Jets and the accretion flow in low-luminosity black holes

Abstract

The X-ray spectra of black hole binaries (BHB) in the low/hard state (LHS) first harden as the flux decreases, then soften. This change in behaviour has been variously attributed to either the X-rays switching from being produced in the flow to being dominated by the jet, or to the flow switching seed photons from the disc to self-generated seed photons from cyclo-synchrotron. Here, we build a simple truncated disc, hot inner flow, plus standard conical synchrotron jet model to explore what this predicts for the X-ray emission mechanism as a function of mass accretion rate. We find that the change in X-ray spectral index can be quantitatively (not just qualitatively) explained by the seed photon switch in the hot flow, i.e. this supports models where the X-rays are always produced by the hot flow. By contrast, standard conical jet models are as radiatively inefficient as the hot flow so there is no transition in X-ray production mechanism with ṁ. Including the effects of electron cooling allows the jet X-rays to drop more slowly with accretion rate and hence overtake the X-rays from the hot flow; however, this produces a corresponding change in the radio–X-ray correlation, which is not observed. We argue that the unbroken radio–X-ray correlation down to quiescence rules out the jet transition model as an explanation for the trend in X-ray spectral index. Our favoured model is then a truncated disc with an inner, hot, radiatively inefficient flow which always dominates the hard X-rays, coupled to a conical synchrotron jet which produces the radio emission. However, even this has issues at low m˙m˙ as the low optical depth and high temperature of the flow means that the Compton spectrum is not well approximated by a power law. This shows the need for a more sophisticated model for the electron distribution in the hot flow.