2011MNRAS.417..280S

We present results on the spectral variability of two Galactic black hole X-ray binaries, GRO J1655–40 and GX 339–4, in the hard state. We confirm a transition in behaviour of the photon index with luminosity, such that the well-known decrease in X-ray photon index with decreasing luminosity only continues down to L_bol∼ 0.01L_E. Below this point, the photon index increases again. For Comptonization models, this implies that the ratio of the Compton luminosity to seed photon luminosity, l_h/l_s, changes with bolometric luminosity, consistent with a scenario where seed photons change from cyclo-synchrotron at the lowest luminosities to those from a truncated disc. Alternatively, the transition could mark the point below which the non-thermal jet starts to dominate, or where reprocessed photons replace the viscous photons in an outflowing corona model.