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arXiv · 1106.1645

Evidence for a change in the X-ray radiation mechanism in the hard state of Galactic black holes

M. A. Sobolewska·I. E. Papadakis·C. Done·J. Malzac
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Abstract

We present results on 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 ~ 0.01 L_Edd . Below this point the photon index increases again. For Comptonisation models, this implies that the ratio of the Compton luminosity to seed photon luminosity, lh/ls, 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 ones in an outflowing corona model.

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M. A. Sobolewska, I. E. Papadakis, C. Done, J. Malzac. 2011-06-08. Evidence for a change in the X-ray radiation mechanism in the hard state of Galactic black holes. https://doi.org/10.1111/j.1365-2966.2011.19209.x

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