Fundamental Plane of Black Hole Activity in the Quiescent Regime

Abstract

A correlation among the radio luminosity ( L R ), X-ray luminosity ( L X ), and black hole (BH) mass ( M BH ) 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, ξ X , changes from ξ X ≈ 0.6 to ξ X ≈ 1.2 – 1.3 when L X / L Edd decreases below a critical value of  ∼ 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 ≲ 10 − 6 ) sample to date, consisting of 75 sources. We find that these quiescent AGNs follow a ξ X ≈ 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 ≳ 10 − 6 ). In this case, the quiescent sources will mix up with those normal ones in L R and L X . The value of ξ X will then be between 0.6 and ∼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.