Radio--X-ray Correlation and the "Quiescent State" of Black Hole Sources

Recently a correlation between the radio and X-ray luminosities is found, $L_{\rm R}\propto L_{\rm X}^{0.7}$, in black hole sources including black hole candidates in our galaxy and active galactic nuclei. We first show that the correlation can be understood in the context of an accretion-jet model developed for explaining the spectral and timing properties of XTE J1118+480. More importantly, we show that when the X-ray luminosity is below a critical value, $\la (10^{-5}$--$10^{-6}) L_{\rm Edd}$, if the jet persists, the correlation should turn and become steeper, $L_{\rm R}\propto L_{\rm X}^{1.23}$, and the X-ray radiation of the system should be dominated by the emission from the jet, rather than by the accretion flow. Possible observational evidence for our predictions is presented and future observations to further test our predictions are proposed.