Jet-dominated states: An alternative to advection across black hole event horizons in 'quiescent' X-ray binaries

We demonstrate that at relatively low mass accretion rates, black hole candidate (BHC) X-ray binaries (XRBs) should enter 'jet-dominated' states, in which the majority of the liberated accretion power is in the form of a (radiatively inefficient) jet and not dissipated as X-rays in the accretion flow. This result follows from the empirically established non-linear relation between radio and X-ray power from low/hard state BHC XRBs, which we assume also to hold for neutron star (NS) XRBs. Conservative estimates of the jet power indicate that all BHC XRBs in 'quiescence' should be in this jet-dominated regime. In combination with an additional empirical result, namely that BHC XRBs are more 'radio-loud' than NS XRBs, we find that in quiescence NS XRBs should be up to two orders of magnitude more luminous in X-rays than BHC XRBs, without requiring any significant advection of energy into a black hole. This ratio is as observed, and such observations should therefore no longer be considered as direct evidence for the existence of black hole event horizons. Furthermore, even if BHCs do contain black holes with event horizons, this work demonstrates that there is no requirement for the advection of significant amounts of accretion energy across the horizon.