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

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

2011-06-08

arxiv.org/abs/1106.1645v1

Astronomy and Astrophysics

Astrophysics

High Energy Astrophysical Phenomena

10 pages, 5 figures, MNRAS, in press

Scientific paper

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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