In recent years , it was found that there are several low/hard state of X-ray binaries ( XRBs ) follow an ‘ outliers ’ track of radio–X-ray correlation ( L _ { R } \propto L _ { X } ^ { b } and b \sim 1.4 ) , which is much steeper than the former universal track with b \sim 0.6 .
In this work , we compile a sample of bright radio-quiet active galactic nuclei ( AGNs ) and find that their hard X-ray photon indices and Eddington ratios are positively correlated , which is similar to that of ‘ outliers ’ of XRBs , where both bright AGNs and ‘ outliers ’ of XRBs have bolometric Eddington ratios \gtrsim 1 \%L _ { Edd } ( L _ { Edd } is Eddington luminosity ) .
The Eddington-scaled radio–X-ray correlation of these AGNs is also similar to that of ‘ outliers ’ of XRBs , which has a form of L _ { 5 GHz } / L _ { Edd } \propto ( L _ { 2 - 10 keV } / L _ { Edd } ) ^ { c } with c \simeq 1.59 and 1.53 for AGNs and XRBs respectively .
Both the positively correlated X-ray spectral evolution and the steeper radio–X-ray correlation can be regulated by a radiatively efficient accretion flow ( e.g. , disk-corona ) .
Based on these similarities , we further present a new fundamental plane for ‘ outliers ’ of XRBs and bright AGNs in black-hole ( BH ) mass , radio and X-ray luminosity space : \log L _ { R } = 1.59 ^ { +0.28 } _ { -0.22 } \log L _ { X } -0.22 ^ { +0.19 } _ { -0.20 } \log M _ { % BH } -28.97 ^ { +0.45 } _ { -0.45 } with a scatter of \sigma _ { R } = 0.51 dex .
This fundamental plane is suitable for radiatively efficient BH sources , while the former plane proposed by Merloni et al .
and Falcke et al .
may be most suitable for radiatively inefficient sources .