A correlation among the radio luminosity ( L _ { R } ) , X-ray luminosity ( L _ { X } ) , and black hole mass ( M _ { BH } ) in active galactic nuclei ( AGNs ) and black hole binaries is known to exist and is called the “ Fundamental Plane ” of black hole activity . Yuan & Cui ( 109 ) predicts that the radio/X-ray correlation index , \xi _ { X } , changes from \xi _ { X } \approx 0.6 to \xi _ { X } \approx 1.2 - 1.3 when L _ { X } / L _ { Edd } decreases below a critical value \sim 10 ^ { -6 } .
While many works favor such a change , there are also several works claiming the opposite .
In this paper , we gather from literature a largest quiescent AGN ( defined as L _ { X } / L _ { Edd } \lesssim 10 ^ { -6 } ) sample to date , consisting of 75 sources .
We find that these quiescent AGNs follow a \xi _ { X } \approx 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 } \gtrsim 10 ^ { -6 } ) .
In this case , the quiescent sources will mix up with those normal ones in L _ { R } and L _ { X } .
The value of \xi _ { X } will then be between 0.6 and \sim 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 .