio galaxies, and BL Lacs—by positing that they all operate in a jet-dominated accretion flow (JDAF) state below a critical accretion rate. The model predicts a universal radio/X-ray correlation when scaled by black hole mass, with non-linear scaling between radio and X-ray luminosities consistent across all source types.

ABSTRACT

We explore the evolution in power of black holes of all masses, and their associated jets, within the scheme of an accretion rate-dependent state transition. Below a critical value of the accretion rate all systems are assumed to undergo a transition to a state where the dominant accretion mode is optically thin and radiatively inefficient. In these significantly sub-Eddington systems, the spectral energy distribution is predicted to be dominated by non-thermal emission from a relativistic jet whereas near-Eddington black holes will be dominated instead by emission from the accretion disk. Reasonable candidates for such a sub-Eddington state include X-ray binaries in the hard and quiescent states, the Galactic Center (Sgr A*), LINERs, FR I radio galaxies, and a large fraction of BL Lac objects. Standard jet physics predicts non-linear scaling between the optically thick (radio) and optically thin (optical or X-ray) emission of these systems, which has been confirmed recently in X-ray binaries. We show that this scaling relation is also a function of black hole mass and only slightly of the relativistic Doppler factor. Taking the scaling into account we show that indeed hard and quiescent state X-ray binaries, LINERs, FR I radio galaxies, and BL Lacs can be unified and fall on a common radio/X-ray correlation. This suggests that jet domination is an important stage in the luminosity evolution of accreting black hole systems.

Motivation & Objective

• Unify diverse low-power black hole systems—X-ray binaries, Sgr A*, LINERs, FR I radio galaxies, and BL Lacs—under a single accretion state.
• Address the long-standing challenge of explaining the observed radio/X-ray correlation across black hole masses and accretion states.
• Establish that jet-dominated accretion flows (JDAFs) dominate radiative output in sub-Eddington systems, replacing disk-dominated emission.
• Demonstrate that non-linear scaling between radio and X-ray luminosities, corrected for black hole mass, collapses disparate sources onto a single correlation.
• Provide a physical basis for the observed radio/X-ray correlation using relativistic jet physics and mass-scaled jet acceleration regions.

Proposed method

• Propose a transition to a radiatively inefficient, jet-dominated accretion flow (JDAF) at sub-Eddington accretion rates, below ~1–10% of Eddington luminosity.
• Apply non-thermal jet emission models to predict spectral energy distributions (SEDs) dominated by flat radio spectra and power-law X-ray emission.
• Use non-linear scaling laws between radio and X-ray luminosities derived from jet physics, assuming a fixed acceleration radius at ~100–1000 gravitational radii.
• Correct observed luminosities by black hole mass to test the universality of the radio/X-ray correlation across sources with vastly different masses.
• Compare observed radio and X-ray luminosities from XRBs, Sgr A*, LINERs, FR I radio galaxies, and BL Lacs to the predicted correlation.
• Use a mass scaling factor of ~3×10⁹ M☉ to normalize luminosities, enabling direct comparison across the full range of black hole masses.

Experimental results

Research questions

• RQ1 Can the observed radio/X-ray correlation across diverse low-power black hole systems be explained by a common physical origin?
• RQ2 Does jet-dominated accretion flow (JDAF) provide a unified framework for sub-Eddington accreting black holes across stellar and supermassive scales?
• RQ3 To what extent does black hole mass modulate the radio/X-ray luminosity scaling in low-power systems?
• RQ4 Is the non-linear scaling between radio and X-ray luminosities consistent with predictions from relativistic jet models?
• RQ5 Can the transition from disk-dominated to jet-dominated accretion explain the spectral and luminosity evolution of black holes across accretion states?

Key findings

• Low-power accreting black holes—including X-ray binaries in hard/quiescent states, Sgr A*, LINERs, FR I radio galaxies, and BL Lacs—fall on a single, mass-corrected radio/X-ray correlation.
• The radio/X-ray luminosity scaling follows a non-linear relationship predicted by jet physics, with a power-law index consistent across all source types.
• The observed correlation is best explained by a common SED dominated by non-thermal emission from a relativistic jet, not by disk emission.
• A critical accretion rate threshold (~1–10% of Eddington) triggers a transition to a jet-dominated state, with the jet's emission dominating over the accretion disk.
• The jet's X-ray emission is likely produced at a fixed mass-scaled radius (~100–1000 Rg), explaining the robust scaling across black hole masses.
• The model successfully accounts for the observed luminosities of Sgr A* and LLAGN when mass corrections are applied, though Sgr A* deviates slightly in flare states due to higher X-ray flux.