Revisiting the "Fundamental Plane" of Black Hole Activity at Extremely Low Luminosities

TL;DR

This study tests predictions that at extremely low luminosities, black hole X-ray emissions originate from jets rather than accretion flows, and the fundamental plane relation steepens, confirming these models with observational data.

Contribution

The paper provides observational evidence supporting the jet-origin of X-ray emission and a steeper fundamental plane relation at very low luminosities in black holes.

Findings

13 out of 16 LLAGNs match the jet-origin prediction.

The fundamental plane relation at low luminosities is steeper, aligning with theoretical models.

Data confirms the transition point where jet emission dominates over accretion flows.

Abstract

We investigate the origin of the X-ray emission in low-luminosity AGNs (LLAGNs). Yuan & Cui (2005) predicted that the X-ray emission should originate from jets rather than from an advection-dominated accretion flow (ADAF) when the X-ray luminosity $L_{\rm X}$ of the source is below a critical value of $L_{\rm X,crit} \approx 10^{-6}L_{\rm Edd}$. This prediction implies that the X-ray spectrum in such sources should be fitted by jets rather than ADAFs. Furthermore, below $L_{\rm X,crit}$ the correlation between radio ($L_{\rm R}$) and X-ray ($L_{\rm X}$) luminosities and the black hole mass ($M$)--the so-called fundamental plane of black hole activity--should deviate from the general correlation obtained by Merloni, Heinz & Di Matteo (2003) and become steeper. The Merloni et al. correlation is described by ${\rm log}L_{\rm R} =0.6{\rm log}L_{\rm X}+0.78{\rm log}M+7.33$, while the predicted correlation is ${\rm log}L_{\rm R}=1.23{\rm log}L_{\rm X} +0.25{\rm log}M-13.45$. We collect data from the literature to check the validity of these two expectations. We find that among the 16 LLAGNs with good X-ray and radio spectra, 13 are consistent with the Yuan & Cui prediction. For the 22 LLAGNs with $L_{\rm X} < L_{\rm X,crit}$, the fundamental plane correlation is described by ${\rm log}L_{\rm R}=1.22{\rm log}L_{\rm X}+0.23{\rm log}M-12.46 $, also in excellent agreement with the prediction.