Fundamental Plane of Black Hole Activity in the. Quiescent Regime

A correlation among the radio luminosity (LR), X- ray luminosity (LX), and black hole (BH) mass (MBH) in active galactic nuclei (AGNs) and BH binaries is known to exist and is called the " fundamental plane" of BH activity. Yuan & Cui predict that the radio/X- ray correlation index, xi(X), changes from xi(X) approximate to 0.6 to xi(X) approximate to 1.2-1.3 when L-X /L-Edd decreases below a critical value of similar to 10(-6). While many works favor such a change, there are also several works claiming the opposite. In this paper, we gather from the. literature the. largest quiescent AGN (defined as L-X /L-Edd less than or similar to 10(-) 6) sample to date, consisting of 75 sources. We find that these quiescent AGNs follow a xi(x) approximate to 1.23 radio/X-ray relationship, in excellent agreement with the Yuan & Cui prediction. The reason for the discrepancy between the present result and some previous works is that their samples contain not only quiescent sources but also "normal" ones (i. e., L-X /L-Edd greater than or similar to 10(-6)). In this case, the quiescent sources will mix up with those normal ones in LR and LX. The value of xX will then be between 0.6 and similar to 1.3, with the exact value being determined by the sample composition, i. e., the fraction of the quiescent and normal sources. Based on this result, we propose that a more physical way to study the fundamental plane is to replace L-R and L-X with L-R /L-Edd and L-X /L-Edd, respectively.