2015MNRAS.453.3447D

It is very controversial whether radio-X-ray correlation as defined in low-hard state of X-ray binaries (XRBs) can extend to quiescent state (e.g. X-ray luminosity less than a critical value of LX,c ∼ 10^–5.5LEdd) or not. In this work, we collect a sample of XRBs and low-luminosity active galactic nuclei (LLAGNs) with wide distribution of Eddington ratios (LX/LEdd ∼ 10^–9-10^–3) to re-explore the Fundamental Plane between 5 GHz radio luminosity, LR, 2-10 keV X-ray luminosity, LX, and black hole (BH) mass, MBH, namely logLR = ξXlogLX + ξMlogMBH + constant. For the whole sample, we confirm the former Fundamental Plane of Merloni et al. and Falcke et al. that ξX ∼ 0.6 and ξM ∼ 0.8 even after including more quiescent BHs. The quiescent BHs follow the Fundamental Plane very well, and, however, FR I radio galaxies follow a steeper track comparing other BH sources. After excluding FR Is, we investigate the Fundamental Plane for BHs in quiescent state with LX < LX,c and sub-Eddington BHs with LX > LX,c, respectively, and both sub-samples have a similar slope, ξX ∼ 0.6, which support that quiescent BHs may behave similar to those in low-hard state. We further select two sub-samples of AGNs with BH mass in a narrow range (FR Is with MBH = 10^8.8±0.4 and other LLAGNs with MBH = 10^8.0±0.4) to simulate the behaviour of a single supermassive BH evolving from sub-Eddington to quiescent state. We find that the highly sub-Eddington sources with LX/LEdd ∼ 10^–6-10^–9 still roughly stay on the extension of radio-X-ray correlation as defined by other sub-Eddington BHs. Our results are consistent with several recent observations in XRBs that the radio-X-ray correlation as defined in low-hard state can extend to highly sub-Eddington quiescent state.