2014ApJ...787L..20D

In recent years, it has been found that several low/hard states of X-ray binaries (XRBs) follow an "outliers" track in the radio-X-ray correlation (L_R ∝ L_X^b and b ∼ 1.4), which is much steeper than the former universal track with b ∼ 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 XRB outliers, where both bright AGNs and XRB outliers have bolometric Eddington ratios ≳ 1%L_Edd(L_Edd is Eddington luminosity). The Eddington-scaled radio-X-ray correlation of these AGNs is also similar to that of XRB outliers, which has the form of L_5 GHz_/LEdd_∝(L2-10 keV_/LEdd_)^ c ^ with c ≃ 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 XRB outliers and bright AGNs in black-hole (BH) mass, radio, and X-ray luminosity space: log L_R = 1.59_–0.22^+0.28 log L_X - 0.22_–0.20^+0.19 log M_BH - 28.97_–0.45^+0.45 with a scatter of σ_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.